Electromyographic effects of fatigue and task repetition on the validity of estimates of strong and weak muscles in applied kinesiological muscle-testing procedures.

The study investigated the effects of fatigue and task repetition on the relationship between integrated electromyogram and force output during subjective clinical testing of upper extremity muscles. Muscles were studied under two conditions differing in the nature and duration of constant force production (SHORT-F) and (LONG-F). The findings included a significant relationship between force output and integrated EMG, a significant increase in efficiency of muscle activity with task repetition, and significant difference between Force/integrated EMG ratios for muscles labeled "Strong" and "Weak" in the LONG-F condition. This supports Smith's 1974 notion that practice results in increased muscular efficiency. With fatigue, integrated EMG activity increased strongly and functional (force) output of the muscle remained stable or decreased. Fatigue results in a less efficient muscle process. Muscles subjectively testing "Weak" or "Strong" yield effects significantly different from fatigue.