COORDINATION OF 2-JOINT RECTUS FEMORIS AND HAMSTRINGS DURING THE SWING PHASE OF HUMAN WALKING AND RUNNING

It has been hypothesized previously that because a strong correlation was found between the difference in electromyographic activity (EMG) o f rectus femoris (RF) and hamstrings (HA; EMG(RF) -EMG(HA)) and the di fference in the resultant moments at the knee and hip (M-k-M-h) during exertion of external forces on the ground by the leg, input from skin receptors of the foot may play an important role in the control of th e distribution of the resultant moments between the knee and hip by mo dulating activation of the two-joint RF and HA. In the present study, we examined the coordination of RF and HA during the swing phase of wa lking and running at different speeds, where activity of foot mechanor eceptors is not modulated by an external force. Four subjects walked a t speeds of 1.8 m/s and 2.7 m/s and ran at speeds of 2.7 m/s and 3.6 m /s on a motor-driven treadmill. Surface EMG of RF, semimembranosus (SM ), and long head of biceps femoris (BF) and coordinates of the four le g joints were recorded. An inverse dynamics analysis was used to calcu late the resultant moments at the ankle, knee, and hip during the swin g phase. EMG signals were rectified and low-pass filtered to obtain li near envelopes and then shifted in time to account for electromechanic al delay between EMG and joint moments. During walking and running at all studied speeds, mean EMG envelope values of RF were statistically (P<0.05) higher in the first half of the swing (or at hip flexion/knee extension combinations of joint moments) than in the second half (or at hip extension/knee flexion combinations of joint moments). Mean EMG values of BF and SM were higher (P<0.05) in the second half of the sw ing than in the first half. EMG and joint moment peaks were substantia lly higher (P<0.05) in the swing phase of walking at 2.7 m/s than duri ng the swing phase of running at the same speed. Correlation coefficie nts calculated between the differences (EMG(RF) -EMG(HA)) and (M-k-M-h ), taken every 1% of the swing phase, were higher than 0.90 for all sp eeds of walking and running. Since the close relationship between EMG and joint moments was obtained in the absence of an external force app lied to the foot, it was suggested that the observed coordination of R F and HA can be regulated without a stance-specific modulation of cuta neous afferent input from the foot. The functional role of the observe d coordination of RF and HA was suggested to reduce muscle fatigue.