2009
ORIGINAL ARTICLE
Endurance Times of Trunk Muscles in Male Intercollegiate
Rowers in Hong Kong
Romy H. Chan, DPT, MS, OCS
ABSTRACT. Chan RH. Endurance times of trunk muscles
in male intercollegiate rowers in Hong Kong. Arch Phys Med
Rehabil 2005;86:2009-12.
Objectives: To establish isometric endurance times of trunk
muscles and their ratios in a group of healthy intercollegiate
rowers in Hong Kong for clinical assessment reference, and to
compare the trunk endurance profile of the rowers in the
current study with that of nonrowers in another study.
Design: Isometric endurance times were measured in 4
different positions in a cross-sectional manner. A subset of 5
subjects was tested 3 times 2 days and 1 week apart to evaluate
reliability.
Setting: Sports medicine department of a national sports
institute.
Participants: Thirty-two subjects selected from a group of
42 male intercollegiate rowers reported to have more than 6
months of rowing experience and without history of low back
pain.
Interventions: Not applicable.
Main Outcome Measures: Trunk muscle endurance times
in seconds and ratios of endurance times normalized to that of
the extensor muscle.
Results: The trunk flexor (mean ⫾ standard deviation,
176.56⫾88.58s) had the best endurance times among all the
trunk muscles tested (extensor mean, 114.28⫾34.62s; left lateral flexor mean, 94.53⫾32.97s; right lateral flexor mean,
98.13⫾41.38s). No significant difference was found between
the left and right lateral flexors (P⬍.05). The lateral flexor and
the flexor endurance times were 85% and 154% of that of the
extensor, respectively. The testing protocol in this group of
rowers showed good to excellent reliability (intraclass correlation coefficient range, .76 –.93).
Conclusions: Intercollegiate rowers in Hong Kong have
better endurance in their trunk flexor than the extensor; the
lateral flexors are of similar endurance capacity. These findings
are different from the endurance profile reported for nonrowers
in a previous study. Such differences should be considered
when evaluating trunk endurance times in rowers for rehabilitation and training.
Key Words: Exercise; Muscles; Physical endurance; Rehabilitation.
© 2005 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and
Rehabilitation
From the Sports Medicine Department, Hong Kong Sports Institute, Hong Kong.
Supported by the Institute of Human Performance, University of Hong Kong.
No commercial party having a direct financial interest in the results of the research
supporting this article has or will confer a benefit upon the author(s) or upon any
organization with which the author(s) is/are associated.
Reprint requests to Romy H. Chan, DPT, MS, OCS, 25 Yuen Wo Rd, Shatin, Hong
Kong, e-mail: romyc@hksi.org.hk.
0003-9993/05/8610-9622$30.00/0
doi:10.1016/j.apmr.2005.04.007
OW BACK PAIN IS NOTORIOUS among rowers and has
L
been reported quite extensively in the literature. In a retrospective analysis of injury data over a 10-year period at the
Australian Institute of Sports, Hickey et al1 found that low back
pain (LBP) comprised 25% and 15.2% of the total injuries among
elite male rowers and female rowers, respectively. Teitz et al2
conducted a large-scale survey to determine the rate of and the
potential etiologic factors for LBP that developed during intercollegiate rowing. These researchers found that the prevalence
of LBP increased over the 20-year period covered by the study,
affecting 32% (526/1632) of the intercollegiate rowers. Based
on their analysis, increased training time on an ergometer and
an overall increased intensity of training are related to the
increase in incidence of LBP. Apart from modification of the
training program as a prevention measure, the authors also
suggested preseason strengthening for the back, hamstrings,
and scapular stabilizing muscles, as well as using good rowing
technique to ensure appropriate force transfer from the lower to
upper limbs to minimize the chance of back injury.
In recent years, there has been growing interest in the use of
stabilization exercises and core training programs in rehabilitation and in athletic training.3 It is believed that core strengthening is beneficial as a preventive regimen,4 as a form of
rehabilitation,5 and as a performance-enhancing program for
various lumbar spine and musculoskeletal injuries.3 According
to McGill,6 the safest and mechanically justifiable approach
through exercises to enhance lumbar stability should emphasize endurance, not strength; ensure a neutral spine posture
when under load (or more specifically, avoid end-range positions); and encourage abdominal cocontraction and bracing in
a functional way.
