EMG and mechanomyogram changes in fresh and fatigued muscle during sustained contraction in humans

Aim of this study was to evaluate the persistence of the fatigue-induced changes in motor unit (MU) electrical and mechanical properties after a short period of recovery. In 7 subjects the force, EMG and mechanomyogram (MMG) were recorded from m. biceps brachii during a 20 s isometric effort at 80% of the maximal voluntary contraction (MVC), before (test 1) and 10 min after (test 2) an intermittent fatiguing exercise (6 s on - 4 s off cycles at 50% MVC). It resulted that: a) the MVC was 412±90 N (mean±SEM) and 304±85 N in fresh and fatigued muscle, resp. (P<0.05); b) during test 1 and 2 the EMG root mean square (RMS) increased linearly from 0.40±0.01 mV to 0.57±0.01 mV (P<0.05) and from 0.48±0.01 mV to 0.63±0.01 mV (P<0.05), resp.; c) during test 1 the MMG RMS dropped from 9.4±0.8 mV to 5.7±0.9 mV (P<0.05); during test 2 constant lower values were observed throughout contraction; d) during test 1 and 2 the EMG mean frequency (MF) declined from 122±7 Hz to 74±7 Hz (P<0.05) and from 106±8 Hz to 60±7 Hz (P<0.05), respectively; e) during test 1 the MMG MF increased in the first 6 s from 19.3±1.4 Hz to 23.9±2.9 Hz (P<0.05), falling to 13.9±1.3 Hz at the end of contraction; during test 2 the MMG MF declined continuously from 18.7±1 Hz to 12.4±0.8 Hz (P<0.05). The lower MVC after the fatiguing exercise and the changes in the EMG and MMG parameters, in particular, the lower initial values of the MMG RMS and the lack of the increase in the MMG MF during the first part of test 2, suggest that even 10 min after the fatiguing exercise some highly fatigable MUs, probably the fast twitch MUs, may not be fully recruitable at their maximal firing rate.