EFFECTS OF MECHANICAL VIBRATION APPLIED IN
THE OPPOSITE DIRECTION OF MUSCLE SHORTENING
ON MAXIMAL ISOMETRIC STRENGTH
HOSANNA R. SILVA, BRUNO P. COUTO,
AND
LESZEK A. SZMUCHROWSKI
Department of Sports, School of Physical Education, Physiotherapy and Occupational Therapy, Laboratory of Load Evaluation,
Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil
ABSTRACT
INTRODUCTION
Silva, HR, Couto, BP, and Szmuchrowski, LA. Effects of
mechanical vibration applied in the opposite direction of muscle
shortening on maximal isometric strength. J Strength Cond Res
22: 1031–1036, 2008—Most studies about human responses
to mechanical vibrations involve whole-body vibration and vibration applied perpendicularly to the tendon or muscle. The aim
of the present study was to verify the effects of mechanical
vibration applied in the opposite direction of muscle shortening
on maximal isometric strength of the flexor muscles of the elbow
due to neural factors. Conventional isometric training with maximal isometric contractions (MVCs) and isometric training with
vibrations were compared. Nineteen untrained males, ages
24 6 3.28 years, were divided into 2 training groups. Group 1
performed conventional isometric training and group 2 isometric training with mechanical vibrations (frequency of 8 Hz
and amplitude of 6 mm). Both groups executed 12 MVCs with
a duration of 6 seconds and 2-minute intervals between the
repetitions. The subjects trained 3 times per week for 4 weeks.
The strength of the group subjected to vibrations increased
significantly by 26 6 11% (p , 0.05), whereas the strength of
the group with conventional isometric training increased only
10 6 5% (p , 0.05). These data suggest that training with
vibrations applied in the opposite direction of muscle
shortening enhances the mechanism of involuntary control of
muscle activity and may improve strength in untrained males.
Since these findings were in untrained males, further studies
with athletes are necessary in order to generalize the results to
athletes’ training, although it seems that it would be possible.
S
KEY WORDS maximal voluntary contraction, muscle spindles,
involuntary control
Address correspondence to Hosanna Silva, hosanna@fcs.fumec.br.
22(4)/1031–1036
Journal of Strength and Conditioning Research
Ó 2008 National Strength and Conditioning Association
trength training causes adaptive responses of
neuromuscular functions (19) due to the capacity
of the skeletal muscle to modify in response to
chronic exercises with load (13,23). Among the
adaptive responses of the muscle, the neural responses play
an important role in the strength gains resulting from this
type of training (19,20). These neural factors are usually
thought to be responsible for the initial enhance of strength,
especially during the first 4 weeks of training (9,14). Training
may cause a decrease in impulses that inhibit the neuromuscular system, permitting improvement in muscle strength
(10). Exercise with vibration exposure is a method of neuromuscular training that has been used both with athletes
and as preventive treatment for such diseases as osteoporosis
and Parkinson’s disease (5,15,17). Most studies about human
responses to mechanical vibrations discuss whole-body
vibrations and vibrations applied perpendicularly to the
tendon or muscle. The use of vibration is based on the fact
that exposure of the skeletal muscular system to mechanical
vibrations may create a muscle contraction reflex (8) called
a tonical vibration reflex (TVR). A TVR involves the stimulation of the muscle spindle, the activation of neural signs and
muscle fibers through large alpha motor neurons (15). It has
been demonstrated that vibration applied directly on the
muscle or tendon at a frequency of 10–200 Hz caused a
response in the form of a TVR (8,12). The frequency that
should be applied in order to obtain a TVR is still controversial, as it has been previously suggested that a TVR may
be obtained with frequencies of 1 to 30 Hz (21).
