The Effects of Static and Dynamic Stretching on Reaction Time and
Performance in a Countermovement Jump
by
Erica Taylor Perrier
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented May 19, 2009
Commencement June 2009
AN ABSTRACT OF THE THESIS OF
Erica Taylor Perrier for the degree of Master of Science in Exercise and Sport Science
presented on May 19, 2009.
Title: The Effects of Static and Dynamic Stretching on Reaction Time and
Performance in a Countermovement Jump
Abstract approved:
________________________________________________________
Mark A. Hoffman
PURPOSE: The purpose of this research was to quantify the effects of a warm-up
with static or dynamic stretching on countermovement jump height, reaction time,
muscle onsets for tibialis anterior (TA) and vastus lateralis (VL), and low back/
hamstring flexibility.
METHODS: Twenty one recreationally-active males (24.4 ± 4.5 yrs), recruited from
the university community, completed 3 data collection sessions. Inclusion criteria
were regular participation (30 minutes, 3 days per week) in exercise including
resistance training, sprinting, or jumping, and no history of lower extremity injury in
the past 6 months. Each session included a 5 minute treadmill jog followed by one of
the stretch treatments: no stretching (NS), static stretching (SS), or dynamic stretching
(DS). After the general warm-up and treatment, the participant performed a sit-andreach test to assess low back and hamstring flexibility. Next, the participant
completed a series of ten maximal-effort countermovement jumps (CMJ), during
which he was asked to jump as quickly as possible after seeing a visual stimulus
(light). The onset of movement and CMJ height were determined from force plate
data, and muscle onsets (TA and VL) were obtained using surface electromyography
(sEMG). Ground reaction forces were recorded at 2000 Hz using a portable force
plate, and filtered at 25 Hz. Outcome measures included maximal jump height,
reaction time, and muscle onset (TA and VL). A repeated measures 3 (treatment) by 8
(jump) ANOVA was used to assess CMJ height. Additional outcome measures
(reaction time, muscle onsets, flexibility) were assessed using separate repeatedmeasures one-way ANOVA.
RESULTS: Results of the 3x8 repeated-measures ANOVA for CMJ height revealed a
significant main effect of treatment (p=.004). Post hoc analysis showed significant
differences between NS (41.4cm) and DS (43.0cm) (p=.0045), and between SS
(41.9cm) and DS (p=.0435), but not between NS and SS (p=.4605). Analysis also
revealed a significant main effect of jump (p=.005) on CMJ height: mean jump height
progressively decreased from the early to the late jumps. No significant interaction
between treatment and jump was observed (p=.571). The analysis of reaction time,
TA and VL sEMG onsets showed no significant effects. Treatment also had a main
effect (p<.001) on flexibility. Post hoc analysis revealed improved flexibility after
both SS (p=.002) and DS (p<.001) compared with NS, with no difference in flexibility
between the two treatments (p=.530).
CONCLUSION: CMJ height was significantly higher during the DS condition
compared to SS and NS, with no difference between NS and SS. Additionally,
reaction time and muscle onsets were not influenced by either stretch technique.
Athletes in sports requiring lower-extremity power should use dynamic stretching
techniques in warm-up to enhance flexibility while improving performance.
© Copyright by Erica Taylor Perrier
May 19, 2009
All Rights Reserved
Master of Science thesis of Erica Taylor Perrier presented on May 19, 2009.
APPROVED:
_____________________________________________________________________
Major Professor, representing Exercise and Sport Science
_____________________________________________________________________
Chair of the Department of Nutrition and Exercise Sciences
_____________________________________________________________________
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.
_____________________________________________________________________
Erica Taylor Perrier, Author
ACKNOWLEDGEMENTS
I would like to express profound appreciation to my advisor, Dr. Mark Hoffman, for
his support, encouragement, and guidance during this project. Additionally, I am
grateful to the members of my committee; Dr. Mike Pavol, Dr. Kim Hannigan-Downs,
and Dr. John Edwards, for their invaluable advice and constructive criticism. Finally,
I would like to thank Leah for her unwavering material and emotional support, as well
as her tireless efforts to get me to spend more time writing when I would rather be
playing outside.
CONTRIBUTION OF AUTHORS
Dr. Mark Hoffman was involved with the writing and editing of the manuscript
(Chapter 3). Drs. Mark Hoffman and Mike Pavol were involved with the data analysis
(Chapter 3). Dr. Heather Barber (University of New Hampshire) was involved with
the editing of the literature review (Chapter 2).
TABLE OF CONTENTS
Page
Chapter 1: Introduction ………………………………………………………… 1
References ……………………………………………………………… 4
Chapter 2: Literature Review …………………………………………………..
7
Introduction: Static Stretching and Exercise …………………………… 8
Static Stretching and Performance …………………………………….. 10
Static Stretching: Mechanisms of Performance Reductions …………... 25
Stretching and Injury Prevention ……………………………………… 30
Dynamic Stretching: An Effective Alternative ………………………... 32
Dynamic Stretching: Mechanisms of Improved Performance ………… 38
Conclusion …………………………………………………………….. 39
References …………………………………………………………….. 41
Chapter 3: Warm-Up with Dynamic Stretching Improves
Countermovement Jump Performance ……………………………. 46
Abstract ……………………………………………………………….. 48
Introduction …………………………………………………………… 50
Methods ……………………………………………………………….. 52
Results ………………………………………………………………… 56
Discussion …………………………………………………………….. 57
Practical Applications ………………………………………………… 61
References …………………………………………………………….. 66
Chapter 4: Conclusion ………………………………………………………… 69
TABLE OF CONTENTS (Continued)
Bibliography ………………………………………………………………….. 71
Appendices ……………………………………………………………………. 77
Appendix A
Institutional Review Board Documents ……………….. 78
IRB Initial Application ………………………………………... 79
IRB Approval Letter …………………………………………... 83
Informed Consent ……………………………………………... 84
Appendix B
Data Output …………………………………………… 87
LIST OF FIGURES
Figure
Page
3.1
Mean jump height (cm) after each stretch treatment ………………………. 63
3.2
Jump height progression for jumps 2 through 9 …………………………… 64
3.3
Mean onsets for movement time, TA EMG, and VL
EMG after each stretch treatment ………………………………………….. 64
3.4
Mean sit-and-reach score (cm) after each stretch treatment ……………….. 65
LIST OF TABLES
Table
Page
2.1
Summary of findings: The acute effects of static stretching on various
measures of performance …………………………………………………... 21
2.2
Summary of findings: The acute effects of dynamic stretching on various
measures of performance …………………………………………………... 36
3.1
Static stretch protocol ……………………………………………………… 62
3.2
Dynamic stretch protocol …………………………………………………... 62
3.3
Results ……………………………………………………………………… 63
Chapter 1: Introduction
2
Athletes traditionally include stretching as part of a pre-activity warm-up in
order to improve performance and decrease the risk of injury. Static stretching, the
most common method (22), involves slowly moving a joint to the endpoint of its range
of motion, just before the onset of pain. The static stretching method is popular for
several reasons: it is simple to learn, can be performed individually, and is effective in
increasing joint range of motion (13). Static stretching has been thought to improve
performance by maximizing joint range of motion and improving coordination (16).
Additionally, static stretching has been advocated as a means to prevent injury to the
musculotendinous unit (MTU) by increasing compliance of the tendon and muscle
fibers, resulting in an increased force transmission capacity (10, 18, 19). Despite this
common practice, there is no conclusive evidence supporting the theory that static
stretching prior to exercise reduces injury risk (17).
Static stretching has recently been shown to decrease performance in measures
of maximal force production (6, 7, 13-15), jump height (4, 21, 25), sprint speed (9,
12), and reaction time and balance (3). Performance reductions following static
stretching have been explained by a combination of mechanical and neurological
factors. Mechanically, static stretching results in increased compliance in the MTU.
As the MTU lengthens and becomes more compliant, contractile elements must
contract over a greater distance, and more forcefully, to “pick up the slack”, resulting
in reduced peak torque and a slower rate of force development (23). A stiffer MTU
would facilitate rapid changes in tension and a faster joint motion response, potentially
improving performance. Neurologically, static stretching may decrease motor unit
activation (1, 5, 13, 15), adversely affecting force production capability. Additionally,
3
studies have shown that static stretching performed on the dominant leg only can
result in decreases in peak torque and motor unit activation in both the stretched and
unstretched limbs (6, 7), lending further support to a neurologically-mediated
performance decline.
In response to this growing body of evidence, strength and conditioning
professionals have began to shift away from static stretching in favor of a functional,
dynamic warm-up before practices and games (2). Dynamic stretching routines
incorporate skipping, directional running, shuffling, and various calisthenics of
increasing intensity that simulate the movement patterns necessary for success in a
particular sport. Recent reports suggest that dynamic stretching prior to activity may
improve performance by increasing joint range of motion and core body temperature,
resulting in increased blood flow to the muscles and faster nerve-impulse conduction
(20). Dynamic stretches that simulate movement patterns used in a sport may also
improve coordination by providing an opportunity for sport-specific skill rehearsal
(12). Performance improvements after dynamic stretching have been documented in
sprinting (8), jumping (11), and peak force generating capacity (24). Currently,
multiple exercises (ballistic stretching, bodyweight squats, and movement drills) have
all been characterized as dynamic stretching in the literature. Further research is
needed to clarify and define the term, as well as to directly compare the effects of both
static and dynamic stretching on CMJ performance and reaction time.
4
References
1.
Avela, J., H. Kyrolainen, and P. V. Komi. Altered reflex sensitivity after
repeated and prolonged passive muscle stretching. J. Appl. Physiol. 86:12831291, 1999.
2.
Baechle, T. R. and R. W. Earle. Essentials of Strength Training and
Conditioning. 3 ed. Champaign: Human Kinetics, 2008
3.
Behm, D. G., A. Bambury, F. Cahill, and K. Power. Effect of acute static
stretching on force, balance, reaction time and movement time. Med. Sci.
Sports Exerc. . 36:1397-1402, 2004.
4.
Behm, D. G., E. E. Bradbury, A. T. Haynes, J. N. Hodder, A. M. Leonard, and
N. R. Paddock. Flexibility is not related to stretch-induced deficits in force or
power. Journal of Sports Science and Medicine. 5:33-42, 2006.
5.
Behm, D. G., D. C. Button, and J. C. Butt. Factors affecting force loss with
prolonged stretching. Canadian Journal of Applied Physiology. 26:261-272,
2001.
6.
Cramer, J. T., T. J. Housh, G. O. Johnson, J. M. Miller, J. W. Coburn, and T.
W. Beck. Acute effects of static stretching on peak torque in women. J.
Strength Cond. Res. 18:236-241, 2004.
7.
Cramer, J. T., T. J. Housh, J. P. Weir, G. O. Johnson, J. W. Coburn, and T. W.
Beck. The acute effects of static stretching on peak torque, mean power output,
electromyography, and mechanomyography. European Journal of Applied
Physiology. 93:530-539, 2005.
8.
Fletcher, I. M. and R. Anness. The acute effects of combined static and
dynamic stretch protocols on fifty-meter sprint performance in track-and-field
athletes. Journal Of Strength And Conditioning Research / National Strength
& Conditioning Association. 21:784-787, 2007.
9.
Fletcher, I. M. and B. Jones. The effect of different warmup stretch protocols
on 20 meter sprint performance in trained rugby union players. J. Strength
Cond. Res. 18:885-888, 2004.
10.
Garrett, W. E., Jr. Muscle strain injuries: clinical and basic aspects. Med. Sci.
Sports Exerc. 22:436-443, 1990.
11.
Holt, B. W. and K. Lambourne. The impact of different warm-up protocols on
vertical jump performance in male collegiate athletes. Journal Of Strength And
Conditioning Research / National Strength & Conditioning Association.
22:226-229, 2008.
5
12.
Little, T. and A. G. Williams. Effects of differential stretching protocols during
warm-ups on high-speed motor capacities in professional soccer players.
Journal of Strength and Conditioning Research. 20:203-207, 2006.
13.
Marek, S. M., J. T. Cramer, L. A. Fincher, L. L. Massey, S. M. Dangelmaier,
S. Purkayastha, K. A. Fitz, and J. Y. Culbertson. Acute effects of static and
proprioceptive neuromuscular facilitation stretching on muscle strength and
power output. Journal of Athletic Training. 40:94-103, 2005.
14.
Papadopoulos, G., T. Siatras, and S. Kellis. The effect of static and dynamic
stretching exercises on the maximal isokinetic strength of the knee extensors
and flexors. Isokinetics and Exercise Science. 13:285-291, 2005.
15.
Power, K., D. Behm, F. Cahill, M. Carroll, and W. Young. An acute bout of
static stretching: effects on force and jumping performance. Med. Sci. Sports
Exerc. 36:1389-1396, 2004.
16.
Shellock, F. G. and W. E. Prentice. Warming-up and stretching for improved
physical performance and prevention of sports-related injuries. Sports Med.
2:267-278, 1985.
17.
Shrier, I. Does stretching help prevent injuries? In: Evidence-based Sports
Medicine. D. MacAuley and T. Best (Eds.) Malden, MA: Blackwell
Publishing, 2007.
18.
Smith, C. A. The warm-up procedure: to stretch or not to stretch. A brief
review. Journal of Orthopaedic & Sports Physical Therapy. 19:12-17, 1994.
19.
Stamford, B. Flexibility and Stretching. The Physician and Sportsmedicine.
12:171, 1984.
20.
Thacker, S. B., J. Gilchrist, D. F. Stroup, and C. J. Kimsey, Jr. The impact of
stretching on sports injury risk: a systematic review of the literature. Med. Sci.
Sports Exerc. 36:371-378, 2004.
21.
Wallmann, H. W., J. A. Mercer, and J. W. McWhorter. Surface
electromyographic assessment of the effect of static stretching of the
gastrocnemius on vertical jump performance. J. Strength Cond. Res. 19:684688, 2005.
22.
Weerapong, P., P. A. Hume, and G. S. Kolt. Stretching: mechanisms and
benefits for sport performance and injury prevention. Physical Therapy
Review. 9:189-206, 2004.
23.
Witvrouw, E., N. Mahieu, L. Danneels, and P. McNair. Stretching and injury
prevention: an obscure relationship. Sports Med. 34:443-449, 2004.
6
24.
Yamaguchi, T., K. Ishii, M. Yamanaka, and K. Yasuda. Acute effects of
dynamic stretching exercise on power output during concentric dynamic
constant external resistance leg extension. Journal Of Strength And
Conditioning Research / National Strength & Conditioning Association.
21:1238-1244, 2007.
25.
Young, W. B. and D. G. Behm. Effects of running, static stretching and
practice jumps on explosive force production and jumping performance.
Journal of Sports Medicine and Physical Fitness. 43:21-27, 2003.
7
Chapter 2: Literature Review
8
Introduction –Static Stretching and Exercise
For years, coaches and strength and conditioning professionals have advised
athletes to stretch prior to physical activity in order to achieve two aims: first, to
improve performance, and second, to decrease the risk of injury. Stretching has been
thought to improve performance for several reasons, including maximizing joint range
of motion (40). A sprinter, for example, lacking sufficient flexibility in the hamstrings
and hip flexors may not be able to maintain an optimal stride length, thereby reducing
speed and negatively affecting performance. Moreover, specific sports such as
gymnastics, and certain events in track and field, require a large degree of flexibility
about specific joints. Shellock and Prentice (40) also indicate that a lack of flexibility
may result in movements that are awkward or uncoordinated. Peak performance in
any sporting event requires an individual to maintain specific biomechanics in order to
maximize speed, efficiency, or power. Thus, a change in biomechanics associated
with uncoordinated movements may ostensibly have an effect on performance.
