Foot and ankle structure, particularly as it contributes to the leverage of the plantarflexor muscles as characterized by the Achilles tendon moment arm (ATma), has been the focus of much recent research due to the plantarflexors’ influence on locomotor function. This dissertation investigated the mechanisms that determine the relationship between ATma and locomotor function under different conditions using a combination of modeling and experimental approaches and also considered the interaction with surrounding limb structure and muscle architecture. The first half of this dissertation focused on an isolated plantarflexion task while the second half considered walking gait, a more complex task. The first study used computer simulations of the plantarflexion task to investigate the relationship between plantarflexor gear ratio and muscle demand (defined as the required muscle activation). Optimal gear ratios were found that balanced the conflicting effects of mechanical advantage and muscle dynamics which determine the required plantarflexor force and the ability of the muscles to produce force, respectively. In the second study, a device was designed and built to enable this task to be studied and the results of the simulations to be tested in controlled human experiments. The third study was an initial investigation into the effect of ATma on walking speed and step length that sets up a future study on these effects in elderly gait. The interaction between ATma and plantarflexor function in gait was further explored through simulations of walking at three different speeds in the fourth study. ATma was found to have a speed-dependent effect on plantarflexor demand, with the demand being less sensitive to variations in ATma at faster walking speeds. These results highlight the task dependence of the structurefunction relationship at the ankle and have important implications for locomotion, including the prediction and reduction of mobility loss among older adults and the design of joint implants and surgical interventions that reconstruct the foot and ankle. Further work is needed to expand the findings of this dissertation into more controlled human experiments that directly study the effects of structure on plantarflexor muscle function.