Fatigue induced changes in phasic muscle activation patterns for fast elbow flexion movements

The present study investigated how muscle fatigue influences single degree-of-freedom elbow flexion movements and their associated patterns of phasic muscle activation. Maximal unfatigued voluntary isometric elbow flexor and extensor joint torque was measured at the beginning of the experiment. Subjects then performed elbow flexion movements over two distances as fast as possible, and movements over the longer distance at an intentionally slower speed. The slower speed was close to what would become the maximal speed in the fatigued state. Subjects then performed a fatiguing protocol of 20 sustained isometric flexion contractions of 25 s duration with 5 s rest at 50% maximal unfatigued voluntary force. After a recovery period they repeated the movements. The fatigue protocol was successful in inducing muscle fatigue, the evidence being decreased isometric maximal joint torque of over 20%. Fatigued movements had lower peak muscle torque and speed. Our principal finding was of changes in the timing of the phasic patterns of fatigued muscle activation. There was an increase in the duration of the agonist burst and a delay in the timing of the antagonist muscle as measured by the centroid of the EMG signals. We conclude that these changes serve as partial but incomplete, centrally driven compensation for fatigue induced changes in muscle function. An additional, unexpected finding was how small an effect fatigue had on movement performance when using a recovery time of 10 min that is long enough to allow muscle membrane conduction velocity to return to normal. This raises questions concerning the behavioral significance of classical laboratory studies of human fatigue mechanisms.