A validation of techniques using surface EMG signals from dynamic contractions to quantify muscle fatigue during repetitive tasks.

The purpose of the current study was to determine the validity of quantifying biceps brachii fatigue with dynamic measures of surface electromyo-graphic (EMG) mean power frequency (MPF) through comparisons with the well-established isometric methodology. Subjects performed repetitive elbow flexion-extension movements with a hand held load of 7 kg until volitional exhaustion. Elbow joint angle and biceps brachii EMG signals were recorded continuously during the fatiguing movement (in 250-ms segments) and during isometric, isotonic contractions (in 1000-ms segments) performed at a 90 degrees flexion angle before and after the trial. The MPF and average EMG amplitude (AEMG) were also calculated with each sample, and a polynomial regression analysis was used to characterize the time history of changes and to determine the rested and fatigued values for the dynamic EMG with: (a) all dynamic samples above 5% MVC and (b) only samples where the elbow joint was between 80 degrees and 100 degrees of flexion. There was a significant increase in AEMG and a decrease in MPF for the isometric contractions and both dynamic methods. When compared to dynamic values at rest and fatigue, the isometric AEMG and MPF were substantially lower and slightly higher, respectively. No significant differences were observed between the AEMG or MPF results from the two methods of processing the dynamic EMG. The decreases in MPF ranged from 25% to 29% and did not differ between methods. The absolute and relative increases in isometric AEMG were substantially lower than with both dynamic methods. The current results support the use of MPF values from surface EMG signals recorded during dynamic contractions to quantify fatigue of the biceps brachii muscle. The proposed methodology can be used to monitor fatigue continuously throughout a dynamic movement with minimal disturbance to the task being performed and without the need to monitor joint angles.