The Torvinen et al. (24) study showed that whole-body
vibration exposure for 4 minutes (at frequencies of 25–40 Hz
and 2 mm of amplitude) did not significantly change the
strength to extend the knee in the case of young, healthy individuals. Conversely, Bosco et al. (2), while investigating the
effects of whole-body vibration on the mechanical behavior
of knee extension muscles for volleyball players (vibration
frequency of 26 Hz for 10 minutes and with an amplitude of 10 mm) revealed that limbs exposed to vibrations
showed a significant increase in average strength, suggesting
that vibration exposure may have caused an increase in
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Effects of Mechanical Vibration on Maximal Isometric Strength
neuromuscular activities. Runge et al. (18) revealed that
whole-body vibration exposure may produce an increase in
muscular performance for elderly people with a 2-month
training program three times per week using a frequency of
27 Hz. In a different study, Bosco et al. (3) evaluated the
influence of vibration on the mechanical properties of elbow
flexors and showed a significant increase in the capacity of
the arm exposed to vibrations (frequency of 30 Hz, 6 mm of
amplitude in 5 isometric activities of 60 seconds each, and an
interval of 60 seconds between each repetition), suggesting
that application of vibrations could possibly stimulate the
neuromuscular system in a superior way compared to other
treatments used to increase neuromuscular properties.
Bongiovanni and Hagbarth (1) found that muscle vibrations
stimulated an increase in strength, electromyographic signs,
and stimulation index of the motor units during isometric
maximal contractions, given that Griffin et al. (7) pointed out
that short periods of muscle stretching, during muscle
activity may prevent muscle spindle activity from decreasing
and consequently prevent a decrease in the nervous stimulation index. That was also discussed by Granit and
Henatsch (6) and Brown et al. (4). In addition, it has been
suggested that vibration exposure activates alpha motor
neurons through efferent motor neurons that cause an
involuntary production of strength (16). Using a different
method in this study, we propose a new type of vibration
exposure. Considering the fact that in eccentric activities,
reflex mechanisms influence the production of strength
(11,22), we hypothesized that vibration exposure in the
opposite direction of muscle shortening may produce short
eccentric effects that could add up to maximal voluntary
contraction (MVC), increasing muscle strength by improvement of neural adaptations.
The aim of the present study was to verify the effects of
mechanical vibrations applied in the opposite direction of
muscle shortening on maximal isometric strength in flexor
muscles of the elbow due to neural factors. The hypothesis is
that the application of vibrations in that direction, at an MVC,
could add an eccentric effect that optimizes neural adaptations and increases the maximal strength compared with
conventional isometric training.
METHODS
Experimental Approach to the Problem
Two different methods of strength training were compared:
conventional isometric training and training based on MVC
with vibration exposure. After a maximal isometric strength
test, untrained men (divided in 2 groups) performed 1 session
of 12 MVCs of the elbow flexors with 2 minutes for recovery
3 times per week for 4 weeks. One group performed
conventional isometric strength training and another group
isometric strength training with the addition of mechanical
vibrations applied in the opposite direction of muscle
shortening. This kind of vibration causes short eccentric
effects that added to MVC stimulus. It is important to note
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that muscular action with the addition of vibration is not
a true isometric muscle action; it is just the difference in
stimulus being tested. At the end of training period, another
maximal isometric strength test was performed. This approach allowed careful monitoring and comparison of MVCs
of each subject before and after strength training. As the first
study using this kind of stimulus to test neural effects, this
study was carried out with untrained subjects because of the
probable major neural adaptations of this group. As in the
study of Bosco et al. (3), the elbow flexor muscles were
chosen due to simplicity of movement and ease of to
applying the stimulus. Moreover, although it is a small
muscle group, it is important in sports such as rowing, lifting,
gymnastics, and others.
Subjects
Nineteen male students, mean age 24 6 3.28 years, who
were untrained and not involved in strength training for
upper limbs, volunteered to participate in this study. We
considered untrained as those volunteers who have never
done strength training or have not done strength training for
the past year. We accepted volunteers without any type of
bone, articular, or muscle damage. Subjects were informed
about the nature of this study and signed an informed consent
form according to the International Review Board for use of
human subjects at research. All procedures were approved by
the Ethical Committee for Research at the Federal University
of Minas Gerais. None of the subjects smoked or was taking
any medication or supplement while participating in the
study.
Procedures
Subjects were divided into 2 groups: group 1 (conventional
isometric training) and group 2 (training with vibrations
addition). After the maximal isometric strength test (pre-test),
the volunteers were classified in decreasing order of the
MVC obtained by elbow flexors (from the 1st to the 19th).
Volunteers assigned an even number composed group 1
(n = 9), and the individuals assigned an odd number formed
the group 2 (n = 10).
