Reliability of a novel neurostimulation method to study involuntary muscle phenomena.

Experimental methods involving painful electrical stimulation of a peripheral nerve showed the existence of a minimum stimulation frequency capable of inducing cramp, termed "threshold frequency" (TF). Our aim was to test an alternative method to induce fasciculations and cramps electrically. Two daily sessions of electrical stimulation of the abductor hallucis muscle were performed in 19 volunteers on 3 days: stimulation trains of 150 monophasic square pulses (duration 152 micros) of increasing frequency (current intensity 30% higher than maximal; frequency of the first trial, 4 pps; recovery between trials, 1 min) were delivered to the main muscle motor point until a cramp developed. Once a cramp was induced the protocol was repeated after 30 min. To verify by electromyography that cramp occurred, a surface electrode array was placed between the motor point and the distal tendon. Ambient and skin temperature were kept constant in all sessions. Fasciculations and cramps were elicited in all subjects. We observed the following median (interquartile range) values of TF: day 1 (session 1), 13 (6) pps; day 1 (session 2), 16 (4) pps; day 2 (session 1), 16 (6) pps; day 2 (session 2), 18 (6) pps; day 3 (session 1), 17 (4) pps; day 3 (session 2), 18 (8) pps. TF intersession intraclass correlation coefficients were 0.82, 0.92, and 0.90 for days 1, 2, and 3, respectively. TF interday intraclass correlation coefficient was 0.85. The absence of pain due to the stimulation and the demonstration of TF reliability support the use of our method for the study of involuntary muscle phenomena.