Stretching has also been advocated as a means to prevent injury to the
musculotendinous unit (MTU) (20, 42, 43). The MTU is composed of both active
contractile elements (muscle fibers) and passive elements (tendon). As a joint moves
through a greater range of motion, or as a larger force is applied to the muscle-tendon
unit, a more compliant tendon can absorb a greater amount of energy, thereby
protecting the active contractile apparatus and reducing injury to the muscle fibers
(50). Muscle strains occur when a muscle is stretched to a critical tensile force (38),
causing tears within the contractile element of the muscle. It would seem reasonable,
therefore, that a more compliant MTU would be able to withstand a greater tensile
9
force, which in turn would be beneficial in terms of injury reduction. This is
supported by the fact that muscle strain injuries tend to occur during the eccentric
phase of muscle contraction, when the forces applied to the MTU can be substantial,
and are most commonly seen in “two-joint” muscles – muscles that cross two joints
and are susceptible to a larger degree of stretch (20). Moreover, strains to the MTU
are most commonly seen in situations involving quick bursts of speed or other
instances where a muscle must generate a large amount of force over a short period of
time (20). Several studies have also indicated that individuals with very little
flexibility are more likely to experience injury in the form of muscle strains (49).
Increasing compliance of the MTU would seem to be a logical way to reduce the
incidence of injury.
Of the various methods of stretching that effectively increase range of motion,
the most common type is static stretching (48). Static stretching involves slowly
moving a joint to the endpoint of the range of motion, typically defined as the point
just before the onset of pain. The National Strength and Conditioning Association
(NSCA) recommends holding a static stretch for 30 seconds (3). In addition, the most
recent edition of Guidelines for Exercise Testing and Prescription, from the American
College of Sports Medicine (ACSM), recommends holding a stretch for 15 to 30
seconds, and contends that no further improvement in flexibility is seen past 30
seconds (1). The static stretching method is advantageous for several reasons: it is
simple to learn, can be performed individually, and is effective in increasing joint
range of motion (29).
10
Despite the prevalence of stretching prior to athletic activity, there is little
evidence supporting that static stretching acutely improves athletic performance, or
that static stretching reduces the incidence of musculoskeletal injury. In fact, much of
the recent literature suggests that static stretching, performed prior to athletic activity,
might actually be detrimental to performance.
Static Stretching and Performance
Static stretching has been shown to affect performance in a wide range of
performance tests, including measures of force production (6, 12, 13, 15, 19, 25, 27,
29, 33, 36, 52), vertical jumping ability (6, 8, 32, 46, 47, 54, 55), sprint speed (17, 18,
41), and balance, reaction time, and movement time (5). Research on the acute effects
of static stretching on performance has, for the most part, examined maximal force and
power production by large lower-body muscle groups, namely the quadriceps and
hamstrings, as well as the plantarflexors. Muscular power is desirable in the vast
majority of sporting activities requiring short bursts of speed, quick changes in
direction, jumping ability, or the ability to move large quantities of weight. An
increase in muscular power allows a muscle to perform a given workload over less
time (35), commonly referred to as ‘explosive power’. The ability to change
directions rapidly, to accelerate quickly, and to jump higher than an opponent are all
contingent on an athlete’s ability to generate sufficient explosive power and can all be
considered important measures influencing performance in a variety of sports.
Additionally, an increase in muscular power allows a muscle to perform a greater
maximal amount of work (35). Sports such as powerlifting rely on a competitor’s
11
ability to generate the greatest amount of force in order to lift the greatest load. Thus,
the ability to generate large muscular power output is relevant to a wide variety of
sports.
Most studies examining stretch-induced force and power decrements have
quantified maximal lower-extremity force production using maximal isokinetic or
isometric muscle contractions or have measured explosive power using jumping tests
(6, 10-15, 19, 24, 25, 29, 32, 33, 36, 45, 47, 54, 55). A wealth of the literature
indicates static stretching can result in acute performance decrements in maximal force
and power. These decrements have been shown to persist up to one hour (19), and
appear to be both neurological (2, 7, 12, 13, 19, 29, 36) and mechanical (15, 19, 47,
50) in origin. Sport-specific decrements in speed and power in sports such as
gymnastics (32, 41) and rugby (18) have also been reported. Balance and reaction
time (5), as well as sprint speed (16-18, 41), also appear to be negatively affected by a
bout of static stretching.
Isokinetic and Isometric Force
Published studies have examined the effect of static stretching on concentric,
isokinetic peak torque and mean power output at both slow and fast velocities. Marek
et al. (29) demonstrated that four repetitions each of four stretching exercises targeting
the leg extensor muscles decreased peak torque and mean power output at 60 and
300o·s-1. Similarly, Cramer et al. (12) documented a decrease in peak torque at similar
velocities (60 and 240 o·s-1). Several other studies have reported similar results when
testing peak torque of the knee extensors (13, 33) and knee flexors (33), and of the
12
biceps brachii (15). In a study examining multiple measures of strength following an
acute bout of static stretching, Behm et al. (6) reported a mean 6.1% to 8.2% decrease
in maximal isometric knee extension and 6.6% to 10.7% reduction in maximal
isometric knee flexion torque. Additional studies focusing on maximal voluntary
isometric contraction (MVIC) of the quadriceps (7, 36), plantar flexors (19, 36), and
hand grip strength (25) all yielded similar decreases in strength following a bout of
static stretching. Moreover, a one repetition-maximum (1RM) test of both knee
flexion and knee extension (27) demonstrated significant differences between pre- and
post-stretch values. Finally, a study examining isotonic leg extensions found that
static stretching reduced leg extension power at light to moderately heavy loads (5%,
30%, and 60% of MVIC) (52). The combined results of these studies indicate that
regardless of the method used to measure maximal force, there is a clear trend toward
reduced maximal force production following a bout of static stretching.
Jumping Performance
In addition to decreases in peak torque and mean power, static stretching has
also been purported to decrease performance in tests of jumping ability. The results,
however, are mixed. Jumping-based studies have measured performance using
various jump types, including the countermovement jump (CMJ), the drop jump (DJ),
and the squat jump (SJ), which involves only a concentric phase of movement. In
comparing the results of studies based on these three jump types, results have varied
depending on which jump style is measured.
13
In examining CMJ performance, Wallmann et al. (47) found a 5.6% mean
decrease in CMJ height after three 30-second stretches of the gastrocnemius muscles.
Similarly, Behm and colleagues (6) reported decreases (-5.5% to -7.5%) in CMJ
height following three 30-second stretches of three lower body muscle groups. Recent
work suggests that even mild stretching can be detrimental to jump performance:
holding static stretches at 50%, 75%, and 100% of the force necessary for the onset of
mild discomfort all yielded significant decreases in CMJ height (2.8%, 3.9%, and
4.2%, respectively) (8). These findings imply that at both mild and moderate
intensities, static stretching causes significant performance decrements that are similar
to those experienced after more intense stretching.
In addition to affecting CMJ performance, static stretching also appears to
hinder performance in the drop jump. An investigation by Young and Behm (55)
reported a 3.2% mean decrease in DJ height after static stretching of the quadriceps
and plantar flexors. Additional work by Young and Elliott (54) and Behm and
colleagues (8) also demonstrated significant decreases in DJ height after static
stretching of the quadriceps, gluteals, hamstrings, and plantar flexors. A fourth study
(6) reported no change in DJ height following static stretching, but observed that DJ
ground contact time had significantly increased following the stretch treatment. The
authors suggested that the increased ground contact time was a compensatory change
in jump strategy in order to combat losses in rate of force production caused by static
stretching. Both a decrease in jump height and an increase in ground contact time
imply that static stretching adversely affects DJ performance.
14
It is less clear whether static stretching affects concentric-only squat jumps
(SJ). Squat jumps incorporate a 2-second pause at peak knee flexion in order to
eliminate the benefits of the stretch reflex on jump height. Three studies with similar
SJ protocols all reported no change in SJ height following static stretching (24, 36,
54), while other studies have reported SJ performance reductions ranging from 3.4%
(55) to 5.7% (8). While the SJ does not rely on the stretch-shortening cycle to
enhance explosive power, jumping activities that involve an eccentric component such
as the drop jump or countermovement jump are adversely affected by prior static
stretching. Many athletic movements, such as running, quick directional shifts, and
jumping, involve both eccentric and concentric muscle action. As such, jump tests
that involve both concentric and eccentric muscle action (CMJ and DJ) might be more
appropriate than the squat jump to gauge the effects of static stretching on actual
athletic performance. The performance reductions in the jumping tests that most
closely reflect specific athletic movements are strong evidence that performance
decrements resulting from static stretching may transfer to more game-like situations.
Static Stretching and Performance in Athletes
The effects of static stretching on recreationally-active individuals have been
the focus of most of the research to date. The majority of the results suggest that for
individuals that are not highly trained, static stretching can decrease performance in
tests of lower-body strength and power. The picture of how static stretching affects
athletic performance is complicated by the mixed conclusions of studies that have
focused on trained athletes as the subject pool. Three studies have examined the
15
impact of static stretching on National Collegiate Athletic Association (NCAA)
athletes. Unick et al. (45) reported that static stretching did not affect performance in
either a countermovement jump or a drop jump in a study of NCAA Division-III
women’s basketball players. Similarly, no changes in peak torque or mean power
output of the leg extensors were observed in a study of NCAA Division-I women’s
basketball players (14). A third study that examined NCAA Division-I athletes in a
variety of sports also found no change in CMJ performance following static stretching,
but reported a significant decrease in performance when the stretching protocol
consisted of proprioceptive neuromuscular facilitation (PNF) stretching (10). In
studies involving athletes in other sports, Knudson et al. (26) observed no change in
tennis serve performance (speed and accuracy) after a bout of static stretching.
Likewise, in a study of competitive athletes from the United States Military Academy
(USMA), McMillan et al. determined that static stretching actually increased
performance on the 5-step jump test, compared to no prior activity (31). Furthermore,
Little and Williams (28) examined the effect of static stretching on professional soccer
players, and concluded that static stretching had no effect on CMJ height or 10 meter
sprint time, and that stretching improved performance on a flying start 20 meter sprint.
There are also an equal number of published studies that have shown static stretchinduced performance decreases in highly trained athletes. Fletcher and Jones (18), in a
study of rugby union players, found a decrease in performance in a 20 meter sprint
after engaging in static stretching. Similarly, slower 50m sprint times have been
reported in elite sprinters after a warm-up including static stretching (17). Moreover,
two separate studies of competitive, highly-trained young gymnasts reported decreases
16
in drop jump performance (32) as well as in sprint speed on the approach to the vault
apparatus (41). The mixed results suggest that stretch-induced performance changes
may vary by the sport, experience level, and specific tests used in the different
protocols.
It has been suggested that athletes may be less susceptible to stretch-induced
performance decreases due to the fact that static stretching is commonly performed
during training. It is possible that performing these stretches regularly as part of an
overall conditioning program confers some protective benefit against the potential
strength deficits. This may also explain why in the study examining both SS and PNF
stretching, the PNF treatment did result in a reduction in vertical jump height, as the
protocol may have been less familiar to the athletes (10). Behm et al. tested the idea
that training status may reduce the effect of static stretching in a study of
recreationally-active individuals. In that study, subjects participated in four weeks of
a five day per week flexibility program involving the quadriceps, hamstrings and
plantar flexors; this training frequency would be similar to the flexibility training
performed by athletes during daily training sessions. Following four weeks of
flexibility training, the subjects continued to demonstrate decreased performance for
MVIC of the hamstrings and quadriceps, as well as decreased performance in
countermovement jump following a bout of static stretching (6). They concluded that
flexibility-training status did not appear to confer any protective benefit.
17
Balance, Reaction Time, and Movement Time
Most of the literature on static stretching and performance has focused on
measures of maximal force or explosive power. While discrepancies exist between
studies, the aggregate results suggest that static stretching negatively impacts
performance in measures of maximal force production and explosive power. Highlevel performance in many athletic events requires an athlete to display high degrees
of speed, balance and coordination. Peterson et al. demonstrated that muscular
strength, peak power output, and vertical jumping ability are all highly correlated with
measures of agility, speed, and acceleration (35). This suggests that if static stretching
can result in decreased performance in explosive power and maximal force output,
there is a strong likelihood that measures of agility, speed and acceleration may also
be adversely affected. Small changes in an athlete’s ability to shift balance quickly or
react to changes in an opponent’s direction may have the potential to impact the
outcome of a competition. Thus, in evaluating the effects of static stretching on
athletic performance, it is essential to take measures of agility, balance, and reaction
time into account, in addition to the more studied measures of maximal power and
jumping performance.
Despite the importance of balance, reaction time, and agility to performance in
many sports, few studies have examined the effects of stretching on these measures of
athletic performance. Behm et al. (5) observed that static stretching of the quadriceps,
hamstrings and plantar flexors resulted in impaired balance, increased reaction time,
and increased movement time in a population of healthy university students. In
contrast, McMillan et al. (31) found that static stretching had no effect on T-drill speed
18
(agility) or medicine ball throw (whole-body power) in a population of USMA cadets.
Little and Williams (28) also reported that static stretching had no effect on zig-zag
drill performance (agility) in a population of professional soccer players. The body of
research examining the performance variables of balance, agility, and reaction time is
limited, and the results have been inconsistent. More research is needed in order to
establish whether static stretching acutely affects these measures of performance.
Duration of Performance Reductions Following Static Stretching
Only a handful of studies have examined the duration and severity of the
performance reductions associated with static stretching. Fowles and colleagues
measured muscle activation pre-stretch, immediately post-stretch, and at 5, 15, 30, 45,
and 60 minutes post-stretch, and concluded that while decreases in MVIC force were
most severe immediately following stretching (-28%), force reductions persisted at
significant levels 60 minutes post-stretching (-9%) (19). In contrast, Bradley and
colleagues measured CMJ height at the same time intervals post-stretch and concluded
that performance decrements seen immediately after stretching (4% decrease in jump
height) had disappeared by 15 minutes post-stretch (9). The difference in outcomes in
the previous two studies is likely due to the intensity and duration of stretches used.
Fowles and colleagues stretched the plantarflexors with 13 repetitions of 2.25 minutes
each – the most extreme protocol seen in the literature – while the stretch protocol
employed by Bradley and colleagues more closely resembles a realistic pre-practice
stretch routine: 4 repetitions of 30 seconds, for 5 lower-body stretches. Both of these
protocols performed their stretching immediately prior to their performance measures,
19
which does not necessarily reflect current practices in sport. It is common for athletes
to perform additional sport-specific warm-up activities following static stretching, yet
little is known about the effects of static stretching following a secondary warm-up. A
recent study by Pearce and colleagues examined the effects of static stretching and a
subsequent, “secondary” dynamic warm-up on CMJ performance (34). Participants’
CMJ height was measured at baseline, following static stretching, and again following
a secondary warm-up at 0, 10, 20, and 30 minutes post-exercise. The authors found a
post-stretch decrease in CMJ height that persisted despite a secondary dynamic warmup, and that CMJ height at 30 minutes post-dynamic warm-up was significantly worse
than at baseline or immediately post-dynamic warm-up. This suggests that even
incorporating static stretching into a broader warm-up strategy including dynamic
exercise can still significantly impact performance.