The study was divided into 4 stages: familiarization, pre-test,
training, post-test. At the familiarization stage, each volunteer
executed 3 MVCs (3 repetitions with each upper limb and
an interval of 5 minutes between each repetition). On another
day, the second stage of the experiment consisted of evaluating
the initial maximal isometric strength of the elbow flexors. To
that end, each volunteer executed 3 isometric maximal actions
with each superior member, with a minimum duration of
6 seconds after having reached maximal strength. There was
a 5-minute interval for recovery between each repetition. The
best result for maximal strength of each limb was chosen
for comparison, and the arm that demonstrated the smallest
MVC was chosen for strength training. At the training
stage, all study participants from both groups performed one
session daily of 12 MVCs with elbow flexors of a duration of
6 seconds, starting from moment when he reached the peak
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strength. There was a 2-minute recovery interval between each
repetition. Each volunteer trained 3 times per week with 48
hours for recovery between the training sessions. The training
lasted for 4 weeks. Group 1 performed conventional isometric
strength training and group 2 performed strength training with
vibrations applied in the opposite direction of muscle
shortening (frequency of 8 Hz and amplitude of 6 mm). A
single standard position was determined for the execution of
the pre-test, strength training, and post-test. The volunteer was
seated on the bench with the armpit area leaning on the support
(the glenohumeral joint angle of 45°). The arm that was
executing the muscle action with elbow flexors was placed
against the support with the elbow bent at a 90° angle and the
forearm tensed. The other arm maintained the elbow extended
and in radioulnar pronation, also placed on the support. Figure
1 illustrates the standard position and the equipment used
during the study.
The post-test consisted of a final evaluation of maximal
isometric strength of elbow flexors. All pre-tests were repeated.
The pre-test was done 72 hours after the last training session.
The only feedback given to the volunteers during all stages of
the experiment was the visualization of strength curve as a
function of time simultaneously with its realization. A computer screen was placed in front of the subject.
Data Acquisition
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Figure 1. Standard position for execution of muscle actions. Adjustments
that could be made in the equipment designed for this study allowed each
individual to adopt a comfortable position.
enabled adjustment of the seat height and the position of
upper limb support. The electromagnetic system is formed
by a reel (with a steel core) fed with a continuous current.
One of extremities of the steel cable is fixed to the core of the
reel and other one attached to a handle that served as a grip
for the hand. After release of the electrical current, a magnetic
field is formed in the spirals of the reel. The field attracts the
core of the reel and consequently the steel cable, applying the
torque on the elbow (maximal strength on the reel 950 N).
When the current is interrupted, after dislocating the steel
cable (6 mm), the torque produced by the reel ceases. Control
of the number of tractions to be executed by the reel is
done through an interface with use of computer software
(Time Trainer 1.0). It is possible to choose the duration of
Values of strength were obtained using a force cell by JBA (Zb
Staniak, Poland) connected to an amplifier of signals (WTM
005–2T/2P, Jaroslaw Doliriski Systemy Mikroprocesorowe,
Poland). The amplifier itself was connected to a computer
with a MAX (version 5.1, JBA) interface that enables analysis
of the strength curve as a function of time (frequency of data
input: 1000 Hz).
The measurements obtained in this
study were reliable owing to the use of
dumbbells of different weights, weighed
on a digital scale, with the precision of
0.001 N as a parameter. We checked the
correlation between values obtained
in 2 measurements performed at the
beginning of the experiment and in
2 measurements performed before the
post-test in order to verify and avoid the
possibility of changes in the precision of
the instrument during the 4 consecutive
weeks of use during the training
sessions.
Special equipment was designed that
enabled both isometric conventional
training and training with vibrations
applied in the opposite direction of
muscle shortening. The equipment consisted of an electromagnetic system
Figure 2. Scheme of the experiment.
controlled by the computer adapted to
a Scott bench. The bench’s design
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Effects of Mechanical Vibration on Maximal Isometric Strength
TABLE 1. Results of pre-test and post-test of maximal
isometric strength (MVC) for group trained in the
conventional way.
TABLE 2. Results of pre-test and post-test of maximal
isometric strength (MVC) for isometric training with
vibrations added group.