Despite the many successes from researchers utilizing sophisticated instruments, such as electromyography,7,8 the open
magnetic resonance imaging scanner,9 and the isokinetic dynamometer10-13 in capturing the muscle performance of the
trunk muscles in the laboratory environment, clinicians are still
struggling to find simple clinical measures with meaningful
constructs. McGill et al14 proposed 3 specific clinical timed
tests for the trunk flexor, extensor, and the lateral flexor based
on their years of research efforts to identify the specific lumbar
stabilizers and means to recruit them. These investigators also
recruited subjects from a university community for endurancetimes testing using these measures in an effort to establish
clinical targets for testing and training. They found these tests
to be of excellent reliability (reliability coefficient, ⬎.97) for
the repeated tests on 5 consecutive days and again 8 weeks later
(reliability coefficient, ⬎.93). They also suggested computing
relative ratios of these muscles to identify endurance deficits
within specific patient groups. The timed endurance testing is
definitely of interest and useful for clinicians who are involved
in a core training program to evaluate muscle function and as
a guideline for clinical decision making for progression of
exercise and functional training.
In sports medicine, sports-specific conditioning plays an
important role in injury prevention. Because there may be
specific muscle adaptation in the trunk muscles of rowers, their
trunk endurance times may differ from those of the general
Arch Phys Med Rehabil Vol 86, October 2005
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TRUNK MUSCLE ENDURANCE COLLEGE ROWERS, Chan
Table 1: Mean Values of Training Characteristics
Training Characteristics
Mean ⫾ SD
Years rowing
Days training per week
Water rowing per week (h)
Ergometer training per week (h)
Weight training per week (h)
1.28⫾0.82
5.31⫾0.86
16.31⫾3.8
1.17⫾0.27
2.13⫾1.68
Abbreviation: SD, standard deviation.
population. It has been the observation of the author that many
rowers exhibit a different trunk endurance profile than that of
other athletes. Currently, there is no study reported in the
literature on trunk endurance times for rowers. The purpose of
this study was to measure the isometric endurance times of a
group of intercollegiate rowers using the clinical tests proposed
by McGill et al14 and to compute the relative endurance ratios.
Such a database is helpful in establishing clinical targets for
rehabilitation and training specific to competitive rowers,
which may have implications in the prevention of LBP among
rowers. As a comparative study, the endurance times of the
rowers in the current study would also be compared with those
in McGill’s study examining nonrowers.
METHODS
Participants
Forty-two male intercollegiate sweep rowers from 5 local
universities (age, 20.4⫾1.16y; weight, 68.9⫾7.11kg; height,
174.98⫾4.7cm) were invited to participate in this study. Before
entry into the study, all subjects were given a standardized
health-status questionnaire for general screening purposes.
Subjects with self-reported rowing experience of less than 6
months and those with a history of LBP were excluded from
the study. Five subjects had rowing experience of less than 6
months and 5 had a history of LBP. These 10 subjects (age,
20.3⫾0.94y; weight, 67.18⫾6.94kg; height, 173.65⫾4.37cm)
were excluded from the study. The remaining 32 subjects (age,
20.52⫾1.23y; weight, 69.47⫾7.18kg; height, 175.42⫾4.79cm)
entered into the study for endurance time measurement. The
training characteristics of the participating subjects are shown
in table 1. The purpose of the study was explained to the
subjects and they signed a consent form approved by the ethics
committee of the University of Hong Kong.
Fig 1. The extensor endurance test.
Arch Phys Med Rehabil Vol 86, October 2005
Fig 2. The flexor endurance test.
Procedure
The timed endurance tests included the extensor endurance test,
flexor endurance test, and the side bridge test, and they were
carried out as described by McGill14 The sequence of testing for
the current study was as follows: flexor endurance test, extensor
endurance test, and bridge test (right side, then left side). Subjects
were reminded to maintain the testing position in each of the tests
as long as possible. Subjects were not given any clues about their
scores until the conclusion of the tests.
The Extensor Endurance Test
Subjects lay prone with the lower body fixed to the test bed
at the ankles, knees, and hips and the upper body extended in
a cantilevered fashion over the edge of the test bed (fig 1). The
test bed surface was approximately 60cm above the floor.
Subjects rested their upper bodies on a stool with wheels before
the exertion. The stool was removed at the commencement of
test. At the beginning of the exertion, the upper limbs were held
across the chest with the hands resting on the opposite shoulders, and the upper body was lifted until the upper torso was
horizontal to the floor. Subjects were instructed to maintain the
horizontal position as long as possible. The endurance time was
manually recorded in seconds with a stopwatch from the point
at which the subject assumed the horizontal position until the
Fig 3. The side bridge test.
