Muscle fatigue and muscle length interaction: effect on the EMG frequency components.

Muscle fatigue can be identified by a decrease in the frequency components of the EMG signal, typically represented by a fall in the center frequency (fc). In addition, changes in muscle length have been demonstrated to independently alter the EMG fc. What remains unknown is the interaction between muscle fatigue and muscle length changes on the EMG fc. To address this question, a human biceps model of muscle fatigue, consisting of repetitive non-isometric contractions until task failure was studied. EMG signals from the biceps brachii were acquired via surface electrodes, and muscle length was determined by recording elbow angle. Five subjects developed biceps fatigue by repeating contractions of 50% maximal force until task failure. Fatigue produced a decrease in EMG fc at all muscle lengths. For the first three quarters of fatigue development there was a similar degree of reduction in EMG fc at all muscle lengths. But as task failure approached, there was a greater reduction in EMG fc at the shortest muscle length relative to the longest muscle length (41% reduction from baseline versus 30% reduction from baseline, respectively). The effect of muscle length independent of fatigue was such that a 20% increase in length would result in approximately an 18% fall in EMG fc. In conclusion, during fatigue development muscle length significantly influences the EMG fc, and it may be an important factor to consider when utilizing the EMG fc to detect muscle fatigue.