Overall, static stretching has been shown to acutely decrease performance in a
variety of performance variables, including peak torque, vertical jumping ability,
balance, reaction time, sprint speed, and movement time. While these effects have
been conclusively demonstrated in recreationally-trained subjects, the literature is
divided on whether trained athletes are susceptible to these performance decreases.
Moreover, research into the effects of static stretching on reaction time, balance, and
agility is scant. Static stretch-induced performance reductions likely last at least 15
minutes and may persist up to 60 minutes post-stretch. Moreover, performance
decrements may also persist despite additional dynamic activity after stretching. A
summary of investigative findings is found in Table 1, which summarizes subject
20
pools, static stretch protocols, and key findings that make up the body of research on
this subject.
21
Isometric Strength
Isokinetic PT and MP
TABLE 1—Summary of findings: The acute effects of static stretching on various measures of performance.
Author
Subjects
Static Stretch Treatment
reps x time in seconds (rest)
Outcome
Cramer et al. (2004) (12)
14 females
mean 22 yrs
4 x 30 (20)
4 leg extensor stretches
Decreased peak torque in leg extension
Cramer et al. (2005) (13)
7 males, 14 females
mean 21.5 yrs
4 x 30 (20)
4 leg extensor stretches
Decreased peak torque in leg extension
Cramer et al. (2006) (11)
13 females
mean 20.8 yrs
4 x 30 (20)
4 leg extensor stretches
No change in eccentric peak torque
No change in eccentric mean power output
Evetovich et al. (2003) (15)
10 males, 8 females
mean 22.7 yrs
4 x 30 (15)
forearm flexor stretch
Decreased peak torque in forearm flexion
Marek et al. (2005) (29)
9 males, 10 females
mean 23, 21 yrs
4 x 30
4 leg extensor stretches
Decreased peak torque
Decreased mean power output
Papadopoulos et al. (2005) (33)
32 males
mean 20.7 yrs
3 x 30 (15)
quadriceps and hamstring stretch
Decreased peak torque in knee extension
Decreased peak torque in knee flexion
Power et al. (2004) (36)
12 males
20-44 yrs
3 x 45 (15)
6 lower body stretches
5.4%-9.5% decrease in quadriceps torque
Behm et al. (2006) (6)
9 males, 9 females
mean 25 yrs
3 x 30 (30)
3 lower body stretches
6.1%-8.2% decrease in MVIC knee flexion
6.6%-10.7% decrease in MVIC knee extension
Fowles et al. (2000) (19)
6 males, 4 females
mean 22.3, 20.3 yrs
13 x 135 (5)
plantarflexors stretch
Decrease in MVIC lasting up to 60 minutes
4 x 10
wrist flexors stretch
Decrease in isometric grip strength at
20-40 seconds of stretching
Knudson et al. (2005) (25)
33 males, 24
females
22
TABLE 1 (cont’d)—The acute effects of static stretching on various measures of performance.
Author
Subjects
Static Stretch Treatment
reps x time in seconds (rest)
Outcome
Behm and Kibele (2007) (8)
7 males, 3 females
mean 26.5 yrs
4 x 30 (30); 3 lower body stretches
50%, 75%, or 100% force to discomfort
Decrease in CMJ, DJ, and SJ height after all
stretching intensities (range: 3.6% to 5.6% decrease)
Behm et al. (2006) (6)
9 males, 9 females
mean 25 yrs
3 x 30 (30)
3 lower body stretches
5.5%-5.7% decrease CMJ height
5.4%-7.4% increase drop jump contact time
No change in drop jump height
10 males, 10
females
mean 23.7 yrs
3 x 15
3 lower body stretches
55% of subjects decreased jumping velocity
35% of subjects increased jumping velocity
12 males
20-44 yrs
3 x 45 (15)
6 lower body stretches
No change in drop jump height
No change in vertical jump height
20 males
mean 20.3 yrs
2, 4, or 6 x 15 (15)
quads, hamstrings, plantarflexors
Decrease in squat jump height post-6
No change in SJ post-2 or post-4
14 males, 12
females
mean 22.0 yrs
2 x 30 (30)
4 lower body stretches
Decreased CMJ height after SS compared with
general warm-up (walk/run) only
Wallmann et al. (2005) (47)
8 males, 6 females
18-34 yrs
3 x 30
gastrocnemius stretch
5.6% decrease in CMJ height
Young and Elliott (2001) (54)
14 males
mean 22 yrs
3 x 15 (20)
3 lower body stretches
No change in squat jump performance
Decrease in drop jump performance
Young and Behm (2003) (55)
13 males, 3 females
mean 26 yrs
2 x 30
4 lower body stretches
Decreased drop jump height
Decreased vertical jump height
Decreased peak concentric force
Decreased rate of force development
Jumping Performance
Knudson et al. (2001) (24)
Power et al. (2004) (36)
Robbins and Scheuermann
(2008) (37)
Vetter (2007) (46)
23
Isotonic
TABLE 1 (cont’d)—The acute effects of static stretching on various measures of performance.
Author
Subjects
Static Stretch Treatment
reps x time in seconds (rest)
Outcome
Yamaguchi et al. (2006) (52)
12 males
mean 23.8 yrs
4 x 30 (20)
6 leg extensor stretches
Decrease in peak power at light and heavy loads
15 males, 15
females
mean 22 yrs
6 x 15 (15)
5 lower body stretches
7.3% decrease in 1RM knee flexion
8.1% decrease in 1RM knee extension
Yamaguchi and Ishii (2005) (51)
11 males
mean 23 yrs
1 x 30
5 lower body stretches
No change in leg press power
Church et al. (2001) (10)
40 NCAA D-I
female athletes
mean 20.3 yrs
SS and PNF
unspecified lower body stretches
No change in CMJ for SS
Decrease in CMJ for PNF
Egan et al. (2006) (14)
11 NCAA D-I
female basketball
mean 20 yrs
4 x 30 (20)
4 leg extensor stretches
No change in peak torque or mean power
Fletcher and Jones (2004) (18)
97 male rugby
mean 23 yrs
1 x 20
unspecified lower body stretches
Decreased performance in 20m sprint
Knudson et al. (2004) (26)
83 tennis players
various skill levels
2 x 15 (10)
7 upper/lower body stretches
No change in serve speed or accuracy
Little and Williams (2006) (28)
18 pro male soccer
1 x 30 (20)
4 lower body stretches
No change in CMJ height
No change in 10m sprint time (stationary start)
Decreased 20m sprint time (flying start)
No change in zig-zag drill time (agility)
Sport-Specific Performance
1RM
Kokkonen et al. (1998) (27)
24
Other Measures
Sport-Specific Performance
TABLE 1 (cont’d)—The acute effects of static stretching on various measures of performance.
Author
Subjects
Static Stretch Treatment
reps x time in seconds (rest)
Outcome
McMillan et al. (2006) (31)
30 USMA cadets
rugby, lacrosse,
strength/conditioning
16 males, 14
females
mean 20.2, 20.4 yrs
1 x 20-30
8 stretches including upper
and lower body
No change in T-drill speed (agility)
No change in medicine ball throw
Increased distance in 5-step jump
McNeal and Sands (2003) (32)
13 competitive
female gymnasts
mean 13.3 yrs
1 x 30
3 lower body stretches
9.6% decrease in drop jump performance
Siatras et al. (2003) (41)
11 competitive
male gymnasts
mean 9.8 yrs
1 x 30
2 lower body stretches
Decreased vault approach speed
Unick et al. (2005) (45)
16 NCAA D-III
female basketball
mean 19.2 yrs
3 x 15
4 lower body stretches
No change in CMJ height
No change in drop jump height
Behm et al. (2004) (5)
16 males
mean 24.1yrs
3 x 45 (15)
3 lower body stretches
9.2% decrease in balance scores
4.0% increase in reaction time
1.9% increase in movement time
Faigenbaum et al. (2005) (16)
60 children
mean 11.3 yrs
2 x 15 (5)
6 lower body stretches
Decreased performance in shuttle-run
Decreased performance in long jump
25
Mechanisms of Performance Reductions
The exact mechanisms by which static stretching impairs performance are still
not clearly defined. It is clear, however, that a combination of mechanical (15, 19, 47,
50) and neural (2, 7, 12, 13, 19, 29, 36) factors play a role in static stretch-induced
force decrements. Several methods of measurement have been employed in order to
examine both mechanical and neural changes after a bout of static stretching.
Electromyography (EMG) is a measure that has been routinely used to examine
changes in muscle activation pre- and post-stretching. A decrease in EMG amplitude
suggests a reduction in motor unit activation, thereby suggesting a neurological
contribution to post-stretch decreases in muscle force. The mechanical counterpart to
electromyography, mechanomyography (MMG) has been used to examine changes in
muscle-tendon unit (MTU) stiffness. MMG is a measure of muscular vibrations
emitted by active muscle, and has been associated with muscle stiffness (4). A stiffer
MTU may dampen muscular vibrations, resulting in lower MMG amplitude. A more
compliant muscle would result in greater MMG amplitude, and would suggest that
increased muscle compliance might have contributed to stretch-induced decrements in
muscle force. The literature indicates a combination of mechanical and neurological
factors likely contribute to the deleterious effects of static stretching on muscular
power output.
Both neural and mechanical factors play a role in decreasing power output
following static stretching. Fowles et al. (19) attempted to quantify the relative
contributions of mechanical and neural factors to stretch-induced decreases in
maximal voluntary isometric contraction (MVIC) of the plantarflexors. Their
26
investigation employed electromyography to study changes in muscle activation prestretch, immediately post-stretch, and at 5, 15, 30, 45, and 60 minutes post-stretch.
The results indicated a 28% mean decrease in MVIC force immediately post-stretch,
with MVIC values remaining 9% below pre-stretch values at a full 60 min poststretching. Decreased EMG amplitudes demonstrated decreased neural activation at 5
min post-stretch, but interestingly, EMG amplitude had recovered to pre-stretch values
by 15 minutes post-stretch. The authors concluded that much of the initial stretchinduced force decrease was neural in origin, but that lingering decreases in force might
be mechanical in origin, as force decrements persisted after EMG amplitudes had
recovered to pre-stretch values. They estimated that immediately post stretch, neural
mechanisms accounted for 60% of the decrease in MVIC, while the remaining 40%
appeared to be mechanically-mediated. By 30 minutes post-stretch, however, neural
deficits accounted for only approximately 10% of the remaining force deficit, while
90% appeared to be mechanical in origin (19). The results of this study support the
theory that both neural and contractile changes influence stretch-induced force
decrements. The direct validity of their results in terms of sports performance,
however, is questionable, as the plantarflexors were stretched for over thirty minutes.
Stretching of a single muscle group for such a long period of time is highly unrealistic
from a competitive perspective – it is unlikely that any sporting event would require
such extensive focus on a particular muscle group. Thus, while their findings are not
directly applicable to a sports performance framework, their theoretical contribution to
the understanding of the origins of force decrements is considerable.
27
Mechanical Factors
Mechanical factors may play a role in decreased force production following a
bout of static stretching. Static stretching results in increased compliance in the MTU,
including a more compliant tendon capable of absorbing a considerable amount of
energy. Witvrouw et al. argue that in some sporting activities, particularly those
involving movements that transfer considerable force to the MTU, a stiff tendon might
be advantageous from a performance standpoint, as it would facilitate rapid changes in
tension and therefore a faster joint motion response. As the MTU lengthens and
becomes more compliant, they argue that contractile elements must contract over a
greater distance to “pick up the slack”, resulting in reduced peak torque and a slower
rate of force development (50). Weerapong et al. support this argument, contending
that increased compliance to the MTU requires increased contractile force to transmit
muscle force to the joint, resulting in a delay in external force generation (48). A
stiffer MTU, therefore, would be more efficient in transmitting contractile force to the
joint and initiating a forceful joint movement response.
Evetovich et al. (15) and Wallmann et al. (47) have published findings
supporting mechanically-mediated changes in force production. Evetovich et al.
supported a mechanical mechanism for decreases in force production in a study of
isokinetic peak torque of the biceps brachii. After static stretching, a decrease in peak
forearm flexion torque was observed along with an increase in MMG amplitude, while
no change was observed in EMG amplitude (15). This suggested that little change in
motor unit activation had occurred, but that the MTU had become more compliant due
to the stretch treatment, resulting in decreased peak torque. Wallmann et al. (47)
28
reported a 17.9% increase in EMG amplitude with no increase in performance
following static stretching of the gastrocnemius muscles, suggesting that additional
motor units had been recruited to compensate for changes to the mechanical properties
of the muscle. From these results, stretch-induced performance changes appear to be
at least partially mechanical in origin.
Neural Factors
Neurally-mediated decreases in power post-stretch have also been documented.
Decreases in motor unit activation have been demonstrated on multiple occasions (2,
7, 29, 36) via decreases in EMG amplitude following a bout of static stretching.
Weerapong et al. also suggested that static stretching may increase presynaptic
inhibition, as well as reducing synaptic transmission during repetitive activation (48),
potentially decreasing performance in repetitive power-based movements. These
results collectively suggest that a reduction in motor unit activation can at least
partially explain acute loss of force following static stretching. Moreover, at least one
study has argued that neural changes may be influenced by the degree of stretching.
Guissard et al. (22) suggested that following moderate static stretching of the soleus,
there was a reduction in presynaptic neural input to the motoneuron pool. However,
with more intense stretching (stretching involving a greater increase in muscle length),
it appeared that postsynaptic reduction in the excitability of the alpha motoneurons
played a larger role in stretch-induced force decrements.
Recent research by Cramer and colleagues lends convincing evidence to
support neurally-mediated changes to muscular force output following static stretching
29
(12, 13). In two separate studies, one performed on a mixed-gender group of subjects
and one performed exclusively on separate female subjects, the leg extensor muscles
of the dominant limb only were stretched using one unassisted and 3 assisted static
stretches. Peak torque, mean power, EMG and MMG data were recorded pre- and
post-stretching for both the dominant (stretched) and non-dominant (unstretched)
limb. In both studies, after stretching the dominant leg only, a decrease in peak torque
was observed in both the stretched and unstretched limb. Moreover, EMG amplitude
for the vastus lateralis and rectus femoris decreased post-stretch for both the stretched
and unstretched limbs, indicating that motor unit activation had decreased in both
limbs, even though only one had been physically stretched. MMG amplitude for both
the stretched and unstretched limb was unchanged pre- to post-stretch, suggesting that
there were no significant changes in MTU compliance in either the stretched or
unstretched leg. Cramer and colleagues concluded that since decreases in maximal
force production were observed in both the stretched and the unstretched limb, a
central nervous system inhibitory mechanism must have been at least partially
responsible for decreasing motor unit activation and/or firing frequency (13).
In summary, the mechanisms by which static stretching acutely decreases force
production have not positively been identified. However, it appears that a
combination of mechanical changes (increased compliance of the MTU) and neural
changes (decreased motor unit activation and/or decreased alpha motoneuron
excitability) play a role in stretch-induced force decrements. Several studies have
suggested that neural deficits appear to recover within 15 minutes post-stretch (2, 19),
30
but that force decrements can persist up to an hour post-stretch (19), again supporting
a combination of neurally and mechanically-mediated changes to the MTU.
