Normal values for median frequency in upper extremity muscles during isometric contraction at 50% of maximal voluntary contraction

Spectral analysis of the electromyogram (EMG) signal is a useful tool for studying the complex phenomena of muscle fatigue. The power density spectrum of the myoelectric signal undergoes a compression towards low frequency as a function of time during a sustained contraction. This compression has been measured using the spectral parameter, median frequency. Normal median frequency values were established for the biceps, triceps and deltoid during a pilot study. The triceps has a lower median frequency slope with fatigue which suggests that this muscle may have more type I fibers (fatigue resistant) than the biceps and deltoids.