Meeting Abstract
Muscles are actuators that exert forces to influence animal movement, but are also active, soft materials that exert forces in response to external perturbations. Rheology, the study of deformation of matter, uses sinusoidal perturbations as an important tool in the study of soft matter, and muscle work loops are a generalization of that tool. Although work loops are often quantified in terms of net work performed by the muscle, the complete shape of a work loop is an important part of a muscle’s effects on animal movement. Muscle rheology varies with both the neural excitation and the external perturbation it receives; the rheology of interest is thus the entire set of possible loop shapes assumed by a muscle across its physiologically relevant excitation patterns and perturbations. We address here the question of classifying loop shapes so as to compare loops measured across different experimental conditions. We present a work loop construction for muscles with a time-varying excitation using ellipses formed from loss/storage moduli as an illustrative example. Each ellipse corresponds to a constant excitation level, and the construction splices together multiple ellipses to form the muscle’s work loop for a time-varying excitation. This motivates a deconstruction of measured loops, if the excitation pattern is known, into ratios of work contributions by elastic, viscous, and ideal forces. All possible loop shapes for muscles undergoing small sinusoidal perturbations are categorized by these ratios. The details differ for other perturbations or if the muscle’s rheology is nonlinear, but the viewpoint of muscle work loops obtained under time-varying excitation as spliced basis loops obtained under constant excitation adds a systematic interpretation of loop shapes and muscle rheology.