Stretching and Injury Prevention
Despite the common perception that stretching prior to activity will reduce an
individual’s risk of injury, little concrete evidence supports this contention. Most
muscle injuries occur within normal limits of range of motion, and occur during the
eccentric phase of muscle contraction (49). Flexibility is a continuum, and individuals
at the extremes, exhibiting either drastically reduced flexibility or extreme flexibility,
bear a greater risk of injury (49). Static stretching is effective at increasing the range
of motion about a particular joint (44, 48-50), increasing static flexibility. This is
contrasted with dynamic flexibility, which involves active muscle contraction to move
a joint through its range of motion (3, 48). To date, there has not been any convincing
evidence linking an increase in passive range of motion achieved by static stretching
to the dynamic flexibility necessary in many athletic activities. Moreover, the
literature fails to credibly establish any direct link between pre-activity stretching and
injury risk.
The literature reviewing the role of static stretching in injury prevention covers
a broad range of activities, each with distinct musculoskeletal demands. Witvrouw et
al. (50) argue that much of the confusion surrounding the role of stretching in injury
prevention stems from the fact that the body of literature examines sports with vastly
different movement patterns. In their extensive review, they argue that different sports
require varying levels of compliance in the MTU. Specifically, they divide sports
31
activities into two broad categories: sports involving high-intensity stretch-shortening
cycle (SSC) movements, and sports involving no or low-intensity stretch-shortening
cycle movements. Sports involving lower-intensity stretch-shortening cycles such as
cycling or swimming might benefit from a stiffer MTU unit. A stiffer MTU would
have the ability to transmit force more quickly across a joint, resulting in a quicker
movement with less energy wasted in picking up the “slack” of a more compliant
MTU. Moreover, as these lower-intensity SSC activities do not place excessive
tensile stress on the MTU, it is unlikely that static stretching would greatly impact
rates of injury. In contrast, sports involving quick bursts of speed, changes in
direction, rebounding, and other higher intensity movements might benefit from a
more compliant MTU in order to store and release adequate elastic energy to produce
explosive movement without causing damage to the muscle. In these activities,
stretching might be beneficial in terms of preventing some injuries by increasing the
amount of energy that the MTU can absorb without rupturing (50). It is important to
note, however, that achieving increased musculotendinous compliance does not
necessarily need to involve static stretching prior to activity.
A more recent direction for injury prevention appears to be a multifaceted
model, which includes an appropriate pre-exercise warm-up, combined with
plyometric, proprioceptive, and strength training (44). Several reasons exist to justify
this shift. Most muscle strain injuries occur during the eccentric phase of muscle
contraction; therefore, it is unclear how increasing range of motion through static
stretching would, in itself, be effective at preventing muscle strains. Thacker et al.
suggest that the general aerobic component of the warm-up is more effective than
32
stretching at increasing blood flow to the muscles, increasing the efficiency of delivery
of energy substrate and oxygen to working muscles, removing waste products, and
increasing the speed of nerve impulses (44). In terms of oxygen supply to muscle
tissue, the Bohr shift dictates that as muscle temperature increases, the amount of
oxygen released from hemoglobin into the working muscles increases (30). A more
active warm-up may not only be beneficial for performance, but may also be
beneficial from an injury standpoint as this gradual progression of activity prepares the
muscles for the more strenuous demands of competition.
Dynamic Stretching: An Effective Alternative
A general aerobic warm-up of 5-10 minutes is more effective than stretching at
increasing blood flow to working muscles, increasing muscle temperature, improving
delivery of oxygen and energy substrate, increasing nerve impulse velocity, and
removing metabolic waste products (44). A study by Behm and colleagues reported
that just 5 minutes of moderate intensity cycling, without any accompanying
stretching exercises, resulted in improved performance in balance, decreased reaction
time and decreased movement time, when compared to measurements taken prior to
engaging in any physical activity (5). These findings are supported by the results of
two studies that examined several combinations of warm-up activities on vertical
jumping performance. Activities included general warm-up exercise (walking and/or
jogging), dynamic stretching, practice jumps, and static stretching. Both studies found
a general tendency for reduced jump height when the warm-up included static
stretching (either alone or in combination with other exercises), and for improved
33
jump height when the exercises included a general warm-up, dynamic stretching or
practice jumps (46, 55). These studies support the importance of including a general
component to the warm-up that increases heart rate and muscle temperature in order to
prepare the body for more intense exercise. Following the general warm-up, several
authors have suggested performing exercises that would function to increase range of
motion without the performance decrements associated with static stretching, and that
might potentially serve to improve performance (18, 21, 28, 31, 51). Dynamic
stretching, which involves actively moving a joint through its range of motion without
holding the movement at its endpoint, may increase flexibility without reducing
neuromuscular activity (48). Exercises such as a walking lunge, high knee pulls,
skipping, carioca, various bounds and jumping exercises, and gradual accelerations are
examples of common dynamic warm-up exercises.
In addition to increasing range of motion, a progression from moderate to high
intensity dynamic movements might also improve performance by increasing balance
and coordination and enhancing neuromuscular function (16, 42). Several studies
have investigated the effect of dynamic stretching on various measures of sport
performance. Fletcher and Jones’ study of 20m sprint performance in rugby union
players compared the effects of static and dynamic stretch treatments. Players were
tested after completing a 10 minute moderate intensity jog, and then once again after
completing either a static stretching session or a dynamic stretching session. As
discussed previously, the players who underwent the static stretch treatment exhibited
significantly slower sprint times. In contrast, the players in the dynamic stretch group
significantly decreased their sprint time (18). A later study found similar results in a
34
group of trained sprinters, whose 50m sprint times improved after a warm-up that
included both a general component and dynamic stretching (17). The authors
suggested that the improvements seen after dynamic stretching may be due to the
rehearsal of specific movement patterns in the dynamic stretching exercises, and also
hypothesized that dynamic stretching may allow for a more optimal switch from
eccentric to concentric muscle action, improving explosive force production (17).
Further evidence suggesting that dynamic warm-up may improve performance
in athletes was published by Little and Williams (28). In their study of professional
soccer players, they concluded that 4 lower body dynamic stretches, performed for 60
seconds each, resulted in improved performance in both a stationary-start 10 meter
sprint and a flying-start 20 meter sprint, as well as improved performance in a zig-zag
drill measuring agility. McMillan et al. (31) also found that a dynamic warm-up
consisting of callisthenic exercises (such as bend and reach, squats, lunges, pushups)
and movement drills (such as shuffling, high-knee jogging, carioca, and gradual
accelerations) resulted in significantly improved performance in a T-drill (agility),
medicine ball throw (whole body power), and 5-step jump (lower body explosive
power) in USMA cadet-athletes. These results suggest that a range of dynamic
stretching exercises may be effective in improving performance across a wide variety
of performance variables.
In addition to the performance improvements reported in athletes, dynamic
stretching also appears to improve performance in recreationally-trained subjects.
Yamaguchi and Ishii (51) reported a significant increase in power in a leg press test
following dynamic stretching consisting of 15 repetitions of 5 lower body stretches.
35
Gourgoulis et al. (21) tested subjects before and after engaging in a gradual
progression of submaximal half squats, ranging from 20% to 90% of 1RM. They
found that after the half squats, subjects demonstrated a significant (+2.4%) increase
in CMJ height. After dividing their subject pool into two groups based on pre-squat
jumping ability, the authors also found that the stronger group (those with greater
jumping ability) increased their CMJ height by an average of 4.01%, compared with
the lower-ability jumping group, which only increased CMJ height by an average of
0.42%. This would suggest that for stronger, potentially better-trained individuals, a
dynamic warm-up can have substantial performance benefits.
In sum, dynamic stretching appears to improve performance in force
production, jumping performance, sprint speed, and agility drills. Table 2 summarizes
the subject pools, dynamic stretch protocols, and key findings that make up the body
of research thus far.
36
TABLE 2 —Summary of findings: The acute effects of dynamic stretching on various measures of performance.
Author
Fletcher and Anness
(2007) (17)
Fletcher and Jones (2004)
(18)
Gourgoulis et al. (2003)
(21)
Holt and Lambourne (2008)
(23)
Little and Williams (2006)
(28)
Subjects
Dynamic Stretching Treatment
Outcome
10 male, 8 female
elite 100m sprinters
3 treatments:
1. Static DS = while stationary (ADS)
2. Static + Active DS = same as above
plus 2 x 5 exercises over 20m (DADS)
3. Static stretching + ADS treatment (SADS)
Improved 50m sprint time after ADS and DADS
compared with SADS
97 male rugby
mean 23 yrs
2 DS treatments:
Active DS = at a jogging pace
Static DS = while stationary
Both groups: 5 lower body
exercises, 20 reps each leg
ADS: Decreased 20m sprint time
SDS: No change in 20m sprint time
20 males
mean 21.2 yrs
5 sets of 2 half-squats
from 20%-90% of 1RM
Increased CMJ height
1. General warm-up only
2. General warm-up + SS: 3 x 5sec (1 sec rest); 5 stretches
3. General warm-up + DS: 10 reps of 6 lower body exercises
4. General warm-up + dynamic flexibility: 10 reps of 8 movements
CMJ height after #1, 3, 4 significant higher than after #2
4 lower-body exercises
60 seconds each
No change in CMJ height
Decreased 20m sprint time (flying start)
Decreased 10m sprint time (stationary start)
Decreased zig-zag drill time (agility)
64 NCAA D-1 male
football
mean 20.7 yrs
18 pro male soccer
37
TABLE 2 (cont’d) —The acute effects of dynamic stretching on various measures of performance.
Author
Subjects
Dynamic Stretching Treatment
Outcome
McMillan et al. (2006) (31)
30 USMA cadets
rugby, lacrosse,
strength/conditioning
16 males, 14 females
mean 20.2, 20.4 yrs
1 x 10 various calisthenics
(bend and reach, lunge,
push-up, squat jump, etc)
plus 20-25 m movement
drills (carioca, shuffle,
gradual accelerations, etc)
Decrease in T-drill completion time (agility)
Increased distance in medicine ball throw
(whole body power)
Increased distance in 5 step jump
32 males
6 x 15 leg swings
No change in isokinetic torque
mean 20.7 yrs
Leg extensors and flexors
14 males, 12 females
4 warm-up components, 6 combinations
CMJ height best after walk/run, DS, practice jumps
mean 22.0 yrs
Walk/run; dynamic stretch; practice jumps; static stretch
CMJ height worst after SS or SS + practice jumps
11 males
mean 22.8 yrs
15 x 2-second repetitions
5 lower body stretches
10% increase in leg extension power
12 males
mean 24.1 yrs
2 x 15 repetitions
4 lower body stretches
Increase in leg extension peak power @ 5, 30, 60% MVC
8.9%, 6.0%, 8.1% respectively
Papadopoulos et al. (2005)
(33)
Vetter (2007) (46)
Yamaguchi and Ishii (2005)
(51)
Yamaguchi et al. (2007)
(53)
38
Dynamic Stretching: Mechanisms of Improved Performance
The performance improvements experienced following dynamic stretching
have been tentatively attributed to several factors, including increased muscle
temperature (44), movement rehearsal (17, 18, 28), and postactivation potentiation
(PAP) (5, 16, 31, 39, 51). The nature of a dynamic stretching protocol is inherently
active – because the subject continues to work at a low to moderate intensity
throughout the dynamic stretch protocol, muscle temperature remains elevated. In
comparison, when an athlete sits down to complete a static stretch protocol, muscle
temperature decreases, and any increases gained by a submaximal jog prior to
stretching may be reduced. The elevated muscle temperature from dynamic stretching
results in increased substrate delivery, waste product removal, and nerve impulse
conductivity, all of which can contribute to improving performance.
It is also possible that the rehearsal of activity-specific movement patterns may
contribute to improved performance. Submaximal rehearsal of specific aspects of the
sprint cycle, for example, is thought to contribute to improved sprint performance by
enhancing movement pattern coordination (18). In addition, the rehearsal of
movements inherent in a dynamic stretching protocol may be beneficial for sports
requiring high-speed movements, such as soccer (28).
Finally, postactivation potentiation (PAP) has been widely postulated as a
mechanism of improved performance following dynamic stretching. PAP increases
the efficiency of muscular contraction by lowering the threshold for recruitment of
motor units (55) and by increasing the rate at which crossbridges form within the
muscle (5). A quicker rate of crossbridge formation would affect the rate of force
39
development, which may in turn affect performance (5, 16). In addition, Behm et al.
speculated that PAP would benefit balance and reaction time by decreasing response
time to shifts in body posture (5). PAP occurs following submaximal or maximal
muscle contraction, and therefore the muscle contractions involved in dynamic
stretching activate this beneficial mechanism. Thus, PAP may partially explain
performance improvements both relating to maximal force and power, as well as
balance, reaction time, and agility measures.
Taken together, studies examining the role of both a general, aerobic warm-up
and of dynamic stretching suggest the following: first, a general, aerobic component to
the pre-activity warm-up is beneficial for increasing muscle temperature, for efficient
substrate delivery and utilization within the muscle tissue, for enhanced neural
impulse conduction, and may also directly improve performance. Second, a dynamic
stretching routine may improve performance by several mechanisms. Dynamic
stretching preserves this elevated muscle temperature, may increase range of motion
without reducing neural input to working muscles, may enhance coordination, and
serves as a rehearsal of sport-specific movements. The net effect is a performance
improvement in events requiring speed, power, and quick reaction time.
Conclusion
Despite traditional recommendations to include static stretching as part of a
pre-game warm-up routine, current evidence suggests that static stretching can be
harmful to performance by affecting rate of force production and peak force
production capacity. Performance reductions have been documented in isokinetic and
40
isometric measures of force, as well as in sprint speed and jump height. Additionally,
little evidence exists to support the notion that static stretching is helpful in reducing
injuries – rather, a more comprehensive model of injury reduction is emerging that
includes a general warm-up combined with plyometric, proprioceptive, and strength
training (44). Dynamic stretching exercises can increase muscle temperature and
blood flow while improving nerve conduction velocity and waste product removal.
Additionally, sport-specific dynamic stretching can provide an opportunity for skill
rehearsal, further improving performance. To date, little is known about the effects of
static and dynamic stretching on ancillary measures of performance such as reaction
time. In competitive sports where the difference between winning and losing may be a
fraction of a second, it is essential that coaches and strength and conditioning
professionals have the knowledge to design the best warm-up routine to maximize
performance.
41
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46
Chapter 3: Warm-Up with Dynamic Stretching Improves
Countermovement Jump Performance
Perrier ET, Hoffman MA
Sports Medicine and Disabilities Research Laboratory, Oregon State University
Erica T. Perrier
101 Women’s Building
Oregon State University
Corvallis, OR 97331
(541) 737-6899
perriere@onid.orst.edu
Keywords: static stretch, warm-up, dynamic stretch, countermovement jump, reaction
time
Manuscript to be submitted to the Journal of Strength and Conditioning Research.