2011
TRUNK MUSCLE ENDURANCE COLLEGE ROWERS, Chan
Table 3: P Values for Differences Between Muscle Groups
Left Side Bridge
Right Side Bridge
P*
94.53⫾32.97
98.13⫾41.38
.533
Extensor
Flexor
114.28⫾34.62
176.56⫾88.58
.001
NOTE. Values are mean ⫾ SD.
*Paired t tests.
Fig 4. Mean endurance times for each of the tests (nⴝ32).
upper body lost control of the test position and came in contact
with the floor.
The Flexor Endurance Test
This is modified from the McGill test in that subjects were
required to sit on a motorized treatment table and place the
upper body against the adjustable back support positioned at an
angle 60° from the test bed (fig 2). The bottoms of the subjects
were positioned about 10cm from the back support. Both the
knees and hips were flexed to 90°. The arms were folded across
the chest with the hands placed on the opposite shoulder and
toes were placed under toe straps. Subjects were instructed to
lift their upper body away from the support and kept it parallel
with the support (as instructed by the examiner). Subjects were
instructed to maintain the body position as long as possible.
The test ended when the upper body fell below the 60° angles
and came in contact with the back support.
The Side Bridge Test
Subjects were instructed to lay on their sides with legs
extended (fig 3). The top foot was placed in front of the lower
foot for support. They were also instructed to support themselves lifting their hips off the surface to maintain a straight
line over their full body length, and support themselves on the
elbow and their feet. The nonsupporting arm was held across
the chest with the hand placed on the opposite shoulder. The
test ended when the hip touched the supporting surface.
Statistical Analysis
Statistical analysis included calculation of the mean and
standard deviation (SD) of each of the timed endurance measurements. Computation of the ratios of endurance times normalized to the extensor muscles were also made in order to
allow comparison across studies. Paired t tests (P⬍.05) were
performed to assess differences of the timed scores between the
left and right side bridge tests, and between the flexor and
extensor tests. A subset of 5 subjects (age, 21.8⫾1.1y; weight,
68.92⫾8.9kg; height, 171.6⫾6.91cm) was tested 3 times 2
days and 1 week apart to evaluate the reliability of the testing
procedures among this group of rowers. The intraclass correlation coefficient (ICC3,1) was computed to assess reliability.
RESULTS
The mean endurance times with their corresponding SDs for
each of the exercises are shown in figure 4. The ratios of
endurance times were normalized to the extensor holding time
to facilitate data comparison with the database established by
McGill et al.14 Data of endurance times and ratios of the
current study are presented along with those from McGill’s in
table 2 for comparison. The mean endurance time for the flexor
is highest among all the muscle endurance tests examined in
this study. A larger variance is also observed in the flexor
endurance time in the current study (as well as in McGill14).
There was no significant between-side difference in the side
bridge endurance time (table 3). The flexor endurance was
significantly better than the extensor endurance (P⫽.001).
ICCs of each of the timed tests in the current study showed
good to excellent reliability (table 4), ranging from .76 to .93.
DISCUSSION
The results of this study showed that young intercollegiate
rowers have better endurance in their trunk flexor compared
with the other trunk muscles. This result is in contrast to the
normal database established by McGill14 (see table 2), which
showed that the extensor holding time was the highest among
the 4 tests. Although the rowers in this study had similar side
bridge holding time compared with the male subjects in the
McGill study, the subjects in the current study showed poorer
extensor endurance. McGill14 recruited subjects in a university
population. McGill did not report specific information regarding the activity level or specific sports preference in this study.
The mean age of the McGill study population was 23⫾2.9
years, which is similar to that in the current study. The majority
(87.5%) of subjects in the current study participated in only
rowing and a rowing-specific training program. They spent an
average of 5 days a week on training, of which 16.3 hours were
spent on water rowing (see table 1). Although the specific
training program was not investigated in the current study, it
would be of interest in future study to correlate the timed
endurance of the trunk muscles with the specific components of
athletes’ training programs. It is possible that this group of
rowers might have a heavier emphasis on training their abdom-
Table 2: Comparison of the Results (Means and Ratios of Endurance Times) of the Current Study With Those of McGill et al14
Exercises
Mean ⫾ SD (s)
Ratio
Mean ⫾ SD14 (s)
Ratio14
Extensor
Flexor
Left side bridge
Right side bridge
114.28⫾34.62
176.56⫾88.58
94.53⫾32.97
98.13⫾41.38
1.00
1.54
0.83
0.86
146⫾51
144⫾76
94⫾34
97⫾35
1.00
0.99
0.64
0.66
Arch Phys Med Rehabil Vol 86, October 2005
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TRUNK MUSCLE ENDURANCE COLLEGE ROWERS, Chan
Table 4: ICC Values of Each of the Timed Tests
Extensor
Flexor
Left Side Bridge
Right Side Bridge
.88
.93
.89
.76
inal musculatures in their program. As a result, they tend to
have a better flexor endurance than that of their extensor.
