Localized muscular fatigue duration, EMG parameters and accuracy of rapid limb movements.

While much is known about the physiological basis of local muscular fatigue, little is known about the kinematic and electromyographic (EMG) consequences of brief fatiguing isometric contractions. Five male subjects performed a horizontal elbow flexion-extension reversal movement over 90° in 250 ms to reversal before and after one of five single maximal isometric elbow flexions ranging in duration from 15-120 s. Surface EMG signals were recorded from the biceps brachii, the long head of the triceps, the clavicular portion of the pectoralis major, and the posterior deltoid. Spatial and temporal errors were computed from potentiometer output. During the fatiguing bouts, maximum voluntary force dropped linearly an average of 4% in the 15 s condition and 58% in the 120 s condition relative to maximum force. The associated biceps rectified-integrated EMG signal increased from the onset of each fatigue bout for 15-30 s, then decreased over the remainder of the longer bouts. Following the fatigue bout, subjects undershot the target distance on the first movement trial in all conditions. Following short fatigue durations (i.e. 15-30 s), the peak biceps EMG amplitude was disrupted and movement velocity decreased, but both measures recovered within seconds. As fatigue duration increased, progressive decreases in peak velocity occurred with increased time to reversal, reduced EMG amplitude, and longer recovery times. However, the relative timing of the EMG pattern was maintained suggesting the temporal structure was not altered by fatigue. The findings suggest that even short single isometric contractions can disrupt certain elements of the motor control system.