Objective: To determine the effects of the voluntary contraction of muscles and magnetic stimulation intensity on the motor evoked potential (MEP) and the silent period (SP).

Method: We studied MEPs and SPs in opponens pollicis muscle in 30 healthy adults (male: 16, female: 14) while varying the amount of the voluntary contraction and the stimulation intensity. We analyzed MEPs and SPs in relation to sex, recording site, opposition power and height.

Results: 1) During the contraction, the latencies of MEP were significantly shorter than during the relaxation. 2) The amplitudes of MEP reached plateau at 30% of maximal voluntary

contraction and increased with increment of stimulation intensity without limitation. The amplitudes of MEP of right hand were bigger than left hand. There were no significant differences according to sex and recording site. 3) The durations of SP were directly proportional to the degrees of voluntary contraction and the stimulation intensity.

Conclusion: Transcranial magnetic stimulation should be performed under the same voluntary contraction and magnetic stimulation intensity. (J Korean Acad Rehab Med 2002; 26: 140-146)

Objectives: To determine the effect of facilitation on H reflex side-to-side amplitude ratio and to determine the effect of averaging under the assumption that an averaging could further stabilize the amplitude ratio in a facilitated condition.

Method: FCR H reflex amplitude ratios of direct and averaged potentials were measured in fifty asymptomatic subjects under three conditions, during rest, during a constant 5 pound isometric contraction under the hand-dynamometer monitoring, and during a moderate constant isometric contraction under the electromyographer's verbal guidance.

Results: The lower limits of the amplitude ratios that encompass 97.5% of subjects by the percentile method under three conditions were as follows; (1) during the rest (n=37); 0.47 (direct) and 0.50 (average), (2) during the constant 5 pound isometric contraction under the hand- dynamometer monitoring; 0.47 (direct) and 0.48 (average), and (3) during the moderate constant isometric contraction under the electromyographer's verbal guidance; 0.48 (direct) and 0.46 (average), respectively.

Conclusion: There seems to be no definite effect of facilitation and averaging on H reflex amplitude ratio. H reflex amplitude ratio measured in facilitated condition without averaging is still useful for the diagnosis of unilateral radiculopathy.

Objectives: To analyze the motor evoked potential (MEP) responses to a degree of voluntary contraction and stimulus intensity and to suggest the standardized optimal stimulation for MEP responses.

Methods: MEPs induced by a cortical stimulation were elicited at the thenar muscles in 15 normal subjects during the rest and gradual voluntary contraction, using the 10% of maximal voluntary contraction (MVC), 30%, 50%, and MVC. During rest and during each contraction, excitability threshold at rest (RET) and at contraction (CET) were determined. Consecutive stimuli were applied, according to the intensity of ratio increment (110% to 150% of excitation threshold).

Results: The RET showed a remarkable decrease (57.1⁑8.2% → 47.4⁑8.7%) after the voluntary contraction (P＜0.05). Shortening of latency reached the saturation level with 10% of MVC, irrespective of stimulus intensity. Amplitude reached a saturation level at 30% of MVC with 62.7% intensity of maximal output, which is equal to 140% intensity of its CET, and to 110% of RET. MEP amplitude at rest and at 10% of MVC were influenced by the excitation threshold (P＜0.05), but those at above 30% of MVC were not related.

Conclusion: The procedure for optimal facilitation for the MEPs is as follows; for minimal latency of MEPs, minimal contraction (10% of MVC) with RET intensity is enough. For maximal amplitude of MEPs, moderate contraction (30% MVC) with 110% intensity of RET is adequate.