MVC (N)
MVC (N)
Group 1
Pre-test
Post-test
Group 2
Pre-test
Post-test
1
2
3
4
5
6
7
8
9
x
SD
174.44
239.12
195.02
229.32
215.60
162.68
156.80
149.94
203.84
191.86
32.59
185.22
269.50
224.42
261.66
223.44
170.52
181.30
156.80
234.22
211.90*
40.28
1
2
3
4
5
6
7
8
9
10
x
SD
170.52
111.72
234.22
232.26
174.44
158.76
207.76
174.44
180.32
166.60
181.10
36.32
231.28
164.64
276.36
263.62
221.48
212.66
270.48
210.70
208.74
189.14
224.91*
36.21
MVC = maximal voluntary contraction; x = arithmetical
average.
* Significant difference between pre-test and post-test
(p , 0.05).
pulses and the number of electrical impulses emitted per
minute, which is characterized by the frequency applied at the
vibration. The strength cell was connected approximately in
the middle of the steel cable (lengthwise). In that way, when
the steel cable is moved by traction (in isometric training or
in training with vibrations addition), this information is collected by the force cell and forwarded to a different computer.
The amplitude of the movement of the steel core of the
reel can be limited by an adjustable screw with a precision of
1 mm (Figure 2 shows the scheme of experiment). Lateral
oscillations were reduced with rubber bands fixed at the
cable (Figure 1).
In order to check the efficiency of the training, we calculated
the strength increase index through the ratio between the
maximal strength obtained during the post-test and the maximal
strength obtained during the pre-test. Strength increase index
is a number that indicates the proportion of increase (or
decrease) of MVC after training. If the value of this index is ,1,
this means that the maximal strength decreased.
MVC = maximal voluntary contraction; x = arithmetical
average.
* Significant difference between pre-test and post-test
(p , 0.05).
RESULTS
A significant difference (p , 0.05) was observed between the
values of MVCs measured at the pre-test stage and at the
post-test stage in both groups, as shown in Tables 1 and 2.
Reliability of measures obtained at pre-test and at after-test
was checked and Average Measure Intraclass Correlation
Statistical Analyses
The normal distribution of the data was verified using
the Kolmogorov-Smirnov test. The comparison between the
averages was done using a Student t-test. In addition to inferential statistics, descriptive statistics of the data were calculated. The reliability of measures was verified by reliability
analysis (two-way mixed, 95% confidence interval). The level
of significance was 5% (p # 0.05). Analyses were conducted
using the Statistical Package for Social Sciences (version
10.0, SPSS, Inc., Chicago, IL). All values are presented as
mean 6 SD.
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Figure 3. Average increase in the index of maximal voluntary contraction
(MVC) (ordinate) obtained in groups 1 and 2. The asterisk indicates
a significant statistical difference (p , 0.05) when compared with the
increase in MVCs in groups 1 and 2.
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was R = 0.98 (p , 0.05) for pre-test measures and R = 0.97
for after-test measures (p , 0.05).
The average of strength increase index was calculated:
1.10 6 0.05 for group 1 and 1.26 6 0.11 for group 2. The
statistical comparison between the groups showed a significant difference between the increase in the MVC obtained in
group 1 and that obtained in group 2 (Figure 3).
DISCUSSION
The principal issue of present study is related to the possibility
of optimizing maximal isometric strength increase through
the application of mechanical vibrations. Various research
obtained positive results regarding the use of TVRs (2,3,15,
17,21) to increase muscle strength. In contrast to the type of
vibration used in these studies, this study applied vibrations
simultaneously with the isometric action, producing consecutive movements opposite to the muscular action and
checks the possibilities of training through this stimulation in
developing maximal isometric strength. This study showed
that the training provoked an increase in the maximal
isometric strength in both groups. As training was conducted
for 4 weeks, this increase in strength may be explained by an
improvement of intramuscular and intermuscular coordination, which would be consistent with the findings of
Häkkinen and Komi (9) and Moritani and De Vries (14),
who observed that neural adaptation to strength training
occurred during first 4 weeks of training. Despite the fact that
we observed a significant increase in strength, in absolute
terms, in both groups, group 2 (training plus mechanical
vibrations) reached a significantly greater index of maximal
isometric strength increase than the group trained conventionally, which allows us to conclude that training with
vibrations added resulted in a higher neural adaptation than
conventional isometric training. The best results of MVC
growth obtained in the group trained with vibrations are
consistent with those of several studies (1–3,15,18) that
suggested that the exposure to vibrations may result in an
increase in neuromuscular behavior of neural regulation.
