This paper evaluates how muscle tensing changes the structural response of the dynamically loaded thorax. Nine porcine thoraces with both ventral (supine) and dorsal (prone) loading were used to quantify the effect. Muscle tensing was assessed using repeated tests on a subject with and without forced muscle contraction. Dynamic (~1.25 m/s) and quasistatic tests were performed with a potentiometer and load cells to calculate effective thoracic stiffness. The results show that the effect of muscle tensing decreases with increasing chest deflection, which is supported by the limited human data available. The peak force increases with muscle tensing for chest deflection levels up to about 20%, after which the peak force changes negligibly when the muscles are tensed. The shape of the force-deflection curve, and therefore the work done by the deforming thorax, do, however, depend upon muscle tensing regardless of the peak deflection level attained. The findings are discussed relative to current thoracic biofidelity corridors, which include an adjustment to account for muscle tensing.