PURPOSE: In the literature, substantial decreases in power output in jumping have been described for both unloading and loading, and these have been attributed to the intrinsic force-velocity-power relationship of muscle. The purpose of this study was to gain a solid understanding of how and why unloading and loading affect power output during jumping. METHODS: Vertical jumps were simulated with a model of the musculoskeletal system, consisting of four rigid segments actuated by six muscles. Muscle stimulation over time was optimized to ensure maximal performance in each loading condition. RESULTS: It was found that, in contrast to what is reported in the literature, unloading by an extra vertical force of -60% of body weight caused a small increase in the peak of the rate of change of the effective energy of the center of mass (dEeff/dt). Loading by an extra vertical force of +60% of body weight caused a decrease in peak dEeff/dt, but this decrease was much smaller than that described in the literature. The small variations in peak dEeff/dt among loading conditions in the simulated jumps were only in part due to the intrinsic force-velocity-power relationship of muscle. Why did the effects of unloading and loading in the simulation model deviate from effects reported in subjects? One possible explanation is that subjects tend to make a smaller countermovement when loaded; in the simulation model, making a smaller countermovement caused a major reduction in peak dEeff/dt. A second possible explanation is that subjects cannot quickly optimize their control and therefore produce submaximal power output in unfamiliar loading conditions. CONCLUSION: The effects of unloading and loading are due only in part to the intrinsic force-velocity-power relationship of muscle. © 2014 by the American College of Sports Medicine.