Electromyographic manifestations of muscular fatigue.

Analyses of surface electromyogram (EMG) power spectra and integrated EMG (IEMG) were performed during isometric fatigue contraction in eight male subjects. Fatigability was determined as the rate of rise in IEMG as a function of time (IEMG slope coefficient or eta). Results indicated that the IEMG slope coefficient for the biceps brachii at 40% of maximal voluntary contraction (MVC) was approximately nine times greater than that of the soleus. The exponential decay of maximal sustaining time (Ts) as a function of IEMG slope coefficient (Log Ts - = 0.895 x eta + 2.60, r = 0.92, P less than 0.001) during different fractions of MVC suggested a neurophysiological link between fatigability of the biceps and their motor unit (MU) activities which increased in an accelerated fashion. Analyses of mean power frequency (MPF) revealed that there was a significant decline in MPF (43.7 Hz, P less than 0.001) for the biceps brachii. Furthermore, the extent of this decline was correlated with MPF obtained during MVC (r = 0.96, P less than 0.000). This correlation indicated that MUs with higher MPF would fatigue to a greater extent than those with relatively lower MPF. Subsequent analyses of MPF during fatigue for the soleus revealed that there was a relatively small decline in MPF (7.3 Hz, P greater than 0.05). It was suggested that non-invasive analyses of power spectra and IEMG slope coefficient could provide a sensitive measure of MU fatigability that may reflect the activities of different types of muscle fibers.