Central motor conduction in relation to contra- and ipsilateral activation.

Differences of central motor conduction between slight activation of the target muscle (ipsilateral: IL) and strong contraction of the contralateral (CL) muscle following magnetic motor cortex stimulation were studied in 18 controls; responses were recorded at abductor pollicis brevis (APB) and tibial anterior (TA). For APB a clearly faster response was obtained with IL activation (mean: 1.7 msec). The amplitude increased only slightly. The reverse was found for TA muscle: amplitude nearly doubled with IL activation, but the latency did not change. In both facilitation procedures a clear correlation was found between left and right normalised amplitude (cortex amplitude/M wave) for APB and TA, but not for the TA response with IL activation. This points to a different mechanism of enhancement for the ipsilateral activation of TA muscle. It is argued that a rise in excitability of spinal motoneurones is largely responsible for the increase in amplitude. In clinical practice contralateral activation as a method of facilitation seems appropriate in most cases: lack of EMG contamination of the baseline makes it easier to read the latency onset. Only in cases of a low TA response can IL activation give a better response.