Surface electromyographic (EMG) activity was recorded from the rectus femoris, vastus medialis, biceps femoris, gastrocnemius and tibialis anterior in the human lower extremity while subjects performed bicycling movements over a range of constant pedalling velocities. Kinematics of knee and hip cyclical movements were analyzed from 16 mm film. The reciprocal pattern of activation in agonist and antagonist muscles and timing of EMG initiation relative to knee joint were studied. Reciprocal activation of rectus femoris and biceps femoris muscles was generally observed to occur during the mid-extension or mid-flexion phase of knee movements. This timing of activation pattern coincided well the period of peak angular velocity and zero angular acceleration. As pedalling speeds approached maximum, activation times of the bifunctional, biarticular rectus femoris, biceps and gastrocnemius muscles were considerably advanced in phase relative to knee joint angles, whereas, EMG initiation of monofunctional, single joint, tibalis anterior and vastus medialis muscles maintained a relatively stable knee position-activation time relationship. At higher velocities, biceps femoris EMG activity was characterized as having a double burst pattern of activation. A less distinctive double burst pattern was seen in the rectus femoris EMG at higher cycling speeds. EMG pattern analysis of the rectus femoris and biceps femoris muscles revealed an earlier onset of activity for both muscles during maximum cycling velocities, relative to cyclical phases of the knee joint angle. Considerable overlapping of the EMG bursts was seen beyond pedalling rates of 1 Hz. Co-contraction between rectus femoris and biceps femoris muscles could be observed during the acceleration period involving an abrupt switch to maximum pedalling performance. When co-contraction was observed, the joint angular acceleration curves observed during the knee flexion period accounted for a larger portion of a single cycle, and were more irregular than the angular accelerations observed during knee extension.
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