Electrically evoked myoelectric signals of back muscles: Effect of side dominance

Electrically evoked myoelectric signals (MES) may be expected to provide information about muscle properties more objectively than voluntary MES (1); To test the sensitivity of electrically evoked MES to small physiological differences some parameters of MES where studied on the right and left longissimus dorsi (RLD and LLD) of five right handed (RH) and five left handed (LH) healthy young subjects. Measurements were repeated five times in five different days for a total of 100 contractions. Supramaximal electrical stimulation was applied with monopolar technique using monophasic current pulses of 0.2 ms width and frequency of 25 Hz, first to the RLD and then to the LLD for 30 s each. A 3x4 cm and a 10x12 cm sponge electrode where applied on the main muscle motor point and on the shoulder blades respectively. Motor points were usually at the T9-T12 level. MES were detected 3 cm to 6 cm caudally with respect to the motor point, approximately at the Tll-Ll level, between two bar contacts 10 mm apart, amplified with a 10-500 Hz bandwidth, sampled at 1024 Hz and converted in 12 bit words. The 25 responses obtained during each second were averaged, the result- ing waveform was zero-padded to 512 Hz, the power spectrum was obtained with the FFT technique and the mean and median frequencies (MDF and MNF) were computed. Details of the technique are provided elsewhere (1).