Medium-latency response to muscle stretch in human lower limb: estimation of conduction velocity of group II fibres and central delay

In standing subjects, ankle dorsiflexion evoked short-latency responses (SLRs) at 41 and 57 ms, on the average, in soleus (Sol) and flexor digitorum brevis (FDB), respectively. Medium-latency responses (MLRs) occurred at 70 and 95 ms. The time between the MLRs was 25 ms and between the SLRs was 16 ms. The difference between these two values represents the extra-time to conduct the FDB volley for MLR from distal to proximal muscle, in excess to that for SLR. The velocity of the afferents mediating the FDB MLR (21.4 m/s on average) was estimated by dividing the distance between the two muscles by the sum of the above extra-time and the conduction time of Ia fibres along the same distance. The central delay of FDB MLR (6.7 ms on average) was obtained by dividing the distance between FDB and spinal cord by the sum of afferent and efferent MLR conduction times. The central delay of FDB SLR (1.4 ms) was analogously obtained. These findings give an estimation of the conduction velocity of the group II afferent fibres in humans and support the hypothesis that the FDB MLR is relayed through a spinal oligosynaptic pathway.