PHASE-DEPENDENT MODULATIONS OF ANTICIPATORY POSTURAL ACTIVITY DURING HUMAN LOCOMOTION

1. The ability of the CNS to coordinate several motor tasks was studied in humans walking on a treadmill while pulling on a handle. Subjects were instructed to respond to an audio signal that was presented in different phases of the step cycle. Electromyograph (EMG) and movements were recorded from the left arm and leg. 2. The activity of the arm muscle was preceded by postural activity in the leg muscles. The pattern of the anticipatory postural activity differed in the various phases of the step cycle. Lateral gastrocnemius and hamstring muscles were activated during responses occurring in the early support phase whereas tibialis anterior and quadriceps muscles were activated when the pull was exerted during the late support phase and during the swing phase. In the middle of the support phase the combination of both muscle activity was present. 3. The temporal sequencing and the spatial distribution of the anticipatory muscle activity changed gradually. Early during the support phase the hamstring muscles were activated before the gastrocnemius muscle, whereas the order was reversed during midstance. The EMG amplitude of the hamstring and gastrocnemius muscles was largest in the beginning of the support phase and then gradually decreased, whereas the amplitude of the tibialis anterior and quadriceps muscles increased during the later parts of the support phase. 4. The anticipatory responses to pulls exerted during the first part of the support phase reduced the ankle flexion during the single support phase. This was followed by a delayed and decreased ankle extension during the push-off at the end of the support phase. Anticipatory responses triggered during later parts of the support phase also delayed and reduced the ankle extension at push-off. 5. The phase-dependent modifications of the postural adjustment suggest an internal representation of the body at different levels of the CNS with a continuous update with respect to the locomotor movements. At a high level, cerebral circuits may program the postural response in conjunction with the pull. The postural response might thereafter be tuned in accordance with the step cycle by central gating, exerted by the locomotor network and peripheral reflex interactions.