Probably the addition of mechanical vibrations to the
exercise potentially increased muscular actions and even
the specific activation of type II muscle fibers, as was
suggested by Rittweger et al. (15). An explanation for the
greater development of maximal isometric strength in the
training done with vibrations should be related to short
periods of muscle stretching added during the execution of
the MVCs. This sudden stretching could have optimized the
action of muscle spindles, especially of the primary endings of
the spindle and the nervous stimulation rate, as has been
suggested by Griffin et al. (7), once the stretching of the
muscle spindles provoke an increase in frequency of
discharge of sensory endings. As the force applied by the
vibration equipment was sufficiently greater than maximal
capacity of voluntary production of strength by muscles
(isometric maximal action), it is possible that this kind of
application of mechanical vibration could have surpassed the
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voluntary limit of strength production and contributed to the
activation of involuntary components of muscle strength
production, characterized by the eccentric action, in this
case, with values sufficiently higher than those obtained
through MVCs. Despite the fact that applying vibrations
stimulated alpha la motor neurons, which stimulates production of strength in an involuntary way (16), Bosco et al.
(3) showed a significant increase in the neural voluntary
activity during the application of vibrations compared to
normal muscle activity. This fact may help explain the
increase in MVC after the training with vibrations done in
this study. The significant increase in muscle strength
obtained after training with vibrations at a frequency of 8
Hz contributes to the controversy about the frequencies of
vibrations that could possibly produce a TVR. The results of
the present study confirmed the findings of Seidel (21) that it
is possible to obtain a TVR with a frequency between 1 and
30 Hz, although it is in contrast to results presented by
Hagbarth and Eklung (8) who reported values between 10
and 200 Hz. Considering that maximal production of
strength depends on neuromuscular capacity of voluntary
opposition to the inhibitory action of Golgi tendon organs
(10), the increase in strength obtained during training with
vibrations may be related to the decreasing activity of these
organs. As the muscle control of the movement is a feedback
system that involves not only the spindle reflex but also more
complex actions, such as the tendon reflex (22), the demand
caused by a higher tension created by the vibration may
provoke as a form of adaptation, the delay in the activity of
the Golgi tendon organs. As electromyography of the
muscles involved in muscle action was not included in this
study, it was impossible to check either the activation of the
muscles or the difference between neural activation of the 2
types of training. Another study is needed to check how
vibrations affect forces at different muscle angles.
The equipment designed for this research allowed the
application of mechanical vibrations in the opposite direction
of muscle shortening, differently from the application of
vibrations normally described in literature. It was demonstrated that isometric training with the addition of this type of
vibration had a positive influence on the development of
maximal isometric strength of the elbow flexors, producing
a greater increase in strength in the group trained with
vibrations than in the group trained in a conventional way.
Confirming the initial hypothesis, the improvement of
strength obtained after training with vibrations was probably
caused by optimization of involuntary mechanisms of muscle
action through sudden and consecutive periods of eccentric
action. In that way, training with the addition of vibrations
seems to interfere with the mechanism of involuntary control
of muscle action.
PRACTICAL APPLICATIONS
The present study demonstrates the efficacy of isometric
training with vibrations specifically applied in the opposite
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Effects of Mechanical Vibration on Maximal Isometric Strength
direction of muscle shortening to improve maximal strength.
Conditioning programs are important and provide potentially
healthy benefits associated with them. Different methods of
training are essential to obtain the adaptive responses of
muscles. This study suggests isometric training with mechanical vibrations applied in the opposite direction of muscle
shortening as another method to be used for healthy people
and demonstrates that it could produce better results of
strength improvement compared to conventional isometric
training. Although this study was done with elbow flexors,
results may be applied to other muscle groups because of
similar behavior of the neuromuscular system. It must be
understood that all subjects in this study were untrained men.
Therefore, the results may not be able to be applicable to
athletes’ training, men who strength train, or women.
ACKNOWLEDGMENTS
This research study was supported by Brazilian Ministry of
Sports and Tourism and from CNPQ (National Agency for
Research Development, Brazil).
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