Responses of leg muscles in humans displaced while standing. Effects of types of perturbation and of postural set

Toe-up or toe-down tilts of a platform on which a subject stands induce early EMG responses in the leg muscles initially stretched by the perturbation and late responses in the antagonist muscles. Early responses are thought to be connected with the stretch of the leg muscle in which they appear. Disagreement exists as to the origin of the late responses occurring in the antagonist muscle. The aims of this study were to assess (1) whether the late responses are induced by afferent volleys from the spindles of the muscle stretched by the initial perturbation, or (2) whether they are connected with the induced overall postural imbalance, and (3) whether the postural set may influence the occurrence of the late responses. Subjects standing on a platform underwent randomized perturbations stretching the soleus (Sol) muscle (upward tilts and backward translations) and tibialis anterior (TA) muscles (downward tilts and forward translations). The platform movement was regulated in order to yield changes in ankle angle of similar extent and velocity during both tilt and translation. Surface EMGs of Sol and TA were recorded bilaterally. An optoelectronic device detected the movements of markers fixed on the body. From these data, movements of the head, and changes in hip, knee and ankle angles, along with variations in the length of Sol, gastrocnemii (Gas) and TA were computed. Both tilts and translations, equally stretching Sol or TA, induced similar early responses in the stretched muscle. Consistent late responses in the antagonist muscle (antagonist reactions, ARs) were induced only by tilts. In spite of similar changes in ankle angles, the most striking differences in body movements between tilts and translations stretching the same leg muscle concerned changes in knee angles and Gas length. Slight differences were also seen in vertical head movements. Standing and holding onto a frame strongly decreased the amplitude and the frequency of occurrence of both early responses and ARs only in the TA muscle, while all Sol responses were not affected. This modulation of TA responses occurred in spite of changes in ankle angle and head movements similar to those occurring under the free-standing condition. It was concluded that early EMG responses are connected with the stretch of the muscle induced by the platform movement. The ARs, on the other hand, appear to be related to the type of overall postural imbalance. The absence of ARs during translations suggests a role in these responses of the afferences from the joint and muscles of the lower limb.