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Purpose : To study the effects of stable and unstable load on stable and unstable surface on EMG activity during the back squat exercise.
Methods : Twenty-four male sport science students, aged range between 18-25 years, were recruited for this study. Each subjects performed a parallel back squat at 75% 1RM under 4 different conditions; 1) a stable load on stable surface (SLSS), 2) a stable load on unstable surface (SLUS), 3) an unstable load on stable surface (ULSS) and 4) an unstable load on unstable surface (ULUS) in a counter-balance order. Surface electromyography (EMG) was used to detect muscle activities from 8 muscles (Biceps femoris, Rectus femoris, Vastus medialis, Vastus lateralis, Soleus, Rectus abdominis, External oblique and Erector spinae) and expressed as percentage of maximum voluntary isometric contraction (%MVIC). Data were analyzed using One-way Analysis of Variance with repeated measure, followed by the Bonferroni as post-hoc test. A level of 0.05 was considered statistical significance.
Results : The result showed that EMG activity of vastus lateralis was significant higher in SLSS than in ULUS (p<0.05), with no differences observed among other conditions. In addition, the EMG activity of soleus was significant higher in SLUS and ULUS than thos in SLSS and ULSS (p<0.05). However, no significant differences in EMG activities were observed in other muscles, regardless of conditions.
Conclusion : The findings suggest that a parallel back squat (75% 1RM) with stable load on stable surface increased activation of the vastus lateralis muscle in order to stabilize the knee joint. In addition, both squat exercise with stable load on unstable surface and unstable load on unstable surface resulted in increased activation of the soleus muscle for stabilizing the ankle joint.
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