Assistive exoskeletons have the potential to aid locomotor recovery and restore walking function in neuromuscularly and musculoskeletally impaired individuals (e.g. post-stroke or spinal cord injury). They also may be used to augment locomotor performance in healthy individuals by reducing the metabolic cost of locomotion or reducing skeletal loading. Whilst these devices have been shown to be effective in replacing joint moments and powers, little is known about how they influence the underlying muscle function. I have collected experimental data on humans examining how ankle exoskeletons affect Soleus muscle function. The purpose of this proposed work is to: (1) Develop an OpenSim-based model of hopping with spring-loaded ankle exoskeletons; (2) Verify that model with the experimental data; and (3) Use the model to examine the mechanics of other muscle groups within the leg.