Surface EMG modifications after eccentric exercise.

The possibility that the surface electromyographic signal (sEMG) from exercised muscle would show significant changes to demonstrate muscle damage after eccentric contraction (EC) was tested in this study. The experiment lasted five consecutive days. On the first day, six sedentary adult subjects performed two rounds of 35 ECs with the biceps brachii of the non-dominant arm, the other arm being used as control. Individual muscle soreness was assessed on a subjective scale. The analysis of sEMG was performed on the signal recorded during isometric contractions at 80% and 50% of maximal voluntary contraction (MVC), choosing root mean square (RMS) and median frequency (MDF) as synthetic sEMG parameters. MVC was also recorded, and plasma levels of creatine kinase were determined in four subjects. The most important findings which resulted from this study were: (a) spectral parameters are less sensitive to error introduced by electrode repositioning than time domain parameters, and are more sensitive to EC-induced sEMG changes than RMS; (b) a significant shift of MDF power spectra towards low frequencies at 80% and 50% MVC (20% and 5% of decay, respectively) was evident as early as 1 h after EC on the exercised arm; and (c) MDF follows the evolution of muscle damage. We concluded from these results that MDF is suitable for the early and non-invasive detection of sEMG changes induced by EC. In addition, we found further evidence that the observed modifications result from a selective or prevalent damage of type 2B muscle fibres.