Facilitation of Motor Evoked Potentials from Magnetic Brain Stimulation in Man: A Comparative Study of Different Target Muscles

The influence of tonic muscle contraction and stimulus intensity on compound muscle action potentials (CMAPs) elicited by magnetic brain stimulation was studied in the biceps brachii (34 subjects), the abductor digiti minimi (11 subjects), the anterior tibial muscle (12 subjects), and the soleus muscle (5 subjects). The muscles were examined at rest and with various degrees of background contraction of up to 60% of maximum force. Stimulus intensity was set at threshold (TSI) or 20% above threshold (1.2 TSI), and in one series additionally at 50% above threshold (1.5 TSI). The effect of voluntary background contraction on CMAP onset latency was similar in the four muscles testedthe latencies shortened by approximately 3 ms when the muscle changed from the relaxed to the contracted state of the 10% of maximum force. An additional increase in the background contraction up to 60% of maximum force induced only few, if any, additional decreases in latency. The uniformity of the latency shift in distal and proximal muscles conflicts with the idea of recruitment of larger and rapidly conducting motoneurons being the cause, since this hypothesis would imply a more pronounced latency reduction in distal than in proximal muscles. The shorter latency during voluntary contraction is more likely due to an enhanced synaptic efficacy at spinal level. Since the motoneurons are brought into an increased state of activity during contraction, they require less temporal summation to reach firing threshold and thus discharge earlier. The CMAP amplitudes of the different muscles were more distinctly affected by voluntary background contraction. When the background contraction was increased, the amplitudes of the anterior tibial muscle rose rapidly and reached a relative plateau at a contraction of 20% of maximum force with suprathreshold brain stimuli. A similar, yet more pronounced amplitude increment with increasing background contraction has been previously described for the abductor digiti minimi. In contrast, the amplitudes of the biceps and soleus muscle did not show this tendency to saturate at lowbackground contraction and showed a more gradual increment with increasing contraction. This did not change much when greatly suprathreshold brain stimuli were used for the biceps.