Physical basis for the shape of muscle work loops

The periodic trajectory taken by a muscle on a force-length plot, i.e. a work loop, is an important measure of its function that arises from its active force generation and resistance to motion. Several types of work loops are routinely observed across different muscle types and across species: a large clockwise loop if work is done on the muscle and dissipated as heat, a large counter-clockwise loop if the muscle does work on its environment, or a figure-8 loop if no work is done and the muscle acts as a mechanical stabilizer. We present here a general construction of work loops, using ellipses formed from storage and loss moduli as an illustrative example, that allows for a qualitative classification of muscle work loops based on its shape into different mechanical behaviours.