Mechanical demand distribution during multi-joint tasks.

摘要

The forceful extension of the hip, knee and ankle, in order to vertically displace the center of mass is common in many activities, including squatting, stepping, and jumping. Moreover, the magnitudes of the demand at the total body level often change during these activities. It was hypothesized that net joint kinetics are scaled proportionally in order to meet the changing demands imposed by the task. The objective of this dissertation was to determine if the relative contributions of the hip, knee and ankle were independent of external loading conditions or movement speeds. To achieve this goal, three experiments were conducted. In the first experiment, it was established that the average net joint moment (NJM) had the highest association with work at the center of mass (COM), while average NJM power had the highest association with power at the COM. In the second experiment, it was determined that, contrary to the initial hypothesis, the contributions of the hip and ankle increased with external resistance, while the contribution of the knee decreased. This redistribution of demand was associated with an anterior shift in the average distance from the ankle joint center to the location of the center of pressure. In the third study, the contributions of the average NJM power at the hip, knee, and ankle to the vertical displacement of the center of mass under rapid conditions were determined to be dependent on movement speed. Contrary to the results of the first study, the contributions of the knee increased with increased movement speed, while the contributions of the ankle and hip decreased. The redistribution was associated with a posterior migration of the location of the COM with respect to the ankle joint center. This strategy was evident when the external resistance was increased, but the magnitude was not as great. Overall, the results suggest that competing mechanisms exist for controlling external resistance and movement speed, which lead to a redistribution of mechanical demand across the three joints of the lower extremity.