Physical Activity, Exercise Physiology, Movement
Effects of neuromuscular fatigue on leg stiffness and joint stiffness during single-leg hopping
(School of Public Health (UMD) Kinesiology Doctoral Student)
The spring-mass model is able to accurately represent hopping spring-like behavior (leg and joint stiffness), and leg and joint stiffness changes can reveal overall motor control responses to neuromuscular fatigue. By understanding leg and joint stiffness modulation, we can determine which variables the nervous system targets to maintain motor performance and stability. Objective: The purpose of this study was to determine how neuromuscular fatigue and the neural and muscular factors of fatigue affect hopping behavior by examining leg and joint stiffness before and after a single-leg calf raise fatiguing protocol. Post-fatigue, leg stiffness decreased for the exercised leg, but not for the non-exercised leg. Approach: Fifteen healthy adults participated in this study. Participants performed single-leg hopping, matching a metronome frequency of 2Hz, for a duration of 30 sec, for each leg before and after an ankle fatiguing protocol for one limb. The fatiguing protocol consisted of 15 sets of 10 repetitions of single-leg calf raises with a 1 sec hold at max plantarflexion on an elevated surface with 30 sec of rest between sets. Leg stiffness was calculated as the change in vertical ground reaction force relative to the center of mass displacement, and joint stiffness for each joint was calculated as the joint moment relative to joint angular displacement. Findings: Ankle and knee joint stiffness did not significantly change for either leg. This indicates that leg stiffness decreases primarily from muscular fatigue, but was not explained by ankle and knee joint stiffness. Conclusion: The decrease in leg stiffness may be an attempt to soften landing impact, while at the same time maintaining performance. Public Health Significance: By understanding the body’s motor control response to fatigue, exercise training and recovery programs can be altered to decrease fatigue and increase recovery time.