This paper evaluates how muscle tensing changes the structural response of the dynamically loaded thorax. Structural models of two porcine thoraces were used to quantify the effect. A quasilinear viscoelastic formulation was used to model the elastic and viscous response, with ramp-hold tests used to determine the model coefficients. The effect of thoracic musculature was assessed using repeated tests on a subject with and without forced muscle contraction. Even maximally contracted thoracic musculature is shown to have a minimal effect on the response, with similar elastic and viscous characteristics exhibited by each subject regardless of muscle tone.