47
WARM-UP WITH DYNAMIC STRETCHING
IMPROVES COUNTERMOVEMENT JUMP
PERFORMANCE
ERICA T. PERRIER, AND MARK A. HOFFMAN
Sports Medicine and Disabilities Research Laboratory, Oregon State University, Corvallis, Oregon
97331
Address all correspondence to:
Erica T. Perrier
101 Women’s Building
Corvallis, OR 97331
541-737-6899
perriere@onid.orst.edu
48
ABSTRACT
PURPOSE: The purpose of this research was to quantify the effects of a warm-up
with static or dynamic stretching on countermovement jump height, reaction time,
muscle onsets for tibialis anterior (TA) and vastus lateralis (VL), and low back/
hamstring flexibility.
METHODS: Twenty one recreationally-active males (24.4 ± 4.5 yrs), recruited from
the university community, completed 3 data collection sessions. Inclusion criteria
were regular participation (30 minutes, 3 days per week) in exercise including
resistance training, sprinting, or jumping, and no history of lower extremity injury in
the past 6 months. Each session included a 5 minute treadmill jog followed by one of
the stretch treatments: no stretching (NS), static stretching (SS), or dynamic stretching
(DS). After the general warm-up and treatment, the participant performed a sit-andreach test to assess low back and hamstring flexibility. Next, the participant
completed a series of ten maximal-effort countermovement jumps (CMJ), during
which he was asked to jump as quickly as possible after seeing a visual stimulus
(light). The onset of movement and CMJ height were determined from force plate
data, and muscle onsets (TA and VL) were obtained using surface electromyography
(sEMG). Ground reaction forces were recorded at 2000 Hz using a portable force
plate, and filtered at 25 Hz. Outcome measures included maximal jump height,
reaction time, and muscle onset (TA and VL). A repeated measures 3 (treatment) by 8
(jump) ANOVA was used to assess CMJ height. Additional outcome measures
(reaction time, muscle onsets, flexibility) were assessed using separate repeatedmeasures one-way ANOVA.
49
RESULTS: Results of the 3x8 repeated-measures ANOVA for CMJ height revealed a
significant main effect of treatment (p=.004). Post hoc analysis showed significant
differences between NS (41.4cm) and DS (43.0cm) (p=.0045), and between SS
(41.9cm) and DS (p=.0435), but not between NS and SS (p=.4605). Analysis also
revealed a significant main effect of jump (p=.005) on CMJ height: mean jump height
progressively decreased from the early to the late jumps. No significant interaction
between treatment and jump was observed (p=.571). The analysis of reaction time,
TA and VL sEMG onsets showed no significant effects. Treatment also had a main
effect (p<.001) on flexibility. Post hoc analysis revealed improved flexibility after
both SS (p=.002) and DS (p<.001) compared with NS, with no difference in flexibility
between the two treatments (p=.530).
CONCLUSION: CMJ height was significantly higher during the DS condition
compared to SS and NS, with no difference between NS and SS. Additionally,
reaction time and muscle onsets were not influenced by either stretch technique.
Athletes in sports requiring lower-extremity power should use dynamic stretching
techniques in warm-up to enhance flexibility while improving performance.
50
INTRODUCTION
Athletes traditionally include static stretching as part of a pre-activity warm-up
in order to improve performance and decrease the risk of injury. Despite this common
practice, no conclusive evidence exists supporting the theory that static stretching
prior to exercise reduces injury risk (23). Additionally, static stretching has recently
been purported to decrease maximal force production (9, 10, 17, 18, 20), jump height
(5, 27, 32), and sprint speed (12, 16), while increasing reaction time and impairing
balance (4). As a result, strength and conditioning professionals have begun to shift
away from pre-practice static stretching in favor of a functional, dynamic warm-up
(2). Dynamic stretching theoretically provides the same flexibility benefits as static
stretching without compromising performance and may even improve performance in
activities involving explosive power (15, 30).
Static stretching-related performance reductions have been explained by a
combination of mechanical and neural factors. Mechanically, static stretching results
in increased compliance in the musculotendinous unit (MTU), including a more
compliant tendon capable of absorbing increased energy (28). As the MTU lengthens
and becomes more compliant, contractile elements must contract over a greater
distance, and more forcefully, to “pick up the slack”, resulting in reduced peak torque
and a slower rate of force development (28). A stiffer MTU would therefore be
advantageous for performance, as it would facilitate rapid changes in tension and a
faster joint motion response. Neurologically, static stretching appears to decrease
motor unit activation (1, 6, 17, 20). Cramer and colleagues reported that static
stretching performed on the dominant leg resulted in decreases in peak torque and
51
motor unit activation (as measured by electromyography amplitude) in both the
stretched and unstretched limbs (9, 10). Their research suggests that since deficits in
maximal force production were observed in both the stretched and the unstretched
limb, a central nervous system inhibitory mechanism must be at least partially
responsible for the observed changes (10).
Recent studies suggest that dynamic stretching prior to activity may improve
performance by increasing joint range of motion and core body temperature, resulting
in increased blood flow to the muscles and faster nerve-impulse conduction (24).
Dynamic stretches that simulate movement patterns used in a sport may also “prime”
the muscles by providing an opportunity for sport-specific skill rehearsal (16).
Dynamic stretching routines incorporate skipping, directional running, shuffling, and
various calisthenics of increasing intensity that simulate the movement patterns
necessary for success in a particular sport. Performance improvements after dynamic
stretching have been documented in sprinting (11), jumping (15), and peak force
generating capacity (30). Recent guidelines recommend strength and conditioning
professionals replace static stretching with dynamic stretching in their pre-activity
warm-ups, suggesting that dynamic stretching is becoming an accepted alternative to
static stretching in pre-game warm-ups.
Successful performance in sport often requires explosive power and quick
reaction time. In elite competition, where success may be affected by incredibly small
performance differences, it becomes essential for the athlete to maximize benefits
from the warm-up. The purpose of this research was to quantify the effects of a warmup with static or dynamic stretching on countermovement jump (CMJ) height, reaction
52
time, muscle onsets – tibialis anterior (TA) and vastus lateralis (VL) – and low back/
hamstring flexibility. We hypothesized that a warm-up including static stretching
would reduce jump height and delay reaction time, while a warm-up with dynamic
stretching would improve jump height and reaction time. Additionally, we
hypothesized that both warm-ups would be equally effective at increasing hamstring
flexibility.
METHODS
Experimental Approach to the Problem
The purpose was to quantify the effects of a warm-up with static or dynamic
stretching on countermovement jump height, reaction time, muscle onsets (TA and
VL), and low back/ hamstring flexibility. Participants completed three testing
sessions, each consisting of a general warm-up followed by one of three treatments: no
stretching (NS), static stretching (SS), or dynamic stretching (DS). After the general
warm-up and treatment, the participant was measured on each of the outcome
variables. First the participant performed a sit-and-reach test to assess low back and
hamstring flexibility. Next, the participant completed a series of ten maximal-effort
countermovement jumps, during which he was asked to jump as quickly as possible
after seeing a visual stimulus (light). The onset of movement and CMJ height were
determined from force plate data. Muscle onsets (tibialis anterior and vastus lateralis)
were obtained using surface electromyography. The order of treatments was
counterbalanced and the three sessions were scheduled 3 to 7 days apart.
53
Subjects
Twenty-one male university students (24.4 ± 4.5 yrs; 1.80 ± .06 m; 81.1 ± 14.0
kg) volunteered for this study. Inclusion criteria included regular participation
(minimum of 30 minutes per day, 3 days per week) in physical activity that included
resistance training, sprinting, jumping, or quick changes in direction. Individuals who
reported low-back or lower extremity injury (strain, sprain, or fracture) in the past 6
months were excluded. Participants were asked to abstain from resistance training for
at least 24 hours prior to testing. The study was approved by an institutional review
board for the protection of human subjects, and all participants gave their informed
consent.
Protocol
Participants completed three testing sessions. A general warm-up was
performed at the start of each testing session. The general warm-up consisted of a 5
minute treadmill jog at self-selected pace (8.6 ± 1.4 km·h-1) that was kept constant for
all three testing sessions, followed by two submaximal countermovement practice
jumps. After completing the general warm-up, participants completed 1 of the 3
treatments (NS, SS, DS) during each visit. During the No Stretching (NS) treatment,
subjects sat quietly for 15 minutes. The Static Stretching (SS) treatment consisted of 7
lower-extremity stretching exercises (Table 1). The selected stretches are
representative of commonly-recommended stretches to target major muscle groups of
the lower extremity (3). Each stretch was held for 30 seconds and was performed
twice (3). Mean time to complete the SS protocol was 14.8 ± 0.4 minutes. The
54
dynamic stretching (DS) treatment consisted of 11 exercises of increasing intensity
performed on a regulation-size volleyball court (18.3m) (Table 2). Mean time to
complete the DS treatment was 13.8 ± 1.7 minutes. Subjects rated the intensity of the
dynamic warm-up as a 5.2 ± 1.2 on the modified Borg RPE scale (1-10).
--- TABLE 1 ABOUT HERE ---
--- TABLE 2 ABOUT HERE ---
After completing the treatment, flexibility was assessed using a sit and reach
box, following a standard protocol (3, 8, 29) Participants removed their shoes and
placed their heels 12 inches apart, with feet flat against the measurement device (Flex
Tester ®, Novel Products, Inc., Rockton, IL). Bending at the waist, and keeping knees
fully extended, participants reached forward with overlapping fingertips, sliding the
indicator forward with their fingertips. The best of three trials was retained.
After the sit-and-reach test, leg dominance was determined using three
functional tests: the ball kick test, step-up test, and balance recovery test (14). Next,
the participant was instrumented with surface electromyography (sEMG) electrodes
and an electrogoniometer. Disposable lubricated surface electromyography (sEMG)
electrodes (Ag/AgCl) were affixed over the vastus lateralis (VL) and tibialis anterior
(TA) of the dominant leg using placement described by Rainoldi et al.(21). An
electrogoniometer (BioPac Systems, Goleta, CA) was affixed to the lateral aspect of
the participant’s dominant knee to monitor knee joint angle during the CMJs.
55
After being instrumented, participants performed a series of 10 CMJs.
Participants stood on a portable force plate (Kistler USA, Amherst, NY) with hands
akimbo, and were instructed to perform a maximal-height CMJ as quickly as possible
after seeing a visual stimulus (light) that was at eye level approximately 2m in front of
the participant. A total of 10 CMJs were performed with 60 seconds rest between
jumps. Ground reaction forces were recorded at 2000 Hz, low-pass filtered at 25 Hz
and processed using a custom program (LabVIEW 8.5, National Instruments, Austin
TX). Recorded ground reaction forces were used to calculate CMJ height and
movement onset. CMJ height was calculated by integrating the area under the forcetime curve from the onset of movement to the instant of take-off, in order to obtain
vertical velocity at take-off, and by integrating velocity to find vertical height of the
participant’s center of mass at take-off. Projectile motion equations were then used to
calculate jump height. The onset of movement, which was used to determine reaction
time in addition to the jump height calculation, was determined to be the point when
vertical ground reaction force fell 3 standard deviations below its baseline value. For
each CMJ, muscle onsets for the TA and VL with respect to the stimulus display were
determined using sEMG collected at 2000 Hz (BioPac Systems, Goleta, CA). EMG
tracings were rectified and smoothed using 10 sample averaging. VL and TA onsets
were defined to be the points when rectified EMG amplitude remained 3 standard
deviations above baseline for 10ms (TA) or 20ms (VL). Onsets selected by a custom
program (LabVIEW version 8.5) were visually confirmed and adjusted if necessary
(66 out of 1260 trials).
56
Statistical Analysis
The purpose of this experiment was to quantify the effects of a warm-up with
static or dynamic stretching on the following dependent variables: countermovement
jump height, reaction time, muscle onsets (TA and VL), and low back/ hamstring
flexibility. Data from jumps 1 and 10 of each testing session was discarded due to
inconsistency with other jumps. Data from two subjects were excluded due to missing
data points and extreme variation between testing sessions. CMJ height was analyzed
with a [3 (treatment) x 8 (jump)] repeated-measures analysis of variance (ANOVA)
(SPSS version 15 for Windows, Chicago, IL). Based on the finding of the initial 3 X 8
ANOVA, the data were collapsed across jumps and a single factor (treatment)
repeated measures ANOVA was used to analyze the remaining dependent variables.
Since flexibility was measured only after each treatment, 1x3 ANOVA was used. All
post hoc analyses were conducted using Bonferroni-adjusted paired t-tests.
Significance was set at p < 0.05.
RESULTS
Results of the 3x8 repeated-measures ANOVA for CMJ height revealed a main
effect of stretch treatment (p=.004) (Fig. 1). Analysis also revealed a significant main
effect of jump on CMJ height: a linear trend of decreasing performance across jumps
existed in all groups (p=.005) (Fig. 2). No significant interaction between treatment
and jump was observed (p=.571). Post-hoc comparisons between the treatments
revealed that mean CMJ height was significantly higher after DS compared to NS and
SS (one-tailed t-tests: p=.0045 and p=.0435). The difference in mean jump height
57
between NS and SS was not significant (p=.4605).
The analysis of reaction time, TA
and VL sEMG onsets showed no significant effects (Fig. 3). Means and p-values for
all variables are listed in Table 3.
A 1x3 ANOVA revealed a significant main effect of stretch treatment on sitand-reach flexibility (p<.001) (Fig. 4). Post-hoc analyses revealed that both SS and
DS significantly increased flexibility compared to NS (SS: p=.002; DS: p<.001). No
difference in sit-and-reach scores was observed between SS and DS (p=.530),
suggesting that both stretch techniques were equally effective in enhancing lower back
and hamstring flexibility.
--- RESULTS TABLE ABOUT HERE (TABLE 3) ---
DISCUSSION
The purpose of this investigation was to compare the effects of a warm-up
including static or dynamic stretching on countermovement jump performance,
reaction time, TA and VL onsets, and hamstring flexibility. Using a repeatedmeasures design, the results revealed that CMJ height was significantly higher after
DS compared with NS or SS (3.9% and 2.5% improvement, respectively). These
results are consistent with previous research that reported increases in CMJ height
after a warm-up consisting of progressive-resistance half-squats (13). Additional
studies have revealed that warm-up treatments including jogging and/or dynamic
stretching and practice jumps resulted in higher CMJ height than warm-ups that
included static stretching (26, 32). Dynamic stretching has also been shown to
58
improve sprint times and agility drill performance. Little and Williams (16) reported
that lower-body dynamic exercises resulted in reduced 10 and 20m sprint times as well
as zig-zag drill time, but reported no change in CMJ performance. Additional research
suggests that dynamic exercise performed at a jogging pace can improve sprint
performance; however, comparable improvements were not observed when these
exercises were performed while stationary (12). These studies collectively suggest
that dynamic stretching exercises, particularly those performed at a jogging pace as
opposed to stationary, can improve performance in measures of power such as
sprinting and jumping.
Our results suggest that static stretching has no effect on jump height, when
compared with no stretching. While many studies investigating the effects of static
stretching on performance have reported decreases in peak torque (9, 10, 17, 18, 20)
and jump height (5, 7, 27, 32), there is still disagreement as to what extent static
stretching reduces performance. Several authors (8, 16, 20, 25) have reported no
change in jump performance after performing lower-extremity static stretching
ranging from 3 repetitions of 15 seconds (25) up to 3 repetitions of 45 seconds (20).
