Reliability of an electrical method to induce muscle cramp

A reliable experimental model to induce muscle cramp is an important first step in developing well-controlled studies that empirically test hypotheses regarding cramp. Experimental methods involving repetitive electrical and magnetic stimulation of a peripheral nerve to induce cramp have been reported.1,2 With either protocol, the frequency of stimulation appears to be the determinant of whether cramp is induced and has been termed the “threshold frequency. ” The reliability of the threshold frequency has not been reported for either protocol. Thus, the purpose of this study was to test a method to electrically induce muscle cramp and the intrasession and intersession threshold frequency reliability of this method. Sixteen (11 men and 5 women) healthy volunteers (height, 175 11.6 cm; mass, 84.4 20.9 kg; age, 23 2.9 yr) were recruited for the study. Participants had no history of neurological, muscular, or vascular pathology of the lower leg or adverse response to electrical stimulation. Each subject reported to the laboratory on three separate days, each approximately 48 h apart from the previous session. The institution’s Human Subjects Committee approved the study, and informed consent was obtained from each subject prior to testing. The MP100 system (Biopac Systems, Inc., Santa Barbara, California) with Ag-AgCl recording electrodes was used to collect electromyographic (EMG) data (sampling rate, 2 kHz) from induced cramps. The Grass S88 Stimulator with SIU5 Stimulus Isolation Unit (Astro-Med, Inc., West Warwick, Rhode Island) and an 8-mm shielded stimulating electrode (Biopac, EL254S) were used to deliver trains of repetitive electrical stimuli. Electromyographic data were analyzed using Acqknowledge 3.7. 1 software (Biopac). Frequency threshold data were analyzed with SPSS 10.0 for Windows software (SPSS, Inc., Chicago, Illinois). After electrode placement sites were shaved (if necessary), debrided, and cleaned, recording electrodes were placed over the midbelly of the flexor hallucis brevis and a ground electrode was placed on the ipsilateral tibial tuberosity. The stimulating electrode was placed over the posterior tibial nerve with a 7-cm dispersive pad over the lateral aspect of the ankle. Subjects were positioned supine with the ankle plantarflexed in a relaxed position. Proper placement of the stimulating electrode was determined as the site at which the most forceful flexion of the hallux was observed following two or three separate 1-ms stimuli. Once in place, the stimulating electrode was secured with tape and an elastic wrap. Upon completion of the session, skin placements of both stimulating and recording electrodes were marked to ensure accurate replacement for subsequent sessions. Stimulus intensity was initially set at 80 V for each subject and voltage was then increased by 10-V increments according to subject tolerance (Table 1). The same stimulus intensity, train duration (2 s), and pulse duration (1 ms) were used for all sessions.2 A stimulus train with an initial frequency of 4 Hz was delivered.2 If a cramp failed to develop, the frequency was increased by 2 Hz and, after 1 min, the stimulus was applied again.2 This method was continued until a cramp was induced as determined by subject feedback, visible flexion of the hallux, and increased EMG activity. Duration of EMG recording was 15 s and consisted of 1-s background EMG, 2-s stimulus, and 12-s poststimulus muscle activity. Any cramp either resolved spontaneously or was arrested by muscle stretch after 10 s. Once a cramp was induced, we waited 30 min and repeated the procedures. Subjects repeated the entire protocol twice. Thus, two cramps were induced on each of three days separated by approximately 48 h. To verify by EMG that cramp occurred, we calculated the root-mean-square (RMS) amplitude of the 2 s following cessation of the stimulus for each cramp. For cramp classification, mean RMS amplitude was greater than 2 standard deviations above baseline EMG mean RMS amplitude. Threshold frequency intrasession and intersession reliability were estimated using intraclass correlation coefficients (ICC [3,1]). One subject was excluded from the study because we were unable to elicit a second cramp during the first session. Thus, results are for 15 subjects. Stimulus intensity © 2002 Wiley Periodicals, Inc.