Different Effect of Height on Latency of Leg and Foot Short- and Medium- Latency EMG Responses to Perturbation of Stance in Humans

In standing humans, platform perturbations evoked short- (SLR) and medium-latency responses (MLR) in soleus (Sol), tibialis anterior (TA) and flexor digitorum brevis (FDB) muscles. The latency of all responses significantly increased with subjects' height. The slope of the regression lines for the MLRs versus height was significantly steeper than that for the SLRs. The conduction velocity of the afferent fibres mediating the FDB MLR, calculated on the basis of the regression, was 17.5 m/s. These findings indicate that the MLRs are transmitted through afferent fibres slower than those mediating the SLRs. While the latter fibres are spindle group Ia afferents, the former are suggested to be group II fibres.