MEDIUM-LATENCY STRETCH REFLEXES OF FOOT AND LEG MUSCLES ANALYZED BY COOLING THE LOWER-LIMB IN STANDING HUMANS

1. In standing subjects, an ankle-dorsiflexing perturbation of the sup porting surface evokes a short-latency response (SLR) and a medium-lat ency response (MLR) to stretch in both soleus (Sol) and flexor digitor um brevis (FDB) muscles. The SLR is the counterpart of the monosynapti c reflex, whilst the MLR might be either mediated by Ia fibres, the de lay being due to a long-loop central circuit, or by fibres of slower c onduction velocity Since small afferents are slowed more than large on es by low temperature, a greater latency increment for the MLR than th e SLR induced by cooling of the limb would point to a peripheral origi n of the MLR. 2. In nine subjects, one limb was cooled by circulating water in a tube wrapped around it for about 120 min. Perturbations wer e delivered to the same limb prior to and during cooling, and after re warming. EMG was recorded by surface electrodes from the Sol and FDB m uscles. 3. The mean increase in latency of MLRs was significantly grea ter than that of SLRs in both muscles. On average, the Sol SLR increas ed from 42.4 to 47.0 ms and the Sol MLR from 72.0 to 82.3 ms. The FDB SLR increased from 58.1 to 66.5 ms and the FDB MLR from 94.9 to 110.5 ms. The mean difference (MLR minus SLR) increased from 29.6 to 35.2 ms for Sol, and from 36.8 to 43.9 ms for FDB at the end of cooling. Afte r 30 min of rewarming, the responses of both muscles recovered towards control values. 4. The greater latency increment of the MLRs than of the SLRs favours the hypothesis of a slower conduction velocity of the responsible afferent fibres. The most likely candidate fibres are the spindle group II afferents.