Meeting Abstract
Skeletal muscle is a composite of fluid-filled muscle fibers and fibrous connective tissues. Recent evidence suggests that the interaction of incompressible cells and tensile extracellular collagen influences fundamental mechanical behaviors of muscle, such as the development of tension during passive deformation. We hypothesized that similar interactions between fluid and collagen could influence muscle force produced during active contraction. To test this, we applied 5 psi squeezes to isolated bullfrog muscles mid-contraction and measured the effect on force. Muscles were squeezed using a small, rapidly inflatable cuff that surrounded the middle third of the muscle belly. In separate experiments, we measured the effect of squeezing a series of physical models of muscle. Models consisted of fluid-filled latex tubes (representing muscle fibers) reinforced by stiff, helically-wrapped thread (representing extracellular collagen fibers). The angle at which collagen wraps muscle fibers varies as a function of muscle length, and models were built with a range of physiologically realistic wrapping angles to represent muscle at various lengths. Squeezing muscle at short lengths reduced isometric contractile force by as much as -11.8% ± 0.05% (average ± SD at 0.9 L0), while squeezing muscle at longer lengths had a neutral or slightly positive effect on force (ex. +3.1% ± 0.01% at 1.3 L0). This pattern was replicated by the physical models, which either decreased or increased force depending on the wrapping angle of their reinforcing fibers. Our results suggest that the distribution and pressure state of fluid within muscle are mechanically important, and that their effect on isometric contractile force depends on the geometry of fibrous collagen in the extracellular matrix.
