EARLY AND LATE STRETCH RESPONSES OF HUMAN FOOT MUSCLES INDUCED BY PERTURBATION OF STANCE

In eight subjects standing on a movable platform, surface EMG activity was recorded from the foot muscles extensor digitorum brevis (EDB) an d flexor digitorum brevis (FDB) and from the leg muscles soleus (Sol) and tibialis anterior (TA) during perturbations of upright stance. Per turbations inducing foot dorsiflexion (upward tilt and backward transl ation) evoked a short-latency response (SLR) and a medium-latency resp onse (MLR) to stretch in the physiological extensors FDB and Sol, and a long-latency response (LLR) in the physiological flexors EDB and TA. Perturbations inducing plantar-flexion (downward tilt and forward tra nslation) evoked the MLR in EDB and TA, and the LLR in FDB and Sol. Th e latency of the FDB and Sol SLR was compared to that of the H and T r eflexes evoked in the same muscles by electrical or mechanical stimula tion, respectively. In both muscles, the T reflex and the SLR followed the H reflex at delays accounted for by the different stimulation mod e, indicating that the SLR induced in both muscles by upward tilt and backward translation was a true autogenetic stretch reflex from spindl e primaries. The time interval between the onset of SLR and of MLR was significantly greater for the FDB than the Sol muscle, suggesting tha t MLR is a spinal reflex travelling through slower peripheral afferent pathways than SLR. From these latency differences and from the distan ce between the muscles, we calculated in four subjects the conduction velocity of the afferent fibres presumably responsible for the MLR in FDB. This was about 29 m/s. LLRs were evoked in TA and EDB during upwa rd tilt and backward translation, and in Sol and FDB during downward t ilt, but not forward translation. LLRs did not adhere to a proximal-to -distal pattern, since these could appear earlier in the foot than in the leg muscles. All responses were modulated by perturbation type (ti lt vs translation) and body posture (normal stance vs forward leaning) . Both the large amplitude of the foot muscle responses and their temp oral pattern indicate that the muscles acting on the toes play a major role in stabilising posture. Their action increases in amplitude and extends in time the foot-ground reaction force, thereby improving the efficiency of the superimposed action of the leg muscle responses.-