In our study, the static stretch protocol consisted of a moderate amount of stretching (2
repetitions of 30 seconds for 7 lower-extremity stretches), which resulted in no change
in CMJ height when compared with no stretching (NS). It is possible that the 15minute waiting period between the general warm-up and jump testing in the NS
condition eliminated any potential differences between general warm-up only and
warm-up with static stretching. It is also possible that 2 sets of 30 seconds was not a
sufficient stretch duration to induce performance deficits. A recent study by Robbins
59
and Scheuermann (22) reported that squat jump performance was affected by 6 sets of
quadriceps, hamstring, and plantarflexor stretches, but that jump height was unaffected
by 2 or 4 sets of the stretches. Their research, however, examined the concentric-only
squat jump, which does not rely on the stretch-shortening cycle to enhance lowerextremity power production and may therefore be less susceptible to stretch-induced
force decrements (31).
One mechanism by which dynamic stretching may improve performance is by
providing an opportunity for rehearsal of specific movement patterns (12, 16). If
dynamic stretching improved performance by allowing for a rehearsal of movement,
participants’ peak performance would be expected to occur during the last several
jumps in the series, as the earlier jumps might provide more opportunity for skillspecific rehearsal. Contrary to this, our results showed a progressive decrease in CMJ
height from the early jumps to the later jumps. This gradual decrease in jump height is
particularly interesting after static stretching. We expected to see mean jump height
increase progressively during the jump series, since the first few jumps had the
potential to function as a secondary warm-up that might gradually increase mean CMJ
height up to the level reached after dynamic stretching. Pearce and colleagues
observed a similar phenomenon to the present while examining the performance
impact of a secondary dynamic warm-up after static stretching (19). The authors
found that performance deficits observed after static stretching continued to worsen
for up to 30 minutes following 10-12 minutes of secondary dynamic movement drills.
From a performance perspective, this suggests that potential deficits in performance
after static stretching are not easily reversed through additional activity.
60
From a neurological perspective, post-activation potentiation (PAP) has been
proposed as a mechanism for improved performance following dynamic stretching.
PAP increases the efficiency of muscular contraction by lowering the threshold for
recruitment of motor units (32) and by increasing the rate of crossbridge formation (4).
Behm and colleagues speculated that PAP may also benefit reaction time by
decreasing response time to shifts in body posture (4). In contrast, our results did not
reveal statistically significant differences in reaction time between the stretch
treatments. Large inter- and intra- subject variability in muscle onsets and insufficient
power in our study may have resulted in an inability to detect possible subtle
improvements.
The observed differences in CMJ height reveal a clear performance advantage
in choosing dynamic stretching prior to exercise. A second important consideration in
warm-up design, however, is whether the chosen stretching method produces the
desired increase in flexibility. Our results suggest that static and dynamic stretching
are equally effective at improving sit and reach performance. Thus, dynamic
stretching may be particularly beneficial in sports requiring a combination of
flexibility and explosive force, as it appears to provide the greatest performance
benefits without sacrificing acute flexibility in the process.
There were some limitations to our study design. First, all subjects were
recreationally active but participated in different sports and trained at different
intensity levels (ranging from 30 minutes of activity, 3 days/week to 2 hours of intense
activity, most days of the week). Additionally, some reported stretching regularly
after every work-out, while others reported no regular stretching practice. Some
61
participants also reported that the dynamic stretch treatment, designed based on a
typical collegiate-level warm-up, was intense enough to potentially cause fatigue.
Participant’s rating of their level of exertion during dynamic stretching ranged from a
3 (“Moderate”) to 6 (“Strong” to “Very Strong”) on the Borg modified RPE scale.
Given these subject characteristics, it is not possible to infer whether static and
dynamic stretching would have similar results on a more highly-trained population.
PRACTICAL APPLICATIONS
Our study, in combination with previous work, suggests that dynamic
stretching prior to exercise can provide a performance advantage in jump height, sprint
speed, and agility. In designing effective warm-up routines for athletes requiring
strength, speed, or power, a general warm-up should be followed by dynamic
stretching that increases muscle temperature and blood flow, while providing the
opportunity for sport-specific movement rehearsal. Dynamic stretches should,
whenever possible, mimic the movement patterns most closely associated with success
in the athlete’s sport. A well-designed warm-up including dynamic stretching can
serve the dual purposes of enhancing acute flexibility while also priming the athlete
for peak performance.
62
TABLES AND FIGURES
TABLE 1 – Static stretch protocol. Stretches were held for 2 repetitions of 30 seconds each.
Stretch
Standing quadriceps
stretch:
Supine hamstring
stretch:
Hip flexor stretch:
Butterfly (groin) stretch:
Piriformis stretch:
Single-knee lower back
stretch:
Standing calf stretch:
Description
while standing, participants flexed one knee and grasped heel,
bringing heel as close as possible to the buttocks, eliciting a
stretch in the quadriceps.
while supine with both legs fully extended, participants raised
one leg, using hands to support both above and below the
knee. Participants were allowed a small amount of knee
flexion.
from a lunge position, participants slowly lowered hips until
o
front knee was flexed to 90 and back leg was extended,
eliciting a stretch in the iliopsoas and rectus femoris muscles.
from a seated position, participants brought the soles of their
feet together and allowed their knees to hang to the sides.
Gentle pressure was exerted by the elbows to lower knees
toward the ground.
while supine, participants crossed one ankle above the
opposite knee and brought the bottom leg towards the chest.
while supine with both legs slightly bent, participants brought
one knee up to the chest.
from a lunge position, participants pressed their back heel
down towards the ground.
TABLE 2 – Dynamic stretch protocol. Each exercise was performed twice over a distance of
18m. Participants walked back to the starting line between repetitions.
Easy skip with arm swings
Skip for distance using arms to drive forward
Skip for height using arms to drive upward
Backward run (extend heel backwards during stride)
Lateral low shuffle (back and forth- no walk - rest 20 seconds between reps)
Step into single leg Romanian dead lift
Walking diagonal lunges
High Knee Pulls (knee to chest, on toe)
Carioca (back and forth - no walk - rest 20 seconds between reps)
Straight leg strides (back and forth - no walk - rest 20 seconds between reps)
Gradual accelerations (1 x 50%, 75%, 90%, walk back between reps)
63
TABLE 3 – Results
General Warm-Up
Only (NS)
Stretch Condition
General WarmUp and Static
Stretch (SS)
General Warm-Up
and Dynamic
Stretch (DS)
Mean Jump
Height (cm)
41.4(6.8)
41.9(6.6)
43.0(6.3)*
†
0.004
Reaction
Time (s)
.307(.039)
.304(.051)
.304(.037)
0.413
TA sEMG
Onset (s)
0.238(.043)
0.235(.035)
0.227(.033)
0.383
VL sEMG
Onset (s)
.509(.079)
.497(.059)
.475(.077)
0.352
Sit-andreach (cm)
30.0(8.3)
32.8(7.8)
†
33.2(7.4)
†
p-value
< 0.001
†
denotes significance (p<.05) compared with NS mean.
* denotes significance (p<.05) compared with SS mean.
Mean Jump Height (cm)
*
47
*
45
cm
43
41
39
37
35
NS
SS
DS
Figure 1 -- Mean jump height (cm) after each stretch treatment. Mean height after DS was
significantly higher than after NS and SS (p=.0045; p=.0435).
64
Mean Jump Height
0.450
NS
0.445
SS
Mean Height (cm)
DS
0.440
Linear (NS)
0.435
Linear (SS)
Linear (DS)
0.430
0.425
0.420
0.415
0.410
0.405
0.400
1
2
3
4
5
Jump
6
7
8
9
10
Figure 2 -- Jump height progression for jumps 2 through 9.
Reaction Time and TA & VL EMG Onsets
0.55
0.5
0.45
ms
0.4
0.35
0.3
0.25
0.2
0.15
NS
SS
DS
Reaction Time
NS
SS
DS
TA EMG
NS
SS
DS
VL EMG
Figure 3 – Mean onsets for movement time, TA EMG, and VL EMG after each stretch
treatment.
65
38
Sit and Reach Score (cm)
*
*
36
34
32
cm
30
28
26
24
22
20
NS
SS
DS
Figure 4 – Mean sit-and-reach score (cm) after each stretch treatment. Mean flexibility after
SS and DS were significantly higher than after NS (p<.001).
66
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Chapter 4: Conclusion
70
The purpose of this investigation was to compare the effects of a warm-up
with static or dynamic stretching on CMJ performance, reaction time, TA and VL
muscle onsets, and flexibility. Participants completed 3 testing sessions, each
consisting of a general warm-up followed by one of three treatments: no stretching
(NS), static stretching (SS), or dynamic stretching (DS). Our results revealed that
CMJ height was significantly higher after DS compared with NS and SS. Stretch
treatment did not appear to influence reaction time or muscle onsets. Sit-and-reach
scores were improved after both SS and DS, suggesting that both treatments are
equally effective at enhancing flexibility.
Our study, in combination with previous work, suggests that dynamic
stretching prior to exercise can provide a performance advantage in jump height,
sprint speed, and agility.
In designing effective warm-up routines for athletes
requiring strength, speed, or power, a general warm-up should be followed by 10 to
15 minutes of dynamic stretching that increases muscle temperature and blood flow,
while providing the opportunity for sport-specific movement rehearsal. Dynamic
stretches should be performed in order of increasing intensity and, whenever
possible, mimic the movement patterns most closely associated with success in the
athlete’s sport. A well-designed warm-up including dynamic stretching can serve
the dual purposes of enhancing acute flexibility while also priming the athlete for
peak performance.
71
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Appendices
78
Appendix A – Institutional Review Board Documents
79
1. Brief Description:
The objective of this study is to compare the effects of static and dynamic stretching
on reaction time and performance in countermovement jump (CMJ). Data collected in
this research will fulfill the research requirement for a MS in Sports Medicine at
Oregon State University, with the intent of submitting a manuscript for publication at
a later date. Specific study aims are to determine the effects of static and dynamic
stretching on the following measures:
Lower back and hamstring flexibility
Reaction time
Jump performance
2. Background and Significance
Previous research has shown that static stretching prior to activity can acutely
decrease performance in activities related to strength and power, specifically affecting
maximal force production and, consequently, jumping performance. Static stretching
also appears to increase reaction time and decrease balance, thereby potentially
causing decrements to athletic performance. Stretching-related performance decreases
have been tentatively explained by a combination of neurological and mechanical
factors.
Recent literature has suggested that dynamic stretching may be effective in
increasing joint range of motion without producing any negative effects on sport
performance. Dynamic stretching routines incorporate skipping, directional running,
shuffling, and various calisthenics in order to increase range of motion and increase
core body temperature, which may result in better performance through greater blood
flow to the muscles, faster nerve-impulse conduction, and "priming" of the muscles by
providing a rehearsal of specific body movements. In fact, several researchers have
indicated that dynamic stretching may improve performance, by providing an
opportunity for the athlete to rehearse sport-specific movements at gradually
increasing intensity.
This study will investigate the effects of static and dynamic stretching on
flexibility, reaction time and performance in a countermovement jump.
3. Methods and Procedures
Subject Recruitment: Male subjects, aged 18-35 years, will be recruited. Eligible
participants will: 1) exercise regularly (a minimum of 30 minutes, three days per week), 2)
be free of leg and lower-back injury (strain, sprain or fracture) for at least 6 months prior
to testing, and 3) current exercise regimen includes include resistance training and/or
exercise that involves jumping, sprinting, or quick changes in direction. Subjects will be
recruited through word of mouth and through flyers posted on campus.
80
Protocol:
- Read and sign consent form (visit 1 only) (~ 5 minutes)
- Determination of height and weight (~ 2 minutes)
- Electrode placement (~ 5 minutes)
- Warm-up, specified below (~ 20 minutes)
- Electrogoniometer placement (~ 5 minutes)
- Sit-and-Reach (~ 5 minutes)
- Countermovement Jumps (~ 15 minutes)
- Clean Up (~ 5 minutes)
- Total Time: ~ 62 minutes
Protocol Specifics:
-
Lab visits: Subjects will complete three testing sessions: one for each warm-up
treatment (see Figure 1). Each testing session will last approximately one
hour. Total time commitment for the study will be approximately 3 hours and
total participation will occur over a maximum of 21 days.
o General Warm-Up (control group): Subjects are instructed to jog at a
self-selected pace for five minutes and then sit quietly for 15 minutes
prior to testing.
o General Warm-Up and Static Stretch (SS): Subjects will complete
general warm-up described above followed by a static stretch series of
seven lower body stretches held for two repetitions of 30 seconds each
(Table 1 and Figure 2).
o General Warm-Up and Dynamic Stretch (DS): Subjects will complete
the general warm-up followed by a dynamic stretch series of 11 lowerbody dynamic stretches (Table 2).
-
Electrode Placement: Three lubricated surface electromyography (sEMG)
electrodes (Ag/AgCl) will be placed on the subject’s dominant leg to monitor
muscle activity: two on the tibialis anterior (3-4 inches below the knee) and
one on the patella (kneecap).
-
Electrogoniometer Placement: A device used to measure knee joint angle will
be affixed to the outside of the subject’s dominant leg.
-
Sit-and-Reach: Subject is instructed to remove shoes and sit on the floor with
the soles of the feet 12 inches apart and flat against the testing apparatus
(Novel Products, Inc., Rockton, IL). Subject will then slowly exhale and
slowly reach forward with both hands to push the metal indicator as far as
possible. The score will be the position of the indicator following three trials.
-
Countermovement Jump: The subject standing on a portable force plate
(Kistler USA, Amherst, NY) will be instructed to jump as quickly and as high
as possible after seeing a red light affixed to the wall at approximately eye
81
level. The subject will be instructed to keep hands resting lightly on hips
throughout the jump. This procedure will be repeated a total of ten times with
60 seconds rest between jumps.
-
Clean Up: Removal of electrodes and electrogoniometer from subject’s
dominant leg.
4. Risks/Benefit Assessment
Risks: The minimal risks to participants in this study include the possibility of injury
while completing the warm-up and stretch treatments, or during jump testing.
Subjects will be instructed as to the proper technique to perform all activities in order
to minimize the risk of injury.
Benefits: There are no direct benefits to subjects participating in this study.
5. Participant Population:
21 healthy male subjects, aged 18-35 will be recruited from the general university
population. Conditions for participation include 1) Recreationally active at least three
days per week, 30 minutes per session, 2) Free of lower extremity injury (strain, sprain or
fracture) for 6 months prior to testing, and 3) Current exercise regimen includes include
resistance training and/or exercise that involves jumping, sprinting, or quick changes in
direction. Participants will initially be screened either via phone or email.
6. Subject Identification and Recruitment:
Subjects will be recruited through flyers and word of mouth. Once a potential subject
is identified, initial screening will occur via phone or email. If potential subject meets
eligibility criteria, a testing schedule will be established with the participant.
7. Compensation:
No compensation is provided for participation in this study.
8. Informed Consent Process:
The informed consent document is attached. Subjects will read and sign informed
consent document prior to the commencement of any testing. Any questions will also
be answered prior to the document being signed.
9. Anonymity or Confidentiality
All forms and files will be coded without any identifiable participant information.
Forms and files will be stored in a locked cabinet in the office of the researcher. All
electronic files will be coded without any identifiable participant information.
