Title: How do we know what muscles do?

Abstract: Muscles actuate movement by development of force, but it remains poorly understood what role individual muscles play in the formation of normal movement. Even less is known about muscle function/dysfunction in pathological movement. Muscle-driven simulations enable us to delve deeper than what can be observed from motion capture experiments by providing estimates of muscle forces that satisfy both observations and physics. The challenge, however, is to analyze a simulation to determine the accelerations caused by each muscle. The crux of this challenge is to separate out the contributions of individual active forces to the system acceleration in the presence of reactive forces. We investigate an approach that substitutes contacts with ideal constraints such that reaction forces are solved for simultaneously as Lagrange multipliers of a constrained mechanism. Unlike previous implementations, several constraint definitions are permitted and explored as replacements for contact forces and applied to human gait where startling differences in muscle function were obtained. I will discuss the implications of these findings.