HUMAN NEURONAL INTERLIMB COORDINATION DURING SPLIT-BELT LOCOMOTION

Human interlimb coordination and the adaptations in leg muscle activit y were studied during walking on a treadmill with split belts. Four di fferent belt speeds (0.5, 1.0, 1.5, 2.0 m/s) were offered in all possi ble combinations for the left and right leg. Subjects adapted automati cally to a difference in belt speed within 10-20 stride cycles. This a daptation was achieved by a reorganization of the stride cycle with a relative shortening of the duration of the support and lengthening of the swing phase of the ''fast'' leg and, vice versa, in support and sw ing duration on the ''slow'' leg. The electromyogram EMG patterns were characterized by two basic observations: (1) onset and timing of EMG activity were influenced by biomechanical constraints. A shortening of the support phase on the faster side was related to an earlier onset and increase in gastrocnemius activity, while a coactivation pattern i n the antagonistic leg muscles was predominant during a prolonged supp ort phase on the slower side. (2) A differential modulation of the ant agonistic leg muscles took place. An increase in ipsilateral belt spee d in combination with a constant contralateral belt speed was associat ed with an almost linear increase in ipsilateral gastrocnemius and con tralateral tibialis anterior EMG activity, while the contralateral gas trocnemius and ipsilateral tibialis anterior EMG activity were little affected. It is concluded that a modifiable timing within the stride c ycle takes place with a coupling between ipsilateral support and contr alateral swing phase. The neuronal control of this coupling is obvious ly based on ipsilateral modulation of leg extensor EMG by propriocepti ve feedback and an appropriate central (e.g. spinal) modulation of con tralateral tibialis anterior EMG activity.