NEUROMUSCULAR ACTIVITY DURING THE SQUAT EXERCISE ON AN UNSTABLE

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					Maior et al.: Neuromuscular activity during squat exercise                              www.brjb.com.br

                                                          ORIGINAL PAPER (ARTIGO ORIGINAL)


           NEUROMUSCULAR ACTIVITY
       DURING THE SQUAT EXERCISE ON
             AN UNSTABLE PLATFORM
Alex Souto Maior1, Roberto Simão2, Belmiro Freitas de Salles3, Humberto Miranda4, Pablo
Brando Costa5

1- Department of Exercise Physiology. Plínio Leite University. Nitéroi, RJ 22000 – BRAZIL.
2 - Universidade Federal do Rio de Janeiro. School of Physical Education and Sports, Rio
de Janeiro, RJ 22941-590 - BRAZIL.
3 - Laboratory for Research in Microcirculation, Department of Physiological Sciences, Rio
de Janeiro, State University of Rio de Janeiro, RJ 20550-170 - BRAZIL.
4 - Institute of Research and Development. Vale do Paraíba University. São José dos
Campos, SP 12244-000 – BRAZIL.
5- Department of Health and Exercise Science. University of Oklahoma, Norman,
Oklahoma – USA.

Corresponding Author:




                                                                                                               Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
Roberto Simão.
E-mail: robertosimao@ufrj.br

Submitted for publication: January 2009
Accepted for publication: April 2009

ABSTRACT
MAIOR, A. S.; SIMÃO, R.; SALLES, B. F.; MIRANDA, H.; COSTA, P. B. Neuromuscular activity during the
squat exercise on an unstable platform. Brazilian Journal Biomotricity, v. 3, n. 2, p. 121-129, 2009. The
purpose of this study was to compare the muscular activity of the quadriceps muscles [vastus lateralis (VL),
vastus medialis (VM) and rectus femoris (RF)] through surface electromyography (EMG) during the squat
exercise with and without the use of an unstable platform (UP). Twenty males (25 ± 3 yrs; 180 ± 5.2 cm; 80 ±
3.2 kg; 24.2 ± 1.6 Kg.m-2) with at least 12 months of experience in resistance training volunteered for the
study. Following a specific warm up of two sets of fifteen repetitions (light and moderate load), EMG
measurements was obtained during the two conditions: maximum voluntary contraction (MVC) on a stable
surface and another MVC on an unstable platform. Three-minute rest intervals between the conditions were
allowed. The Wilcoxon test revealed significantly greater (p < 0.05) muscle activation of the quadriceps
muscles during the squat exercise on the UP (VL = 21%; RF= 18%; VM = 16%). The results of this study
revealed the UP may be incorporated in some periods of resistance training to increase the activity of the
quadriceps muscle.
Key Words: Electromyography; muscle strength; weight lifting.


INTRODUCTION
The functional training on an unstable platform (UP) has been an important method for
joint rehabilitation and neuromuscular conditioning, consequently, providing an
improvement of coordination and pattern of neuromuscular recruitment (HOLM et al.,
2004; MATTACOLA & DWYER, 2002; STRONJNIK et al., 2002). In addition,


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neuromuscular mechanics play an important role in balance not only when motionless, but
also during movement (HOLM et al., 2004; MATTACOLA & DWYER, 2002).
The UP training provides the largest activation of the proprioceptive system in a mainly
static activity through the afferents fibers (MAGNUSSON et al., 1996; VERHAGEN et al.,
2005). In addition, researches have also demonstrated this potential effect in dynamic
activities (HEITKAMP et al., 2001; SODERMAN et al., 2000). Thus, regular training
prevents possible joint injury and is efficient in the improvement of muscle strength,
reaction time, and balance (ANDERSON & BEHM, 2005; BEHM et al., 2002).
Furthermore, strength gains from this type of training can be attributed to increases in
muscle cross-sectional area (ANDERSON & BEHM, 2005).
During the training with the UP, the instability of movements provide unstable conditions to
the joints, hence, activating proprioceptive impulses which are integrated in several
sensorial-motors centers and regulate the automatic contraction of postural muscles
maintaining general postural balance (MATTACOLA & DWYER, 2002; SODERMAN et al.,
2000). The intrafusal muscle fibers, Golgi Tendons, and other proprioception forms of
feedback assist in the maintenance of balance and detection of body position (COOKE,
1980). As a result, the acute changes in the length of muscle-tendon units, tension, muscle
strength production, and neuromuscular activity can alter the ability to detect (afferent
proprioception) and to respond (muscle activity efferent) to immediate changes in balance
(BEHM et al., 2004; IVANENKO et al., 1997). In this manner, the sensory-motor centers
supply the necessary feedback of muscle status to the central nervous system (HOLM, et
al., 2004; COOKE, 1980). Furthermore, in agreement with training progression, trained
individuals perform movements demanding an exceptional degree of neuromuscular




