Interaction between central programs and afferent input in the control of posture and locomotion.

Two basic aspects of the neuronal control of bipedal stance and gait have been discussed, namely the interlimb coordination and the antigravity function of leg extensors. During stance and gait both legs act in a cooperative manner as each limb affects the strength of muscle activation and the time-space behaviour of the other. There are indications that interlimb coordination is mediated by spinal interneuronal circuits, which are themselves under supraspinal (e.g. cerebral and cerebellar) control. Proprioceptive reflexes involved in the maintenance of body equilibrium depend on the presence of contract forces opposing gravity. Extensor load receptors are thought to signal changes of the projection of body's centre of mass with respect to the feet. According to recent observations in the spinal cat, this afferent input probably arises from Golgi tendon organs and represents a newly discovered function of these receptors in the regulation of stance and gait.