I adopted the 60° flexor test proposed by McGill et al14 in
their study. Among the 32 subjects, 10 (31%) subjects achieved
an endurance time of more than 200 seconds. The 60° flexor
test was reported to have a large variance and mainly involved
the abdominal oblique muscle and the rectus abdominis muscle
with the fulcrum at L4-5.15 Chen et al15 recommended using an
alternative 45° flexor test, which has less variance with the
same location of the fulcrum at L4-5. By requiring the subjects
to recline another 15°, it makes the test more demanding for the
rectus abdominis and the oblique muscles by varying the length
of the moment arm and muscle length-tension relationship.
Another explanation for the higher abdominal endurance time
is related to how this test is conducted. Based on our established
procedure, the test ended when the subject fell below 60°. It was
observed that some subjects could maintain the 60° angle by
further flexing their lower lumbar spine, primarily using their hip
flexors to maintain the test position. Because no attempt was made
to monitor the subjects’ trunk movement in the transverse plane,
some subjects might have slightly rotated their spine in the transverse plane to different sides at different time (ie, time-sharing
between the abdominal muscles) without falling. These types of
rotatory compensatory movements may explain the higher flexor
endurance time in some subjects and the larger variance in this
test. However, this cannot be verified in the current study. Future
studies may consider using electromyography for monitoring
these types of compensatory movements during the testing procedure. It is more difficult to compensate for the horizontal hanging position in the extensor test because most of the posterior trunk
muscles are arranged parallel with the spine. It therefore tends to
have a less variant test results in this study as well as in other
studies.14,15
All of the subjects in the current study were right-arm
dominant. However, I did not find any significant difference in
the side-bridge holding time between the left and right side.
This may imply that arm dominance has minimal effect on the
endurance times of the trunk side flexor and should not be a
factor in considering the side-to-side difference in endurance
time measurements.
Based on the result of the current study, intercollegiate
rowers in Hong Kong demonstrated a higher flexor to extensor
ratio (1.54) than healthy nonrower male subjects in a university
community in North America14 (.99) of similar age (see table
2). This result may be a consequence of relatively better flexor
endurance and poorer extensor endurance of the rowers in
Hong Kong. It is not clear whether this is a rowing-specific
flexor-to-extensor endurance ratio or a result of flexor-biased
training in this group of rowers. Further study looking into the
specific components of the athletes’ training program and collecting endurance time measures on other groups of rowers
may be helpful in understanding their relationships.
It may also be of interest to compare rowers with a history
of LBP with healthy rowers to verify if they would have a
different trunk-endurance profile. A longitudinal study looking
into future development of LBP in this group of subjects may
also be helpful in understanding the implications of the specific
endurance profile found in the current study.
Lee et al12 studied the relation of trunk muscle isokinetic
strength and future development of LBP. They found that
Arch Phys Med Rehabil Vol 86, October 2005
subjects with an imbalance in trunk muscle strength (lower
extensor strength than flexor muscle strength) might be more
prone to develop LBP in the future. However, no such relation
with respect to trunk muscle endurance measures in the athletic
population has been established. Future studies may also look
at this specific parameter (ie, extensors/flexors endurance ratio)
in relation to the future incidence of LBP in different athletic
groups. This can be of importance in developing better backconditioning programs for athletes who are prone to LBP.
CONCLUSIONS
By means of measuring isometric endurance times, this
study found that intercollegiate rowers in Hong Kong achieved
better endurance times with the flexor muscle than with the
extensor muscle. The left and right trunk side flexors were of
similar endurance capacity. The endurance profile among rowers is apparently different than that of nonrowers, as reported in
the previous study of McGill,14 and should be considered when
evaluating the trunk endurance times of rowers for rehabilitation and training.
Acknowledgment: I thank Mr. Michael Tse, Strength and Conditioning Department, Hong Kong Sports Institute, for his assistance in
recruiting rowers for this study.
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