Preferred rates of limb movement in locomotor tasks have long been associated with energy optimization. In human walking, the preferred stride rate at most walking speeds results in minimal energy expenditure. In cycling, however, the preferred cadence is often 50% higher than the energetically-optimal rate. Mechanical and energetic properties of the lower limb muscles have often been suggested as determinants of preferred rates in human locomotion, but have received little direct attention. The present research sought to better understand how muscle properties influence optimal and preferred rates o f limb movement. In Study One, a model of muscle energy expenditure was developed and validated that permits estimates of metabolic energy during simulations of human movement. In Study Two, computer simulations were used to examine the effects of muscle fiber type distribution on the pedaling cadences which minimize energy expenditure and maximize power output. It was found that a greater proportion of fast-twitch muscle fibers leads to higher optimal cadences for both submaximal energy expenditure and maximal power output. Further analysis revealed that differences in mechanical efficiency of the leg muscles may be important for cadence selection. In Study Three, experimental and simulation techniques were used to investigate the effects of stride rate on minimization of energetic and mechanical demand in walking. Experimentally, the mechanical demand placed on the lower limb muscles was minimized at a stride rate close to, but lower than (by 4-5 stride-min'1), the preferred and energetically-optimal rates, which were not different from each other. Simulations further revealed that metabolic cost during walking was dominated by the cost of generating muscular force, and this factor was influential in determining the minimum energy stride rate. The results suggest that humans may select a cycle rate in locomotor activities that comes close to minimizing the energetic and/or mechanical loads on the muscles primarily responsible for producing the movement. During walking or pedaling, an individual may choose a cycle rate that minimizes the demands placed on the lower limb muscles, even if this results in greater whole body energy expenditure as in cycling.