Characteristics of the muscle mechanoreflex during quadriceps contractionsin humans

We examined muscle sympathetic nerve activity (MSNA) in the nonexercising l ower limb during repetitive static quadriceps contraction paradigm at 25% m aximal voluntary contraction in eight men. Subjects performed 20-s contract ions with 5-s rest periods for up to 12 contractions. Although the workload was constant, we found that MSNA amplitude rose as a function of contracti on number [0.6 In (amplitude/min)/contraction]; this suggests chemical sens itization of the muscle reflex response. We employed signal-averaging techn iques and then integrated the data to examine the onset latency of the MSNA response as a function of the 25-s contraction-rest period. We observed an onset latency of similar to 4-6 s. Moreover, although the onset latency di d not appear to vary as a function of contraction number, the rate of MSNA increase took approximately four contractions to reach a steady-state rate of rise; this suggests contraction-induced sensitization. The onset latency reported here is similar to findings in recent animal studies, but it is a t odds with latencies determined in prior human handgrip contraction studie s. We believe our data suggest that I) mechanically sensitive afferents con tribute importantly to the MSNA response to the paradigm employed and 2) th ese afferents may be sensitized by the chemical products of muscle contract ion.