The Role of Muscle Tensing on the Force-Deflection Response of the Thorax and a Reassessment of Frontal Impact Thoracic Biofidelity Corridors

Abstract

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 (53.9 ± 2.9 cm/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, a fact which is supported by the limited human data available. The peak force increases with muscle tensing for chest deflection levels up to about 20 per cent, 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. A simplified finite element model is used to elucidate the mechanisms for the experimental findings. This model shows that the changes in force-deflection response of the thorax that occur with muscle tensing are due primarily to the increased modulus of the muscle tissue itself, rather than to any stiffening effect from pretensioning the muscle membrane around the bony structure. The findings are discussed relative to current thoracic biofidelity corridors, which include an adjustment to account for muscle tensing.