The passive tensile mechanical properties of living rabbit skeletal muscle were investigated in these studies. An experimental test system was developed to study these responses under in vivo conditions. Techniques were also developed to quantify local tissue deformations and surface strains in order to develop an injury criterion for skeletal muscle.

The time-dependent, nonlinear structural properties of two muscles with different fiber architecture (Tibialis Anterior - TA, and Extensor Digitorum Longus - EDL) were studied. The viscoelastic responses were measured and modeled using a quasi-linear viscoelastic constitutive relation with a continuous relaxation spectrum. Rate sensitivity of the force-time history was observed in response to constant velocity testing at rates from 2 Hz to 0.01 Hz. Average hysteresis energy, expressed as a percentage of maximum stored strain energy, was 39.3 ±5.4%, and was relatively insensitive to deformation rate. The quasi-linear model, with constants derived from relaxation testing, was able to describe and predict the measured constant velocity responses with correlation exceeding the 99% confidence interval for the 132 constant velocity tests performed (r_mean = 0.9263 ±0.0373). The rate insensitivity of hysteresis energy was predicted by the model;​ however, the absolute value of the hysteresis was underestimated (30.2 ± 4.0%). The two muscles demonstrated strikingly different elastic responses. Geometric normalization of these responses (stress and strain) did not result in a single elastic function for both muscles. This suggests that the elastic function depends on internal structure (fiber orientation) as well as the geometry of the muscle.

The effects of loading rate on the structural failure properties of both muscles was also investigated. It was found that structural properties (load to failure, total deformation, and energy absorption) were rate sensitive, however maximum axial strain at the site of injury was insensitive to loading rate for both muscles. It was concluded therefore that the St. Venant or Maximum Strain Theory applies locally at the site of injury. Finally, the site of muscle failure was found to vary with the rate of tissue loading and could be accurately predicted by the location of the maximum axial strain.