Fig. 4

Representative sarcomere length traces as a function of time for all individual sarcomeres of a single myofibril. The myofibril in this experiment was actively stretched from an initial average sarcomere length on the plateau of the force-length relationship to a final length on the descending limb of the force-length relationship. Note that the individual sarcomeres are at vastly different lengths that are associated with active force differences of up to 100%, but the sarcomere lengths are perfectly stable (constant) despite these predicted force differences. The cross-bridge theory, as well as the sarcomere instability theory predict that the longest (weakest) sarcomeres are pulled quickly beyond actin myosin filament overlap (lengths greater than 3.9 μm in this preparation), at the expense of the shortest (strongest) sarcomeres, but this clearly does not happen. Therefore, there must be stabilizing elements in single, serially arranged sarcomeres in a myofibril that have not been considered in the cross-bridge theory