Attachments
- Recruitment Materials
- Institutional Review Board Approval Letter
- Informed Consent Document
82
Script for Word-of-Mouth Recruitment
“We are enrolling subjects for a study examining the effects of stretching on reaction
time and jumping performance. If you meet our screening criteria, we would like to
give you the opportunity to participate in the study. If you are eligible, and if you
choose to participate, you would attend three one-hour testing sessions over a
maximum period of 21 days. Would you like to see if you are eligible to participate?”
Screening Tool
Question
1. Are you male, between the ages of 18 and 35?
2. In the past 6 months, have you had any injuries to your legs or
lower back?
3. Do you currently exercise?
If yes, how many times per week? __________________________
How long do you exercise for? ____________________________
4. Does your current exercise regimen include resistance training or
exercise that involves jumping, sprinting, or quick changes in
direction?
If
yes,
what
type
of
exercise?
_______________________________
-
Yes
No
□
□
□
□
□
□
□
□
83
84
85
86
87
Appendix B – Data Output
First
Visit SID
DS
01
NS
03
DS
04
SS
05
SS
06
DS
07
NS
08
NS
09
SS
10
DS
11
NS
12
SS
13
DS
14
NS
15
SS
16
DS
17
NS
18
SS
19
DS
20
NS
21
SS
22
No Stretch
Static Stretch
Dynamic Stretch
Total Stretch Sit/ Total Stretch
Sit/ Total Stretch Sit/
Days/ Height Weight
Time Time Reach Time Time Reach Time Time Reach
(kg)
BMI Leg Treadmill (min) (min)
(cm) (min) (min)
(cm) (min) (min) (cm) RPE
Age NS Date SS Date DS Date Visits (m)
22 01/30/08 02/07/08 01/24/08
7.0 1.75
83.0 27.1 L
5.3
40
15
33
41
14.75
36.5 40
10
35
6
22 01/30/08 02/04/08 02/11/08
6.0 1.74
69.0 22.9
L
5
39
15.0
30.5
40
15
32.5 45
12.5
32
3
36 02/11/08 02/18/08 02/04/08
7.0 1.76
86.5 28.1 R
4.5
39
15.0
17.5
41
14.75
32.5 41 12.75
25
5
27 02/13/08 02/05/08 02/08/08
4.0 1.87 119.0 34.0 R
4
41
15
31
46
15.25
32.5 43 14.25
30
6
32 02/22/08 02/11/08 02/18/08
5.5 1.89
76.5 21.4 R
6.7
41
15.0
19
41
14.75
21.5 40 13.75 23.5
7
27 02/18/08 02/21/08 02/15/08
3.0 1.89
98.0 27.4 R
6
41
15
40
41
15.25
42 43
11
42
5
22 02/07/08 02/14/08 02/21/08
7.0 1.77
78.0 24.9
L
5.5
42
15
26
40
15
30 43
13.5
32.5
5
22 02/14/08 02/21/08 02/28/08
7.0 1.79
65.5 20.4 R
4.9
42
15.0
33
40
15
30 46 16.75 36.5
6
23 02/22/08 02/11/08 02/14/08
5.5 1.84
95.0 28.2 R
6.2
43
15
28
42
14
32 40 12.25 34.5 5.5
29 03/04/08 03/07/08 02/29/08
3.5 1.83
81.5 24.5
L
4.5
41
15
40.5
42
14.8
41.5 45
13.5
39
5
19 03/03/08 03/06/08 03/10/08
3.5 1.71
73.5 25.1 R
5.3
41
15
27.5
42
14.8
29.5 42
13
32
6
25 04/18/08 04/11/08 04/15/08
3.5 1.70
61.0 21.1 R
4.4
44
15
13.5
42
14.5
11.5 47
15
15.5
6
21 04/16/08 04/22/08 04/11/08
5.5 1.75
86.0 28.1 R
6
39
15
41.5
41
14.5
42.5 42 12.25 42.5
7
31 04/11/08 04/16/08 04/19/08
4.0 1.84
78.0 23.2 R
8
42
15
27.5
43
15
30.5 43
15.5
32.5
3
20 04/23/08 04/16/08 04/30/08
7.0 1.76
75.0 24.4 R
5
42
15
40.5
42
14
43 42 14.75 44.5
3
23 04/30/08 05/02/08 04/25/08
4.0 1.86 105.0 30.4 R
4.5
43
15
32.5
42
14.75
38.5 43 13.75 41.5
6
19 04/30/08 05/02/08 05/13/08
7.0 1.82
71.5 21.6 R
5.9
43
15
27
43
15.25
33 43 14.75 31.5
4
24 06/04/08 05/28/08 05/30/08
3.5 1.79
69.5 21.7 R
5
42
15
43
40
15
42 44 16.75
41
5
21 05/30/08 06/04/08 05/28/08
3.5 1.82
87.0 26.3
L
4.8
42
15
33.5
42
14.25
36.5 42
15.5
35.5
6
23 05/28/08 05/30/08 06/04/08
3.5 1.76
68.0 22.0 R
5
43
15
26
38
14.5
26 43
14
26
5
24 06/11/08 06/02/08 06/04/08
4.5 1.79
77.0 24.0
L
6
42
15
19
42
15
24.5 42 14.25 24.5
5
88
Subject Data
89
Jump Data
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
Test_Order
2
3
1
1
2
3
3
1
2
2
3
1
2
3
1
3
1
2
1
2
3
1
2
3
2
3
1
3
1
2
1
2
3
2
3
1
3
1
2
1
2
3
2
1
3
3
1
2
1
2
3
2
3
1
3
1
2
1
2
3
2
3
1
Condition
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1_t_onset
0.155
0.189
0.121
0.345
0.213
-0.225
0.208
0.131
0.235
0.223
0.276
0.208
0.249
0.176
0.212
0.246
0.261
0.285
0.191
0.327
0.267
0.229
0.191
0.206
0.179
0.211
0.261
-0.258
0.158
0.260
0.179
0.218
0.267
0.217
0.221
0.174
0.348
0.337
0.284
0.251
0.263
0.379
0.221
0.131
0.253
0.100
0.172
0.281
-0.234
0.376
0.222
0.122
0.140
0.161
0.312
0.156
0.101
0.356
0.155
0.410
1_height
0.430
0.460
0.478
0.300
0.460
-0.282
0.456
0.326
0.401
0.450
0.452
0.345
0.339
0.346
0.411
0.385
0.484
0.362
0.384
0.431
0.318
0.486
0.411
0.455
0.407
0.450
0.329
-0.384
0.508
0.524
0.503
0.329
0.480
0.380
0.466
0.467
0.489
0.303
0.319
0.354
0.481
0.378
0.402
0.477
0.468
0.581
0.371
0.482
-0.347
0.315
0.434
0.476
0.482
0.494
0.440
0.459
0.633
0.411
0.270
0.503
1_TA_emg
0.238
0.230
0.235
0.337
0.205
0.221
0.189
0.191
0.203
0.258
0.196
0.296
0.220
0.320
0.202
0.176
0.272
0.269
0.325
0.259
0.242
0.185
0.130
0.185
0.171
0.276
-0.231
0.251
0.278
0.217
0.208
0.217
-0.182
0.193
0.173
0.171
-0.235
0.278
0.244
0.342
0.211
0.184
0.186
0.217
0.209
0.195
0.191
0.173
0.266
0.241
0.195
0.197
0.196
0.206
0.193
0.200
0.181
0.260
0.197
0.190
1_VL_emg
0.295
0.417
0.341
0.469
0.430
0.575
0.213
0.350
0.409
0.495
0.625
-0.491
0.593
0.567
0.622
0.244
0.190
0.602
0.608
0.228
0.707
0.506
0.525
0.233
0.500
0.254
0.504
0.328
0.353
0.425
0.443
0.463
-0.711
0.475
0.614
0.663
-0.918
0.603
0.774
0.732
0.674
0.531
0.644
0.583
0.544
0.512
0.637
0.538
0.518
0.525
0.502
0.451
0.421
0.542
0.650
0.500
0.507
0.596
0.575
0.633
90
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
2_t_onset
0.279
0.245
0.227
0.282
0.254
-0.456
0.258
0.478
0.347
0.299
0.305
0.310
0.256
0.196
0.338
0.093
0.318
0.327
0.383
0.303
0.223
0.370
0.307
0.281
0.325
0.285
0.281
0.292
0.302
0.265
0.291
0.264
0.311
0.265
0.270
0.267
0.276
0.320
0.335
0.334
0.312
0.363
0.418
0.415
0.227
0.261
0.192
0.297
0.229
0.286
0.313
0.424
0.326
0.269
0.261
0.238
0.356
0.266
0.236
0.351
0.300
0.281
2_height
0.460
0.455
0.492
0.409
0.337
-0.318
0.340
0.351
0.384
0.455
0.425
0.345
0.340
0.399
0.493
0.294
0.522
0.373
0.379
0.405
0.361
0.321
0.394
0.544
0.448
0.472
0.410
0.352
0.399
0.523
0.495
0.614
0.359
0.424
0.329
0.422
0.446
0.488
0.329
0.300
0.310
0.449
0.427
0.445
0.512
0.509
0.511
0.465
0.432
0.442
0.342
0.371
0.371
0.477
0.465
0.488
0.469
0.467
0.455
0.436
0.485
0.475
2_TA_emg
0.231
0.229
0.199
0.212
0.324
0.198
0.387
0.248
0.388
0.265
0.235
0.232
0.293
0.233
-0.210
0.251
0.213
0.296
0.319
0.220
0.191
0.204
0.180
0.214
0.266
--0.249
0.205
0.219
0.231
0.224
-0.191
0.201
0.173
0.191
0.182
0.185
0.191
0.199
0.352
0.255
0.248
0.233
0.205
0.150
0.177
0.196
0.176
0.175
0.351
0.224
0.221
0.205
0.205
0.199
0.199
0.202
0.193
0.228
0.234
2_VL_emg
0.543
0.401
0.315
0.576
0.450
0.492
0.605
0.489
0.398
0.382
0.633
-0.499
0.539
-0.556
0.356
0.175
0.495
0.572
0.202
0.483
0.576
0.480
0.471
0.424
0.195
-0.264
0.513
0.475
0.460
0.450
-0.601
0.450
0.594
0.363
0.625
0.699
0.420
0.507
0.513
0.464
0.550
0.495
0.481
0.500
0.532
0.435
0.566
0.508
0.478
0.502
0.433
0.359
0.531
0.604
0.496
0.529
0.614
0.574
0.559
3_t_onset
0.322
0.225
0.213
0.238
0.312
0.248
0.265
-0.356
0.447
0.345
0.334
0.269
0.296
0.247
0.354
0.362
0.367
0.366
0.301
0.368
0.253
0.399
0.305
0.297
0.267
0.251
0.319
0.273
0.302
0.238
0.254
0.259
0.317
0.360
0.267
0.284
0.270
0.265
0.325
0.395
0.309
0.413
0.385
0.406
0.205
0.279
0.201
0.282
0.240
0.264
0.312
0.412
0.389
0.225
0.228
0.238
0.217
0.249
0.199
0.358
0.272
0.351
3_height
0.480
0.470
0.480
0.408
0.416
0.304
0.295
-0.359
0.418
0.498
0.441
0.354
0.354
0.359
0.478
0.397
0.500
0.358
0.415
0.431
0.302
0.435
0.370
0.498
0.449
0.471
0.381
0.339
0.400
0.529
0.511
0.559
0.350
0.339
0.333
0.463
0.487
0.471
0.313
0.383
0.365
0.430
0.419
0.492
0.514
0.522
0.416
0.449
0.465
0.474
0.328
0.372
0.358
0.479
0.508
0.457
0.439
0.481
0.533
0.447
0.462
0.514
3_TA_emg
0.382
0.209
0.195
0.200
0.223
0.187
0.190
0.160
0.232
0.239
0.207
0.197
-0.228
0.209
0.236
0.198
0.234
0.302
0.233
0.302
0.192
0.247
0.201
0.188
0.201
-0.271
0.185
0.237
0.185
0.208
0.217
-0.222
0.210
0.182
0.192
0.172
0.177
0.330
0.235
0.355
0.246
0.190
0.195
0.234
0.190
0.195
0.241
0.178
0.196
0.244
0.316
0.220
0.207
0.177
0.170
0.182
0.180
0.189
0.259
0.204
91
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
3_VL_emg
0.520
0.417
0.409
0.627
0.653
0.538
0.405
0.528
0.393
0.745
0.403
--0.505
0.388
0.549
0.206
0.217
0.450
0.434
0.374
0.458
0.609
0.594
0.233
0.462
0.593
0.352
0.516
0.485
0.448
0.462
0.458
-0.375
0.400
0.585
0.428
0.449
0.666
0.728
0.543
0.539
0.500
0.707
0.445
0.549
0.519
0.555
0.459
0.517
0.405
0.453
0.537
0.458
0.434
0.510
0.501
0.560
0.604
0.596
0.657
0.643
4_t_onset
0.438
0.241
0.235
0.272
0.269
0.259
0.233
0.287
0.323
0.352
0.308
0.457
0.319
0.255
0.253
0.344
0.340
0.362
0.383
0.323
0.312
0.225
0.291
0.322
0.304
0.300
0.335
0.354
0.319
0.327
0.239
0.315
0.251
0.341
0.385
0.296
0.284
0.254
0.276
0.367
0.274
0.319
0.420
0.398
0.332
0.246
0.285
-0.276
0.268
0.261
0.346
0.370
0.345
0.256
0.214
0.249
0.236
0.267
0.235
0.341
0.326
0.413
4_height
0.460
0.463
0.468
0.390
0.368
0.393
0.326
0.349
0.354
0.380
0.465
0.432
0.360
0.323
0.368
0.481
0.446
0.510
0.366
0.377
0.370
0.355
0.340
0.363
0.490
0.402
0.455
0.323
0.373
0.363
0.561
0.522
0.399
0.325
0.345
0.351
0.463
0.477
0.489
0.312
0.248
0.336
0.466
0.436
0.436
0.499
0.506
-0.452
0.499
0.416
0.279
0.335
0.369
0.467
0.478
0.467
0.473
0.478
0.455
0.443
0.470
0.530
4_TA_emg
0.430
0.223
0.227
0.239
0.184
0.190
0.150
0.150
0.224
0.245
0.216
0.195
0.210
0.189
0.191
0.202
0.255
0.321
0.305
0.300
0.247
0.200
0.160
0.189
0.265
0.202
-0.231
0.350
0.238
0.184
0.263
0.207
-0.216
0.195
0.182
0.197
0.172
0.209
0.224
0.235
0.329
0.258
0.248
0.203
0.218
0.150
0.178
0.175
0.160
0.256
0.256
0.232
0.224
0.185
0.223
0.179
0.227
0.206
0.179
0.269
0.237
4_VL_emg
0.533
0.403
0.427
0.515
0.531
0.405
0.584
0.527
0.405
0.470
0.397
-0.450
0.527
0.416
0.284
0.227
0.205
0.703
0.357
0.202
0.538
0.498
0.533
0.229
0.484
0.245
0.468
0.514
0.265
0.498
