The forceful extension of the hip, knee, and ankle in order to vertically displace the body’s center of mass is common in many activities, including squatting, stepping and jumping. This movement is so common that it may represent a generalized motor program. The GMP hypothesizes that the net joint kinetics are scaled proportionally in order to meet the demands imposed by the task. Although exercise interventions are usually based on this premise, it has never been determined experimentally that this is how the movement is controlled. The objective of this dissertation was to determine if the relative contributions of the hip, knee, and ankle are independent of external loading conditions or movement speeds. To achieve this goal, three experiments were conducted, hi the fust experiment, the associations between the mechanical objectives of two movements (one to perform work and the other to produce power at the center of mass) and six measures of lower extremity function were established. For the second experiment, a detailed analysis of the contributions of the average net joint moments at the hip, knee and ankle toward the vertical displacement of the center of mass under four different loading conditions was conducted to test the hypothesis that these contributions would be independent of the external resistance. Contrary to this hypothesis, the contributions of the hip and ankle increased with external resistance, while the contribution of the knee decreased. This redistribution of demand was accompanied by an anterior shift in the average distance from the ankle joint center to the location of the center of pressure. Additional analyses revealed that the average hip net joint moment had the greatest contribution under each loading condition, and showed the least amount of variability. The average ankle net joint moment increased in response to not only the increased ground reaction force, but also the increase in distance from the ankle joint center to the center of pressure. The average ankle net joint moment also showed a high degree of variability, which was highly correlated to the variability in the location of the center of pressure. In the third study, the contributions of the average net joint moment power at the hip, knee, and ankle to the vertical displacement of the center of mass under rapid conditions were determined. Like the first study, the contributions were not independent. Contrary to the results of the first study, the contributions of the knee increased with increased movement speed, the contributions of the ankle decreased, and the contributions of the hip remained constant. This redistribution was associated with a posterior migration in the location of the center of pressure with respect to the ankle joint center. This strategy was evident when the external resistance was increased, but the magnitude of the differences was not as great. Overall, the results suggest that competing mechanisms exist for controlling external resistance and speed.