The electromyographic fatigue threshold is not a valid tool to assess muscle function

Abstract

The present study aimed at determining the electromyographic fatigue threshold (EMG FT) from the EMG activity level and the EMG frequency content of the First Dorsal Interosseous. Thirty-seven healthy subjects performed seven isometric index abductions at randomly ordered percentages of maximal voluntary contraction (i.e., 20%, 25%, 30%, 35%, 40%, 50% and 60%). During these bouts, surface EMG was measured using a linear electrodes array (i.e., seven EMG channels) in the First Dorsal Interosseous. For each subject the EMG FT was determined from both Root Mean Square (RMS) and Mean Power Frequency (MPF) values, only if the following criteria were met: (i) significant positive linear regression ( P < 0.05) between force and slope coefficient, (ii) an adjusted coefficient of determination for force versus slope coefficient relationship greater than 0.85, and (iii) a standard error for the EMG FT below 5% of MVC. The results showed the inability to determine an EMG FT in all of the 37 subjects from both RMS (9 out of 37 subjects) and MPF (27 out of 37 subjects). In addition, for the 12 subjects tested twice, the reproducibility of the EMG FT determination was weak (ICC = −0.029 and SEM = 7.5% of MVC for EMG FT determined from MPF). The present results suggest that the EMG FT is not a valid tool to assess muscle function.