Stimulated and voluntary fatiguing contractions of quadriceps femoris differently disturb postural control.

Muscle fatigue affects muscle strength and postural control. However, it is not known whether impaired postural control after fatiguing muscular exercise depends on the nature of the muscle contraction. To answer this question, the present study analyzes changes in postural control after two fatiguing exercises of equal duration and intensity but that induced different magnitudes of strength loss. The effects of fatiguing contractions of the femoris quadriceps were compared for voluntary muscular contraction (VOL) and neuromuscular electrical stimulation (ES) on muscle strength and postural control. Seventeen subjects completed these two fatiguing exercises. Maximal voluntary contraction (MVC) and postural control were recorded using an isokinetic dynamometer and a force platform that recorded the center of foot pressure. Recordings were performed before and after the completion of both fatiguing tasks. Results indicate that, after a fatiguing exercise, the ES exercise affected MVC more than the VOL exercise. Inversely, postural control was disturbed more after VOL exercise than after ES exercise. In conclusion, postural control disturbance is influenced by the nature of the muscular contraction (voluntary vs. non-voluntary) and the type of the motor unit solicited (tonic vs. phasic) rather than by the magnitude of strength loss.