0.517
0.493
-0.391
0.558
0.616
0.588
0.303
0.600
0.500
0.432
0.532
0.301
0.651
0.512
0.578
0.552
0.501
0.475
0.510
0.394
0.559
0.464
0.469
0.419
0.476
0.627
0.517
0.483
0.543
0.763
0.629
5_t_onset
0.267
0.243
0.352
0.283
0.308
0.301
0.273
0.294
0.318
0.396
0.319
0.345
0.297
0.275
0.308
0.362
0.345
0.348
0.375
0.303
0.359
0.264
0.364
0.305
0.291
0.323
0.276
0.357
0.445
0.267
0.252
0.315
0.247
0.259
0.331
0.281
0.291
0.251
0.278
0.368
0.274
0.365
0.347
0.378
0.415
0.226
0.251
0.239
0.288
0.277
0.329
0.332
0.350
0.291
0.281
0.226
0.238
0.314
0.266
0.195
0.550
0.295
0.379
5_height
0.409
0.460
0.461
0.408
0.419
0.363
0.352
0.311
0.330
0.386
0.463
0.304
0.368
0.331
0.396
0.439
0.476
0.514
0.354
0.391
0.390
0.369
0.337
0.369
0.521
0.431
0.449
0.325
0.331
0.408
0.569
0.512
0.550
0.369
0.330
0.362
0.485
0.486
0.482
0.295
0.328
0.331
0.400
0.464
0.467
0.548
0.506
0.548
0.425
0.479
0.476
0.362
0.328
0.382
0.455
0.484
0.482
0.451
0.465
0.479
0.419
0.425
0.510
92
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
5_TA_emg
0.241
0.203
0.342
0.214
-0.566
--0.195
0.276
0.268
0.260
0.226
0.215
0.180
0.215
0.222
0.253
0.314
0.243
0.278
0.191
0.200
0.201
0.192
0.256
-0.337
0.502
0.215
0.208
0.262
0.191
-0.183
0.186
0.175
0.184
0.172
0.287
0.201
0.286
0.337
0.214
0.272
0.216
0.216
0.198
0.181
0.251
0.322
0.277
0.277
0.207
0.238
0.196
0.201
0.259
0.221
0.178
0.426
0.395
0.273
5_VL_emg
0.463
0.443
0.564
0.488
0.531
0.480
0.600
0.625
0.363
0.551
0.444
-0.327
0.440
0.423
0.300
0.194
0.176
0.647
0.460
0.260
0.470
0.581
0.566
0.342
0.537
0.533
0.490
0.530
0.462
0.457
0.492
0.488
-0.359
0.400
0.554
0.545
0.526
0.629
0.593
0.422
0.487
0.361
0.697
0.518
0.535
0.417
0.616
0.446
0.567
0.290
0.521
0.462
0.432
0.398
0.477
0.641
0.511
0.413
0.806
0.540
0.547
6_t_onset
0.367
0.268
0.262
0.264
0.291
0.284
0.257
0.262
0.324
0.401
0.293
0.343
0.254
0.235
0.291
0.354
0.313
0.315
0.332
0.299
0.350
0.301
0.289
0.295
0.296
0.362
0.226
0.352
0.278
0.348
0.246
0.269
0.279
0.286
0.329
0.299
0.279
0.293
0.277
0.416
0.302
0.392
0.283
0.449
0.351
0.269
0.388
0.215
0.271
0.322
0.273
0.368
0.312
0.340
0.221
0.199
0.236
0.289
0.356
0.208
0.337
0.281
0.418
6_height
0.467
0.444
0.463
0.420
0.435
0.382
0.382
0.490
0.370
0.406
0.491
0.464
0.281
0.342
0.360
0.536
0.335
0.464
0.387
0.353
0.365
0.367
0.316
0.424
0.453
0.468
0.501
0.316
0.338
0.413
0.553
0.528
0.523
0.375
0.324
0.335
0.454
0.457
0.469
0.327
0.307
0.330
0.469
0.465
0.491
0.544
0.482
0.533
0.462
0.437
0.438
0.353
0.329
0.356
0.471
0.474
0.492
0.434
0.524
0.494
0.445
0.471
0.500
6_TA_emg
0.334
0.247
0.229
0.219
0.194
0.233
0.187
0.240
0.236
0.220
0.243
0.233
0.190
0.201
0.258
0.183
0.246
0.221
0.296
0.252
0.288
0.173
0.226
0.182
0.172
0.264
-0.248
0.206
0.207
0.198
0.220
0.246
-0.229
0.187
0.173
0.191
0.185
0.276
0.208
0.286
0.231
0.320
0.231
0.251
0.321
0.181
0.160
0.263
0.184
0.276
0.233
0.199
0.187
0.194
0.195
-0.228
0.177
0.181
0.202
0.321
6_VL_emg
0.451
0.415
0.454
0.416
0.533
0.420
0.402
0.503
0.504
0.725
0.454
-0.442
0.372
0.588
0.327
0.271
0.180
0.614
0.460
0.368
0.520
0.468
0.542
0.290
0.444
0.696
0.484
0.326
0.556
0.475
0.402
0.470
-0.627
0.610
0.565
0.500
0.504
0.600
0.725
0.451
0.542
0.384
0.576
0.528
0.546
0.463
0.496
0.412
0.501
0.557
0.444
0.499
0.437
0.414
0.433
-0.698
0.489
0.569
0.510
0.650
7_t_onset
0.228
0.262
0.266
0.389
0.315
0.301
0.296
0.275
0.434
0.311
0.313
0.391
0.277
0.272
0.275
0.311
0.333
0.338
0.433
0.389
0.324
0.413
0.356
-0.302
0.318
0.298
0.332
0.295
0.279
0.273
0.356
0.248
0.305
0.312
0.303
0.274
0.244
0.277
0.356
0.347
0.287
0.467
0.477
0.434
0.245
0.255
0.279
0.267
0.261
0.249
0.334
0.334
0.344
0.242
0.214
0.259
0.265
0.248
0.218
0.295
0.297
0.419
93
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
7_height
0.460
0.467
0.447
0.427
0.349
0.275
0.386
0.310
0.356
0.373
0.472
0.463
0.332
0.350
0.331
0.421
0.437
0.489
0.379
0.384
0.347
0.357
0.327
-0.459
0.480
0.507
0.323
0.377
0.415
0.533
0.494
0.551
0.351
0.311
0.366
0.469
0.473
0.478
0.324
0.377
0.314
0.444
0.436
0.439
0.523
0.528
0.555
0.426
0.445
0.456
0.347
0.346
0.355
0.467
0.495
0.478
0.471
0.488
0.497
0.427
0.508
0.425
7_TA_emg
0.198
0.236
0.241
0.258
0.201
0.200
0.217
0.160
0.342
0.247
0.190
0.239
0.207
0.235
0.206
0.202
0.212
0.231
0.386
0.353
0.255
0.315
0.150
0.173
0.172
0.193
-0.351
0.243
0.347
0.228
0.298
0.204
-0.203
0.220
0.171
0.175
0.183
0.281
0.324
0.189
0.339
0.357
0.244
0.206
0.201
0.242
0.199
0.230
0.229
0.226
0.267
0.306
0.212
0.220
0.195
0.235
0.213
0.183
0.191
0.252
0.447
7_VL_emg
0.383
0.406
0.471
0.329
0.266
0.667
0.222
0.517
0.336
0.467
0.460
-0.508
0.576
0.475
0.565
0.199
0.181
0.717
0.436
0.217
0.618
0.553
0.721
0.507
0.529
0.245
0.599
0.500
0.500
0.452
0.523
0.470
-0.808
0.535
0.677
0.450
0.239
0.671
0.669
0.398
0.506
0.600
0.569
0.489
0.625
0.456
0.470
0.491
0.725
0.296
0.473
0.526
0.482
0.375
0.433
0.448
0.696
0.568
0.533
0.673
0.694
8_t_onset
0.368
0.253
0.245
-0.307
0.276
0.300
0.295
0.314
0.352
0.329
0.389
0.268
0.254
0.299
0.324
0.350
0.364
0.361
0.365
0.285
0.220
0.348
0.332
0.330
0.325
0.285
0.413
0.286
0.303
0.256
0.276
0.260
0.317
0.284
0.315
0.284
0.262
0.273
0.367
0.358
0.305
0.420
0.467
0.411
0.249
0.259
0.225
0.310
0.273
0.272
0.442
0.383
0.338
0.232
0.256
0.248
0.247
0.251
0.201
0.333
0.408
0.362
8_height
0.454
0.436
0.434
-0.304
0.397
0.335
0.325
0.358
0.440
0.434
0.413
0.364
0.343
0.353
0.460
0.264
0.468
0.356
0.340
0.365
0.354
0.366
0.368
0.489
0.461
0.468
0.351
0.357
0.398
0.543
0.508
0.533
0.351
0.308
0.371
0.473
0.480
0.459
0.328
0.323
0.308
0.402
0.446
0.412
0.503
0.522
0.519
0.374
0.458
0.441
0.372
0.361
0.345
0.457
0.492
0.453
0.457
0.496
0.464
0.411
0.454
0.495
8_TA_emg
0.343
0.230
0.239
0.218
0.259
0.208
0.179
0.15
0.227
0.281
0.323
0.289
0.258
0.241
0.228
0.192
0.246
0.251
0.312
0.367
0.221
0.236
0.210
0.204
0.172
0.189
-0.423
0.281
0.367
0.203
0.246
0.202
-0.176
0.244
0.194
0.170
0.198
0.294
0.290
0.205
0.368
0.323
0.222
0.211
0.209
0.173
0.261
0.246
0.172
0.295
0.290
0.349
0.208
0.225
0.222
0.181
0.221
0.175
0.181
0.397
0.285
8_VL_emg
0.557
0.473
0.455
0.352
0.507
0.556
0.541
0.548
0.407
0.461
0.727
-0.498
0.540
0.524
0.611
0.271
0.257
0.640
0.500
0.416
0.485
0.551
0.410
0.241
0.407
0.213
0.720
0.524
0.500
0.469
0.380
0.504
-0.550
0.638
0.563
0.475
0.296
0.669
0.533
0.422
0.634
0.466
-0.488
0.596
0.396
0.569
0.447
0.538
0.500
0.500
0.564
0.420
0.546
0.457
0.450
0.400
0.525
0.547
0.651
0.651
94
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
9_t_onset
0.264
0.269
0.217
0.248
0.310
0.300
0.404
0.313
0.318
0.365
0.302
0.323
0.305
0.220
0.239
0.335
0.314
0.398
0.316
0.379
0.303
0.238
0.312
0.291
-0.293
0.273
0.313
0.306
0.277
0.243
0.269
0.233
0.307
0.278
0.356
0.284
0.256
0.258
0.373
0.337
0.403
0.320
0.494
0.456
0.247
0.303
0.231
0.258
0.273
0.227
0.335
0.378
0.351
0.232
0.234
0.232
0.339
0.311
0.232
0.346
0.408
0.384
9_height
0.468
0.437
0.473
0.420
0.398
0.373
0.333
0.453
0.371
0.397
0.296
0.422
0.350
0.355
0.297
0.456
0.349
0.498
0.355
0.347
0.372
0.336
0.269
0.387
-0.477
0.476
0.325
0.345
0.433
0.534
0.508
0.561
0.347
0.321
0.371
0.480
0.451
0.474
0.341
0.294
0.336
0.410
0.450
0.454
0.523
0.502
0.534
0.419
0.458
0.423
0.345
0.362
0.344
0.445
0.479
0.464
0.437
0.476
0.485
0.443
0.454
0.515
9_TA_emg
0.229
0.246
0.204
0.183
0.203
0.306
0.274
0.145
0.229
0.323
0.240
0.267
0.255
0.220
0.191
0.197
0.186
0.239
0.258
0.350
0.224
0.237
0.247
0.110
0.204
0.175
0.254
0.208
0.210
0.355
0.181
0.210
0.177
-0.184
0.219
0.182
0.188
0.179
0.328
0.256
0.374
0.236
0.284
0.269
0.192
0.271
0.150
0.182
0.205
0.214
0.312
0.274
0.316
0.209
0.195
0.208
0.200
0.221
0.176
0.185
0.196
0.204
9_VL_emg
0.469
0.406
0.392
0.358
0.256
0.615
0.334
0.653
0.494
0.509
0.636
-0.312
0.398
0.456
0.395
0.180
0.279
0.556
0.354
0.186
0.408
0.501
0.370
0.228
0.335
0.615
0.431
0.503
0.499
0.446
0.383
0.445
-0.308
0.342
0.590
0.334
0.196
0.629
0.550
0.471
0.620
0.389
0.378
0.506
0.673
0.474
0.564
0.538
0.547
0.413
0.491
0.521
0.443
0.435
0.389
0.618
0.601
0.523
0.590
0.556
0.580
10_t_onset
0.268
0.240
0.249
0.314
0.269
0.291
0.277
0.283
0.344
0.325
0.332
0.323
0.310
0.254
0.236
0.314
0.413
0.399
0.360
0.333
0.316
0.412
0.250
0.274
0.304
0.287
0.257
0.317
0.308
0.240
0.266
0.283
0.246
0.305
0.287
0.310
0.275
0.282
0.298
-0.339
0.323
0.390
0.450
0.454
0.252
0.239
0.210
0.284
0.329
0.244
0.353
0.368
0.293
0.244
0.189
0.242
-0.323
0.217
0.325
0.249
0.347
10_height
0.466
0.457
0.447
0.432
0.406
0.401
0.376
0.488
0.354
0.410
0.577
0.432
0.352
0.341
0.369
0.487
0.324
0.483
0.378
0.369
0.397
0.377
0.259
0.358
0.494
0.472
0.482
0.333
0.406
0.420
0.561
0.550
0.539
0.378
0.340
0.342
0.500
0.473
0.500
0.303
0.345
0.464
0.451
0.432
0.514
0.523
0.542
0.379
0.451
0.464
0.335
0.322
0.393
0.448
0.491
0.470
-0.492
0.470
0.404
0.488
0.477
10_TA_emg
0.249
0.224
0.239
0.200
0.278
0.183
0.242
0.130
0.249
0.226
0.213
0.284
0.264
0.207
0.225
0.209
0.282
0.250
0.349
0.305
0.236
0.262
0.203
0.208
0.222
0.186
0.197
0.318
0.273
0.223
0.218
0.239
0.201
-0.196
0.194
0.183
0.181
0.200
0.328
0.275
0.234
0.242
0.293
0.258
0.231
0.192
0.174
0.155
0.221
0.232
0.239
0.310
0.254
0.264
0.205
0.208
0.175
0.207
0.176
0.187
0.213
--
95
Subject_ID
1
1
1
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
9
9
9
10
10
10
11
11
11
12
12
12
13
13
13
14
14
14
15
15
15
16
16
16
17
17
17
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
10_VL_emg
0.473
0.363
0.460
0.686
0.548
0.377
0.455
0.616
0.531
0.443
0.620
-0.464
0.604
0.552
0.564
0.243
0.220
0.393
0.476
0.201
0.600
0.436
0.351
0.232
0.356
0.479
0.560
0.529
0.440
0.459
0.481
0.484
-0.326
0.516
0.587
0.606
0.339
0.746
0.556
0.372
0.468
0.335
0.469
0.481
0.598
0.621
0.499
0.485
0.569
0.342
0.477
0.452
0.484
0.394
0.429
0.525
0.550
0.479
0.500
0.499
0.680
96
97
98
99
100
101
102
103
104
105