Exoskeletons improve walking economy by steering muscle dynamics

Exoskeletons reduce whole-body net metabolic rate by modifying limb-joint biomechanics. Yet, muscles, not limb-joints, are the primary consumers of metabolic energy during locomotion. Perplexingly, it is unestablished how exoskeletons alter muscle dynamics to improve locomotion economy. We investigated the relationships between the rates of active soleus force, work, and volume to net metabolic rate from participants walking with springy bilateral ankle exoskeletons. Across exoskeleton stiffness (kexo) values, rates of soleus force, work, and active muscle volume explained 50.9, 16.1, and 54.1% of the change in net metabolic rate, respectively (p=0.006). Thus, we suggest that exoskeletons improve locomotion economy primarily by affecting rates of active muscle volume rather than muscle work.