VOLUNTARY CONTRACTION SHORTENS PERIPHERAL CONDUCTION TIME IN RESPONSETO TRANSCRANIAL MAGNETIC STIMULATION OF THE BRAIN

We investigated the effects of voluntary contraction on peripheral con duction time in response to transcranial magnetic stimulation (TMS) of the brain in 10 normal subjects. We obtained surface recordings of co mpound muscle action potentials (CMAP) from the abductor digiti minimi muscle (ADM) and nerve action potentials (NAP) from the ulnar nerve, at rest and during contraction (10% of maximal voluntary contraction) in response to TMS delivered at 100% output using a coil shaped like a figure 8. The distance between the two recording electrodes was 10 cm . The distal latency in response to TMS was calculated by subtracting the NAP latency from the CMAP latency. Distal latency was also measure d by recording ADM responses to supramaximal electrical stimulation (E S) 10 cm proximal to the recording electrode. TMS-induced distal laten cy was significantly shorter during voluntary contraction than at rest (P < 0.00001). There was no significant difference between TMS-induce d distal latency during contraction and ES-induced distal latency. TMS -induced distal latencies at rest and during contraction were correlat ed with the ES-induced distal latencies (r(2) = 0.468, P = 0.028 and r (2) = 0.769, P = 0.0009, respectively). Our results showed that the pe ripheral conduction time in response to TMS was related to the activit y of the target muscle and to the fastest conduction velocity of the t arget nerve. Voluntary contraction reduced the peripheral conduction t ime in response to TMS.