CHARACTERIZATION OF THE PASSIVE RESPONSES OF LIVE SKELETAL-MUSCLE USING THE QUASI-LINEAR THEORY OF VISCOELASTICITY

The tensile viscoclastic responses of live, innervated rabbit skeletal muscle were measured and characterized using the quasi-linear model o f viscoelasticity. The tibialis anterior (TA) and extensor digitorum l ongus (EDL) muscles of anesthetized New Zealand white rabbits were sur gically exposed and tested under in vivo conditions. Rate sensitivity of the force time history was observed in response to constant velocit y testing at rates from 0.01 to 2.0 Hz. Average hysteresis energy, exp ressed as a percentage of maximum stored strain energy, was 39.3 +/- 5 .4% and was insensitive to deformation rate. The quasi-linear model, w ith constants derived from relaxation testing, was able to describe an d predict these responses with correlation exceeding the 99% confidenc e interval for the 132 constant velocity tests performed (r(mean) = 0. 9263 +/- 0.0373). The predictive ability of this model was improved wh en compressive loading effects on the muscle were neglected, r(mean) = 0.9306 +/- 0.0324. The rate insensitivity of hysteresis energy was pr edicted by the model; however. the absolute value of the hysteresis wa s underestimated (30.2 +/- 4.0%). Both muscles demonstrated strikingly different elastic functions. Geometric normalization of these respons es (stress and strain) did not result in a single elastic function cap able of describing both muscles. Based on these results. the quasi-lin ear model is recommended for the characterization of the structural re sponses of muscle; however. further investigation is required to deter mine the influence of muscle geometry and fiber architecture on the el astic function.