Fatigue-induced changes of impedance and performance in target tracking

Abstract

Kinematic variability is caused, in part, by force fluctuations. It has been shown empirically and numerically that the effects of force fluctuations on kinematics can be suppressed by increasing joint impedance. Given that force variability increases with muscular fatigue, we hypothesized that joint impedance would increase with fatigue to retain a prescribed accuracy level. To test this hypothesis, subjects tracked a target by elbow flexion and extension both with fatigued and unfatigued elbow flexor and extensor muscles. Joint impedance was estimated from controlled perturbations to the elbow. Contrary to the hypothesis, elbow impedance decreased, whereas performance, expressed as the time-on-target, was unaffected by fatigue. Further analysis of the data revealed that subjects changed their control strategy with increasing fatigue. Although their overall kinematic variability increased, task performance was retained by staying closer to the center of the target when fatigued. In conclusion, the present study reveals a limitation of impedance modulation in the control of movement variability.

Running injury and stride time variability over a prolonged run

“…One possible explanation provided for this reduction is an increased need for adjustments occur with fatigue. This explanation is plausible given loss of motor unit firing and force production and the more variable motor unit firing that occurs with fatigue [16,20,21]. Interestingly, no significant differences existed between the middle and end portions of the run.…”

“…In a study of upper extremity target tracking, fatigue increased distributional measures of movement variability [16]. Temporal long-range correlations have been specifically examined in standing posture and a seated repetitive push/pull tasks.…”

Temporal strategy and performance during a fatiguing short-cycle repetitive task

“…Earlier studies, however, have shown inconsistent effects of fatigue on performance. In a tracking task, performance in terms of percentage time on target was not affected after a fatigue protocol (Selen et al 2007), whereas Huysmans et al (2008) showed a decrease in performance during a similar task. Also studies of activities closer to real occupational tasks have shown conflicting effects of fatigue.…”

Shoulder muscle loading and task performance for overhead work on ladders versus Mobile Elevated Work Platforms

“…This may result in recruitment of other muscles (Cote et al 2002;Selen et al 2007) in the exertion. Antony and Keir (2009) concluded that performing a simultaneous shoulder exertion and hand grip led to posture specific redistribution of shoulder muscle activity for both isometric and dynamic conditions.…”