                                                                                                 Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
coordination involving automatic interactions of voluntary motor commands and postural
stability of upper and lower body muscles (BEHM et al., 2004; IVANENKO et al., 1997;
BLOEM et al., 2000). Thus, instability can be incurred through both stable and unstable
platforms and forms of resistance.
Research regarding resistance training on unstable surfaces has been very scarce in the
scientific literature, posing a question to be answered: Which condition (unstable or stable)
provides greater muscle activation for the neuromuscular system?
The aim of this study was to compare the neuromuscular activity of the quadriceps
muscles [vastus lateralis (VL), vastus medialis (VM), and recto femoris (RF)] through the
use of surface electromyography (EMG) in males performing the squat exercise with and
without an unstable surface.


METHODS
- Participants
Twenty males (25 ± 3 yrs; 180 ± 5.2 cm; 80 ± 3.2 kg; 24.2 ± 1.6 kg·m-2) with at least 12
months of experience in resistance training voluntarily participated in the study. The
participants were assigned to stable or unstable conditions in a counterbalanced and
randomized fashion in order to nullify any treatment order effects. Participants exclusion
criteria included participants who were bearer of any of the following conditions: a)
cardiovascular disease; b) joint injury in the past six months; c) muscular contracture in the
past six months; d) joint surgery in the past 12 months; e) labyrinthitis; f) accentuated
instability of the knees or ankles; g) disc hernia; or h) severe degenerative joint disease.
- Equipment
The exercise used for the EMG assessment was the squat on the Smith Machine (Life


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Fitness equipment; Franklin Park, IL) (Figure 1). The UP exercise was performed on an
unstable surface measuring 15.3 cm x 74 cm x 56 cm and weighing 12.5 kg (Core Board
Training, Reebok-USA) (Figure 2). EMG signals were then amplified (1000x), filtered (20–
500 Hz), smoothed (10 samples), and stored on a personal computer after being directed
through an analog-digital converter. All data were recorded at a sampling rate of 2.000 Hz
and analyzed with a software program (Matlab version 6.0, Mathworks, Massachusetts
USA).




                                                                                             Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
                                 Figure 1 - Stable squat exercise




                                Figure 2 - Unstable squat exercise




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- Testing Procedures
All the participants answered the Physical Activity Readiness Questionnaire – PAR-Q
(Shepard, 1988) and signed an informed consent form before participating in the study
according to the Declaration of Helsinki. The research study was approved by the ethics
commission in experiments with human subjects of the Vale do Paraíba University (São
Paulo, Brazil).
Participants took part in six familiarization session in alternated days (2 weeks) before data
collection began in order to become more familiar with the test in unstable and stable
condition and other testing assessments. The measurements of EMG for both stable and
unstable conditions were recorded during three repetitions of maximum voluntary
contraction (MVC). Hence, test protocol was as follows in: 1) Specific warm up with two
sets of 15 repetitions (light and moderate load); 2) An MVC for the stable condition (Figure
1); 3) An MVC for the unstable condition (Figure 2). MVCs were performed in balance
cross over design for five seconds and three minutes rest intervals were allowed between
MVCs (stable and unstable).
The exercise progressed through the following stages: initial position, eccentric phase, and
concentric phase. The eccentric phases were performed starting from the top position. a)
Initial position - The individual standing up, legs parallel with a small lateral rotation of the
feet, feet approximately 30-40 cm apart, knees extended, and elbows aligned with
shoulders, holding the bar with the load of the training supported on the shoulder; b)
Concentric phase - starting from the end of the eccentric phase (at 90º of knee flexion), the
concentric phase reversed the eccentric movement, extending the knees and hips.
To minimize possible errors in the EMG measures, the following strategies were adopted:




