NEURONAL MECHANISMS OF HUMAN LOCOMOTION

The surface electromyogram (EMG) of human leg muscles was recorded during running at different speeds. The EMG was compared with the simultaneously recorded vertical force exerted by the foot and with the angle of the ankle joint. During running, the electrical activity of the gastrocnemius muscle increased sharply 35-45 ms after ground contact and reached its maximum at the end of muscle stretch. This activity was superimposed on a slowly increasing level of activation, which began 120-180 ms before ground contact. At the end of the stance phase, gastrocnemius became inactive and, simultaneously, there was a sudden increase in tibialis anterior activity. The assumption that the steep increase in the gastrocnemius EMG reflects the spinal stretch reflex of α-motoneurons is supported by the following findings. The peak level of gastrocnemius EMG in the stance phase of fast running was 2-3 times higher than the activity during maximum voluntary contraction. With stimulation of the tibial nerve at different rates, the maximum isometric force was about 30-40% higher than the maximum voluntary isometric contraction. The increase in EMG at 35-45 ms after ground contact was markedly diminished during running, after partial blockage of Ia afferents by ischemia, at a time when the strength of voluntary contraction was shown to be uninfluenced by the ischemia. The gastrocnemius activity during running was simulated by electrical stimulation of the tibial nerve. The rate of stimulation was varied so as to approximate to the EMG profile during running. This indicated that a spinal stretch reflex could become mechanically effective within the shortest stance phase measured in a fast sprint (ca. 120 ms).