| These studies attempted to characterize the mechanical behavior of skeletal muscle following injury, utilizing both experimental and theoretical methods. Experiments were conducted to create and evaluate an eccentric muscle stretch injury, or “muscle pull”, in the Tibialis Anterior muscle of living rabbits. Modifications to an existing experimental system increased the control and reproducibility of the imposed mechanical insult. Both “moderate” and “large” injuries were studied in vivo. Linear analysis of tensile test data showed that stiffness and load at failure appeared to follow the same trend: the largest reduction in injured muscle at day 1, followed by a steady decrease in the injured/control difference 3 and 7 days post injury.; To include the stiffening and softening behavior of skeletal muscle, a nonlinear Maxwell fluid model was developed. This model was combined, in parallel, with a contractile element and a length-dependent linear spring to produce a phenomenological model of skeletal muscle capable of describing both active and passive nonlinear large deformation responses.; This novel viscoelastic model was fit with data of non-destructive uniaxial elongations. These experiments were conducted in the muscle's physiological range, for three different stretch rates, in active, passive and chemically relaxed muscle. Statistically significant effects of stretch rate and stimulation state were observed solely in the second-order spring constant. Additionally, the muscle load-time response appeared to flatten, or reduce order, upon stimulation.; The nonlinear model was also fit with the data from the injury experiments, in an effort to improve on the prior linear treatment. The post hoc analysis demonstrated a statistically significant decrease in the percent difference of the linear spring constant following injury, as well as notably different low-load behavior between injured and contralateral control muscles. Interestingly, the behavioral variations in the toe-in region produced only negligible changes in the linear-range slope, such that a linear analysis would fail to observe the differences.; These studies have demonstrated the need for nonlinear analyses for the study of skeletal muscle's mechanical behavior and recovery from injury, and have developed a unique, nonlinear, phenomenological model to do so. |