                                                                                                    Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
(a) all the participants received standard instructions on the general routine of data
assessment and the exercise performance techniques before testing, (b) the exercise
technique of all the participants during all testing sessions was monitored and corrected as
needed, (c) all the participants received verbal encouragement during testing, (d) feet were
positioned parallel and maintained at 0º abduction in order to maintain within-participant
consistency between exercises, and (e) individuals maintained the base of support
between 30 and 40 cm apart (feet parallel) during all tests.
EMG activity was measured during two protocols of varied conditions (unstable and stable)
and included the analysis of the VL, RF, and VM muscles. Skin preparation for all
electrodes included removal of dead epithelial cells with an abrasive paper around the
designated areas, followed by cleansing with an isopropyl alcohol swab. Muscle activation
was detected with a bipolar arrangement placed on the surface of the skin over the muscle
with EMG electrodes placed two centimeters apart. To maximize EMG sensitivity,
electrodes were aligned parallel to the muscle fiber orientation. Bipolar surface stimulating
electrodes were secured over the proximal and distal portions of the quadriceps. The
electrodes presented the width from four to five cm and length was sufficient to wrap the
width of the muscle belly. All muscles monitored were from the subject’s dominant side as
determined by leg kicking preference. Hence, all surface electrodes were placed on the
right side of each participant.
The maximum amplitude of the smoothed root mean square (RMS) of the EMG signal was
evaluated over the duration of the unstable and stable conditions of the squat exercise.
EMG activity was normalized based on an MVC for each muscle (De Luca, 1997). The
computer software program rectified and integrated the EMG signal over a 500-millisecond
period during an MVC.




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- Statistical analyses
Wilcoxon test was used (nonparametric) for comparisons of muscle groups (VL, VM, and
RF) between the two squat exercise conditions (stable and unstable). Differences were
considered significant at p < 0.05. Statistical analysis was performed using GraphPad
Prism, 4.0 version (Graphpad Software Inc., San Diego, USA).


RESULTS
Statistical analysis revealed significant differences for all the muscles (VL, VM, and RF)
demonstrating greater muscle activation for the unstable than the stable condition (p <
0.05) for all three muscles analyzed (Figures 3 and 4). The muscles RF, VL, and VM had
greater (p < 0.05) muscle activation during the stable condition of 18, 21, and 16%
respectively (Figure 3).




                                                                                                       Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
Figure 3 - EMG activity (mean ± SD) for the muscles rectus femoris, vastus lateralis, vastus
medialis, respectively. (*) p = 0.0001; (**) p = 0.02




Figure 4 - The analysis for time of the signal RMS in the EMG for muscles femoral (scale of
variation from 0 to 5 seconds). Style dot line = unstable platform; Straight line = stable platform.


DISCUSSION
The results of the current study revealed significantly greater muscle activation of the three
quadriceps muscles analyzed in unstable condition (VL = 21%; RF = 18%; VM = 16%)
when compared to the stable condition. To our knowledge, studies reporting EMG activity


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of the quadriceps muscles during the squat exercise in the Smith Machine comparing
stable and unstable conditions were lacking. However, during stable conditions, Isear et al.
(1997) examined EMG activity of the lower limb muscle groups during a squat exercise
measured through an MVC. Their results showed higher muscle activation of the VM
(68%) when compared to VL (63%). The EMG response in other exercises involving the
quadriceps muscles (knee extension and leg press) have been examined by Alkner et al.
(2000) using percentages of the MVC (20, 40, 60, 80, 100%). Their findings revealed the
VL presented less muscle activation when compared to the VM during each intensity of the
MVC.
CIintra & Furlani (1996) reported with the use of EMG that the VM was active along the
entire arch of the movement in the knee extension, from 0º up to 90º of knee flexion. Their
conclusion revealed activation of the VM muscle is important to reestablish the normal
functioning of the knee joint and increase in the rigidity and dynamic stabilization against
the forces that could move patella laterally (CONLAN et al., 1993 ; GRABINER et al.,
2001).
Muscle activity of the VL during the squat exercise in stable and unstable conditions,
without a direct comparison of the VM, has been examined by Anderson & Behm (2005).
Their testing protocol compared muscle activity during the squat exercise in the smith
machine, free weight, and free weight on an unstable surface (Swiss balls). Their results
revealed significantly greater muscle activity of the VL in the smith machine exercise
compared to the free weight squat and the free weight squat on the unstable surface.
These results are contradictory to our findings possibly because of the three variations of
the same exercise. In addition, the VL muscle plays a reciprocal and synergistic role in the




