The effects of joint angle on electromyographic indices of fatigue.

The purpose of this study was to examine the effect of manipulation of joint angle on electromyographic (EMG) fatigue curves at different sites over the quadriceps muscle group. Eight subjects performed isometric knee extensions at 0.26, 0.79, and 1.31 rad from full extension for 1 min at 50% of maximum. EMG signals were recorded with a branched electrode lead system at proximal and distal sites over the vastus lateralis and vastus medialis. The 1-min contractions were analyzed for changes in integrated EMG (IEMG) and median power frequency (MPF) over time. The results showed that the fatigue slopes for IEMG were greatest at 0.79 rad. However, the MPF data showed the greatest slopes at 0.26 rad. We hypothesize that the decline in MPF at 0.26 rad is due to activation failure while the increase in IEMG at 0.79 rad is driven by contractile failure. In addition, the EMG fatigue rates within each joint angle were similar at all sites.