MEDIUM-LATENCY RESPONSE TO MUSCLE STRETCH IN HUMAN LOWER-LIMB - ESTIMATION OF CONDUCTION-VELOCITY OF GROUP-II FIBERS AND CENTRAL DELAY

In standing subjects, ankle dorsiflexion evoked short-latency response s (SLRs) at 41 and 57 ms, on the average, in soleus (Sol) and flexor d igitorum brevis (FDB), respectively. Medium-latency responses (MLRs) o ccurred at 70 and 95 ms. The time between the MLRs was 25 ms and betwe en the SLRs was 16 ms. The difference between these two values represe nts the extra-time to conduct the FDB volley for MLR from distal to pr oximal muscle, in excess to that for SLR. The velocity of the afferent s mediating the FDB MLR (21.4 m/s on average) was estimated by dividin g the distance between the two muscles by the sum of the above extra-t ime and the conduction time of la 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 ef ferent MLR conduction times. The central delay of FDB SLR (1.4 ms) was analogously obtained. These findings give an estimation of the conduc tion velocity of the group II afferent fibres in humans and support th e hypothesis that the FDB MLR is relayed through a spinal oligosynapti c pathway. (C) 1998 Published by Elsevier Science Ireland Ltd. All rig hts reserved.