                                                                                                 Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
stabilization of the patella to a lesser extent than the VM (WILK & REINOLD, 2001). The
RF (Figure 3) showed a pattern of significantly lower muscle activation than the VL or the
VM. This occurred possibly due to the fact that the RF presents greater muscle activity
during hip flexion, consequently, the RF’s primary muscle action of hip flexion was greater
than knee extension.
Kornecki et al. (2001) explained that the quadriceps muscles present greater muscle
activation in unstable conditions, such that this could be related to mechanism of
anticipatory actions to the movement. Therefore, this statement confirms the reports of
Blackburn et al. (2000) and Johnston et al. (1998) that comment on the motor control
system action in the use of complex processes involving sensorial-motors components,
consequently, adjusting the dynamic movement. In addition, maintenance of postural
balance includes sensorial detection of body movement, integration of sensory-motor
information in the central nervous system, and appropriate skeletal muscle responses for
movement execution. Furthermore, while unstable resistance training should certainly tax
the proprioceptive control of posture, it has not been established whether any positive
adjustments would be mediated through anticipatory postural adjustments (nervous
system processing) (KORNECKI et al., 2001; JOHANSON, 1988). This provides a larger
role of the central nervous system that promotes increases of the motoneuron activity (co-
activation), consequently, greater activation of the antagonist muscles.
It is important to mention that an increase in neural activation occurs, which promotes
increases in afferent pathway activities to muscle groups and joints involved in instability.
In addition, Townsend et al. (1978) stated these afferent pathways aid directly in the
adjustment of changes in the center of gravity through lateral oscillations in relation to the
initial movement position, when the joints and muscle groups are exposed to unstable
conditions. Hence, the hypothetical answer to determine the actual position of the center of
gravity to be moved is done through afferent nervous responses (TOWNSEND et al.,


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1978).
The behavior of the muscle strength during the two conditions (unstable and stable) has
been investigated by Heitkamp et al. (2001), in which 30 individuals were examined with
the purpose of comparing strength gains through isokinetic dynamometry between groups
that performed instability training (n = 15) and the group that performed resistance training
without instability (n = 15). Both groups trained two times per week during six weeks for 25
minutes. The group that trained instability used mini trampoline, rollers skates, balls, and
the other group resistance training performed leg press and knee extension exercises. The
results demonstrated similar knee extension and flexion strength gains for both groups.
The researchers concluded that training with instability is effective in increasing balance
and muscle strength. Thus, muscle actions with instability without previous nervous
system adaptation (acute movement response) lead to proprioceptive adaptations, of
which one of the most important tasks is to control the stabilization of the joint’s range of
motion unused in a given motor task by stimulation of antagonistic muscles and correction
of muscle strength deficit.
Regarding muscle strength deficit, Behm et al. (2002) analyzed young men performing
knee extensions. The participants executed an MVC while EMG was being analyzed.
Exercises were performed with stability (seating on the seat of machine) and instability
(seating on a Swiss ball). The results revealed muscle activation during the stable
condition was significantly greater than the unstable condition (EMG was 11.3% lower
during the unstable condition). With respect to the antagonists' action, it showed a
significant increase during the unstable condition when compared with the stable condition
(p < 0.05). Concerning muscle strength with and without instability of the upper body,




                                                                                                  Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
Anderson & Behm (2004) reported a significant decrease of 59.6% during the unstable
condition in relation to the stable condition in the chest press exercise during an MVC.
Behm et al. (2002) and Anderson & Behm (2004) stated three hypotheses exist for the
lower muscle strength obtained on unstable conditions: 1) control, maintenance, and
balance of the joints and limbs involved; 2) activation of the afferents fibers Ib (interneuron
inhibitory Ib) that arise in the Golgi tendon organs, being inhibitory on agonists and
excitatory on antagonists; 3) EMG measurements without the occurrence of the nervous
system adaptation. Thus, these acute measures of muscle strength increase the effort of
the neural response in controlling these two variables (balance and strength) during the
training.
The contractile reaction process of a muscle fiber is controlled by the addition of excitatory
and inhibitory neural impulses that transmit continually from neurons, hence, determining
the potential for excitement. This physiological response facilitates increases in motor
cortex activity, greater synchronization, and excitability of the motoneurons (AAGAARD et
al., 2002).


CONCLUSION
Bilateral contractions of the lower body under an unstable surface can lead to increased
muscle activation. In addition, through the use of UP, training-induced increases in
balance and muscle coordination can be experienced with a functional training. In
conclusion, the present study found greater muscle activation during the unstable
condition compared to the stable condition in VL and VM muscles. However, as this was
only an acute response to an unstable exercise, one should use caution when making
recommendation as to possible training effects.



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ACKNOWLEDGMENTS
Dr. Roberto Simão would like to thank the Brazilian National Board for Scientific and
Technological Development (CNPq) and Research and Development Foundation of Rio de
Janeiro State (FAPERJ) for the research grant support.
Ms. Humberto Miranda is grateful to CAPES for the financial support.


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Description: In the power sector, the squat is recognized as the ultimate measure of strength, while leg strength is recognized as the measure of body size or strength of the mark. This is because the power of leg strength accounted for the largest proportion of the body, and the most practical.