Different strategies to compensate for the effects of fatigue revealed by neuromuscular adaptation processes in humans M. Bonnard, A.V. Sirin, L. Oddsson, A. Thorstensson

Introduction In daily tasks, many times we need to maintain stable performance levels, which can gradually increase fatigue Compensatory mechanisms at various levels of the neuromuscular system act to delay the effects of fatigue so that the motor activity can be prolonged

Task for this study Subjects were required to maintain an initially submaximal hopping activity as long as possible (until the task became the maximal performance) at a constrained global power output (hopping frequency and amplitude were imposed)

Justification Can show within-joint versus between joints compensating mechanisms: Ankle extensors are the main contributors to the vertical propulsion in this task, and with prolonged activity the ankle extensor muscles will most likely be affected by fatigue, which may lead to reorganization of neuromuscular control between the joints of the leg.

Study Aim Analyze the differences in movement patterns and EMG activity of the main extensor leg muscles at the beginning of the hopping activity and just before the subjects became unable to maintain their activity any longer at the constrained frequency and amplitude.

Methods 4 active and healthy adult subjects Instructed to maintain a two-legged hopping task for as long as possible Hopping frequency was set at 2Hz Hopping amplitude was controlled at 30% of the maximal hopping height

Methods Continued The four subjects were able to maintain the hopping task for 23, 34, 37, and 44 minutes. Data was recorded for the first 2 minutes and just before subjects stopped Electrogoniometer, force plate, 3-D motion capture, and EMG

Results: Inter-Joint Compensation Ankle maximal negative moments increased significantly Knee maximal negative moments were not significantly different Ankle maximal positive moment increased, but does not compensate for the increase in the maximal negative moment at the ankle Knee maximal positive moment increased

Results: Hopping Cycle The total hopping cycle duration was not different when comparing pre and post- fatigue. The relative duration of the ground contact phase slightly decreased

Results: Kinetics Peak vertical force increased and occurred earlier during the contact phase when fatigued Center of pressure location did not change as a result of fatigue

Results: Kinematics When fatigued, subjects landed with knees more flexed Knee and ankle joints reached a smaller minimum angle during the ground contact phase Maximal flexion of the ankle occurred earlier with fatigue With fatigue, the ankle began extension before the knee and hip, compared to without fatigue, where all 3 joints extended simultaneously

Results: EMG IEMG decreased in the medial gastrocnemious during the eccentric phase and the pre- activation phase, but vastus lateralis activity increased No change in IEMG during the eccentric phase, but there was an earlier activation of the medial gastrocnemious prior to ground contact

Discussion This study shows that hopping can be maintained with the same global power output for extended periods of time, with relatively small changes in movement pattern All subjects increased IEMG in the biarticular rectus femoris

Discussion This study suggests 2 different strategies are effective to maintain a hopping task mainly involving the ankle extensor muscles for extensive periods of time – Preactivation at the medial gastrocnemious before ground contact – Trade-offs between muscles across joints and operates during the eccentric phase