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 (%EMGmax) to that during maximal voluntary contraction. The %EMGmax 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 %EMGmax).
Results: QF %EMGmax 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 %EMGmax 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.