Maior et al.: Neuromuscular activity during squat exercise
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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
Corresponding Author: Roberto Simão.
E-mail:
[email protected]
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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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.
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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).
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 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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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).
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Figure 1 - Stable squat exercise
Figure 2 - Unstable squat exercise
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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).
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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).
To minimize possible errors in the EMG measures, the following strategies were adopted: (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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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.
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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
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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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).
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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.
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 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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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).
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1978).
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, 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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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.
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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. REFERENCES AAGAARD, P.; SIMONSEN, E. B.; ANDERSEN, J. L.; MAGNUSSON, P.; DYHRE POULSEN, P. Neural adaptation to resistance training: changes in evoked V-wave and H reflex responses. J Appl Physiol, v. 92, p. 2309-2318, 2002. ANDERSON, K. G.; BEHM, D. G. Maintenance of EMG activity and loss of force output with instability. J Strength Cond Res, v. 18, p. 637-640, 2004. ANDERSON, K. G.; BEHM, D. G. Trunk muscle activity increases with unstable squat movements. Can J Appl Physiol, v. 30, p. 33-45, 2005.
BEHM, D. G.; ANDERSON, K. G.; CURNEW, S. Muscle force and neuromuscular activation under stable and unstable conditions. J Strength Cond Res, v. 16, p. 416-422, 2002. BEHM, D. G.; BAMBURY, A.; CAHILL, F.; POWER, K. Effect of acute static stretching on force, balance, reaction time, and movement time. Med Sci Sports Exerc, v. 36, p. 1397 1402, 2004. BLACKBURN, T.; GUSKIEWICZ, K. M.; PETSCHAVER, M. A.; PRENTICE, W. E. Balance and joint stability: The relative contribuitions of proprioception and muscular strength. J Sport Rehabil, v. 9, p. 315-28, 2000. BLOEM, B. R.; ALLUM, J. H. J.; CARPENTER, M. G.; HONEGGER, F. Is lower leg proprioception essential for triggering human automatic postural response? Exp Brain Res, v. 130, p. 375-391, 2000. CIINTRA, A. I. D.; FURLANI, J. Electromyographic study of quadriceps femoris in man. Electromyogr Clin Neurophysiol, v. 21, p. 539-554, 1996. CONLAN, T.; GARTH, W. P.; LEMONS, J. E. Evaluation of the medial soft- tissue restraints of the extensor mechanism of the knee. J Bone and Joint Surg, v. 75, v. 682 603, 1993. COOKE, J. D. The role of stretch reflexes during active movements. Brain Res, v.181, v. 493-497, 1980. DE LUCA, C. J. Use of the surface electromyography in biomechanics. J Appl Biomec, v.13, p. 135-163, 1997. GRABINER, M. D.; KOH DRAGANICH, T. J. Neuromechanics of the patellofemoral joint. Med Sci Sports Exerc, v. 26, p. 10-21, 1994. HEITKAMP, H. C.; HORSTMANN, T.; MAYER, F.; WELLER, J.; DICKHUTH, H. H. Gain in strength and muscular balance after balance training. Int J Sports Med, v. 22, p. 285-290, 2001.
Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
ALKNER, B. A.; TESCH, P. A.; BERG, H. E. Quadriceps EMG/force relationship in knee extension and leg press. Med Sci Sports Exerc, v. 32, p. 459-463, 2000.
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Maior et al.: Neuromuscular activity during squat exercise
www.brjb.com.br
HOLM, I.; FOSDAHL, M. A.; FRIIS, A.; RISBERG, M. A.; MYKLEBUST, G.; STEEN, H. Effect of neuromuscular training on proprioception, balance, muscle strength, and lower limb function in female team handball players. Clin J Sport Med, v. 14, p. 88-94, 2004. ISEAR, R.; JEROME, A.; ERICKSON, C.; TEDDY, W. EMG analysis of lower extremity muscle recruitment patterns during an unloaded squat. Med Sci Sports Exerc, v. 29, p. 532-539, 1997. IVANENKO, Y.; LEVIK, Y.; TASLIS, V.; GURFINKEL, V. Human equilibrium on unstable support: The importance of feet-support interaction. Neurosci Lett, v. 235, p. 109-112, 1997. JOHANSON, H. Rubrospinal and rubrobulbospinal influences on dynamic and static gama-motoneurons. Behav Brain Res, v. 28, p. 97-101, 1998. JOHNSTON, B.; RICHARD, B.; HOWARD, M. E.; CAWLEY, P. W.; LOSSE, G. M. Effect of lower extremity muscular fatigue on motor control performance. Med Sci Sports Exerc, v. 30, p. 1703-1707, 1998. KORNECKI, S.; KEBEL, A.; SIEMIENSKI, A. Muscular cooperation during joint stabilization, as reflected by EMG. Eur J Appl Physiol , v. 85, p. 453-461, 2001.
MATTACOLA, C. G.; DWYER, M. K. Rehabilitation of the ankle after acute sprain or chronic instability. J Athl Train v. 37, p. 413-429, 2002. SHEPARD, R. J. Par-Q. Canadian home fitness test and exercise screening alternatives. Sports Med, v.5, p. 185-195, 1988. SODERMAN, K.; WERNER, S.; PIETILA, T.; ENGSTROM, B.; ALFREDSON, H. Balance board training: prevention of traumatic injuries of the lower extremities in female soccer players? A prospective randomized intervention study. Knee Surg Sports Traumatol Arthrosc, v. 8, p. 356-363, 2000. STRONJNIK, V.; VENGUST, R.; PAVLOVIC, V. The effect of proprioceptive training on neuromuscular function in patients with patellar pain. Cell Mol Biol Lett, v. 7, p. 170-171, 2002. TOWNSEND, M. A.; LAINHART, S. P.; SHIAVI, R.; CAYLOR, J. Variability and biomechanics of synergy patterns of some lower-limb muscles during ascending and descending stairs and level walking. Med Biol Eng Comput, v. 16, p. 681-688, 1978. VERHAGEN, E.; VAN DER BEEK, A.; VAN TULDER, M.; VAN MECHELEN, W. An economic evaluation of a proprioceptive balance board training programme for the prevention of ankle sprains in volleyball. Br J Sports Med, v. 39, p. 111-115, 2005. WILK, K. E.; REINOLD, M. M. Principles of patellofemoral rehabilitation. Sports Med Arthrosc Rev, v. 9, p. 325-336, 2001.
Brazilian Journal of Biomotricity, v. 3, n. 2, p. 121-129, 2009 (ISSN 1981-6324)
MAGNUSSON, S. P.; SIMONSEN, E. B.; KJAER, M. Biomechanical responses to repeated stretches in human human hamstring muscle in vivo. Am J Sports Med, v. 24, p. 622-628, 1996.
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