Effects of fatigue of plantarflexors on control and performance in vertical jumping.

INTRODUCTION We investigated the effects of a mismatch between control and musculoskeletal properties on performance in vertical jumping. METHODS Six subjects performed maximum-effort vertical squat jumps before (REF) and after the plantarflexors of the right leg had been fatigued (FAT) while kinematic data, ground reaction forces, and EMG of leg muscles were collected. Inverse dynamics was used to calculate the net work at joints, and EMG was rectified and smoothed to obtain the smoothed rectified EMG (SREMG). The jumps of the subjects were also simulated with a musculoskeletal model comprising seven body segments and 12 Hill-type muscles, and having as only input muscle stimulation. RESULTS Jump height was approximately 6 cm less in FAT jumps than in REF jumps. In FAT jumps, peak SREMG level was reduced by more than 35% in the right plantarflexors and by approximately 20% in the right hamstrings but not in any other muscles. In FAT jumps, the net joint work was reduced not only at the right ankle (by 70%) but also at the right hip (by 40%). Because the right hip was not spanned by fatigued muscles and the reduction in SREMG of the right hamstrings was relatively small, this indicated that the reduction in performance was partly due to a mismatch between control and musculoskeletal properties. The differences between REF and FAT jumps of the subjects were confirmed and explained by the simulation model. Reoptimization of control for the FAT model caused performance to be partly restored by approximately 2.5 cm. CONCLUSION The reduction in performance in FAT jumps was partly due to a mismatch between control and musculoskeletal properties.