Meeting Abstract
Muscle is energetically versatile, achieving widely varying work outputs depending on the task. Many steady state measurements of muscle properties (force-length, force-velocity, and twitch response) are used to predict how it will behave under dynamic conditions. However, even when these properties are known, it can still be impossible to predict dynamic workloops. Two muscles in the cockroach, Blaberus discoidalis have nearly identical classical steady state muscle properties and share innervation, but one is a brake and one is a bifunctional motor. Previously, we discovered the first consistent difference between these two muscles – a 1 nm difference in the actin-myosin spacing within their sarcomeres. However, it is still unclear how these passive differences could affect muscle dynamically without causing differences in their steady state macroscopic behavior. Using the BioCAT x-ray beamline at the Advanced Photon Source at Argonne National Lab, we measured the lattice spacing of both muscles during dynamic workloops and isometric twitches. We found that in isometric twitches, one muscle’s lattice spacing increased to the same value as the other muscle, causing the spacing difference to vanish under steady state conditions. A difference in spacing under passive but not active conditions means that during cyclic workloops the lattice spacing changes were larger in one muscle than the other. This increased transient correlated with an earlier and higher rise in force that enables a region of positive work in the workloop. A 1 nm transient difference in actin-myosin spacing mediates the difference in the work output of the two cockroach muscle but also predicts their identical quasi-static properties. These results indicate that multiscale dynamics from the nanoscale to the macroscale can mediate categorical changes in muscle function during locomotion.
