Closed-loop coupling of a muscle to a robotic device for dynamic assessment of muscle function

Abstract

The contributions of individual muscles to the performance of functional tasks are difficult to evaluate using traditional isolated muscle protocols. During movements, skeletal muscles work against a variety of environmental loads that influence their energetics and function. In turn, these changes in muscle length and muscle velocity alter the forces that the muscle can generate. Classic single-muscle experiments clamp at least one muscle state (length, velocity, force) such that it is independent of the other states, interrupting the dynamic interactions between the muscle and its environment. The purpose of this study was to design and build a real-time feedback system to virtually couple an isolated muscle to a robotic device. Using this approach, the muscle length is not prescribed, but results from the dynamic interactions between the muscle and a physical environment. Therefore our device facilitates the study of how physical interactions between a muscle, limb, and environments alter the force and motion produced by the muscle during controlled muscle activation. To demonstrate the utility of our system, we replicated some salient features of frog swimming, we coupled a frog plantaris longus muscle to a one-degree of freedom \"limb\" that drove a frog foot through water. We demonstrate that under identical muscle stimulation parameters, changes to muscle moment arm, environmental viscosity, and muscle fatigue can significantly alter the resulting muscle force, length, and work.