Association between Knee Extensor Strength and EMG Activities during Squat Movement

FUJITA, E., H. KANEHISA, Y. YOSHITAKE, T. FUKUNAGA, and H. NISHIZONO. Association between Knee Extensor Strength and EMG Activities during Squat Movement. Med. Sci. Sports Exerc., Vol. 43, No. 12, pp. 2328-2334, 2011. Purpose: The present study aimed to clarify how the force-generating capability of quadriceps femoris (QF) is associated to its surface EMG activity during a body mass-based squat movement. Methods: Isometric knee extension torque (KET) during maximal voluntary contraction and EMG activities of the rectus femoris and vastus lateralis muscles during a body mass-based squat movement were determined in 53 men and 48 women age 19-90 yr, including 18 frail elderly persons who used the long-term care insurance system. The rectified EMG signals during the squat movement were averaged and normalized as the relative value (%EMG(max)) to that during maximal voluntary contraction. The %EMG(max) values for rectus femoris and vastus lateralis were averaged and used as an index representing the level of muscular activities of QF during the squat movement (QF %EMG(max)). Results: QF %EMG(max) was nonlinearly related to KET relative to body mass (KET/BM). Linear piecewise continuous regression analysis showed that there was a breakpoint of 1.9 N.m.kg(-1) in the relationship between the two variables. In individuals with KET/BM less than 1.9 N.m.kg(-1), QF %EMG(max) rapidly increased as KET/BM decreased. Conclusions: The current results indicate that the activity level of QF during a body mass-based squat movement is influenced by its force generation capability. For individuals with a KET/BM less than 1.9 N.m.kg(-1), body mass-based squat movement is considered to be a fairly high-intensity exercise. The breakpoint of 1.9 N.m.kg(-1) may be assumed to be a threshold level of knee extensor strength, which should be maintained for performing the activities of daily living without great difficulty.