77.5 Sunday, Jan. 6 Three-dimensional strain patterns in aponeuroses AZIZI, E*; ROBERTS, TJ; Brown University; Brown University email@example.com
In many muscles, force generating fibers insert onto broad tendinous sheets (aponeuroses). Muscle force and displacement can act to stretch the aponeurosis along the muscle’s line of action (longitudinal), storing elastic energy, which can be recovered as the tissue is unloaded. Unlike tendons, aponeuroses cover a large proportion of a muscle’s surface and therefore must also expand in directions orthogonal to the muscle’s line of action (transverse) as a shortening muscle swells to maintain a constant volume. Here we characterize three-dimensional deformations in aponeuroses in order to understand the determinants of mechanical loading in this biological spring. Since muscle forces are oriented along the line of action, we hypothesized that contraction force determines longitudinal strain in the aponeurosis. Transverse strains may be influenced more by changes in fiber diameter during contraction. Thus, we hypothesized that transverse strain is proportional to muscle fiber shortening. To test these hypotheses we used an in situ muscle preparation in the lateral gastrocnemius of wild turkeys. Muscle fiber length was measured with sonomicrometry and muscle force and displacement were measured with a servomotor during contractions at varying force levels. In addition, the aponeurosis was covered with an array of radio-opaque markers and imaged using high-speed biplanar fluoroscopy. The three-dimensional positions of the markers were used to quantify instantaneous aponeurosis strain during contractions. Across all contractions, transverse strains were consistently higher than longitudinal strains. Longitudinal aponeurosis strain increased with increasing muscle force, as predicted. Transverse strain, however, was not simply determined by fiber strain. We speculate that transverse strains may also be influenced by the tissue’s Poisson’s ratio (long. strain/trans. strain) at high forces.