One of the major (but the least studied!) functions of skeletal muscle is protecting the skeletal system from external impacts by absorbing and redistributing the energy of mechanical shock in time and space. During muscle contraction, its elasticity modulus is greatly increased, which partly unloads adjacent bones and skeletal joints. Muscle viscosity is also greatly increased helping to absorb and dissipate dangerous shocks. Elasticity and viscosity data may be obtained using the measurement of shear wave velocity and attenuation. In this study, we investigated changes in the velocity and attenuation of shear acoustic waves in an anisotropic tissue phantom mimicking skeletal muscle under different level of tension of the fibers imbedded in the phantom. Stretching the fibers simulates the muscle contraction. It is shown that both velocity and attenuation of shear waves propagating along the fibers significantly increase with fiber tension while they are negligibly affected in the case of wave propagation across the fibers. We developed a theory for propagation of shear waves in anisotropic medium simulating the muscle and muscle contraction. Equations for the speed and attenuation of various modes of shear acoustic waves are derived. Theoretical predictions are in agreement with experimental data. [NIH R21AR065024.]