Work loop

A work loop is a technique used in muscle physiology to evaluate the mechanical work and power output of muscle during cyclical contractions.

Work loops were first used by Josephson to evaluate properties of katydid flight muscles. Previously, attempts to understand muscle function during locomotion relied upon characterizing various aspects of muscle physiology in isolation, which made determining their interactions difficult - for instance, force-velocity relationships are evaluated at constant velocities and loads, which is rarely the case in nature, and power measurements obtained from these tests could not take into account the activation and relaxation times of the muscle (which can comprise a significant time portion of the limb's overall movement cycle). By driving the muscle through cycles of a natural range of motion at a range of frequencies observed in natural behavior, all of these aspects would be integrated in a single resultant graph of force vs. displacement.

Since work is defined as force multiplied by distance, the area of the graph could determine mechanical output of the muscle. In a work-generating instance, the muscle would show a rapid curvilinear rise in force as it shortened, followed by a slower decline during or shortly before the muscle begins the lengthening phase of the cycle. The area under the shortening curve would give the total work done by the shortening muscle, while the area underneath the lengthening curve would represent the work absorbed by the muscle and turned into heat (done by either environmental forces or antagonistic muscles). Subtracting the latter from the former would give the net mechanical work output of the muscle cycle, and dividing that by the cycle duration would give mechanical power output. This technique allowed greater appreciation for the role of activation & relaxing kinetics in muscle power and work output - for instance, if a muscle turns on and off more slowly, the shortening and lengthening curves will be shallower and closer together, resulting in decreased work output. "Negative" work loops were also possible, showing a lengthening curve at higher force than the shortening curve and resulting in net energy absorption by the muscle, as in the case of deceleration or constant-speed downhill walking.

Originally, workloops imposed a sinusoidal length change on the muscle, with equal time lengthening and shortening. However, in vivo muscle length change often has greater than half the cycle shortening, and less than half lengthening. Imposing these "asymmetrical" stretch-shorten cycles can result in higher work and power outputs, as shown in treefrog calling muscles .