In cricket fast bowlers an increased incidence of stress fractures or lesions in the L4 pars interarticularis is observed, which shows a strong statistical correlation with the presence of hypertrophy in the contralateral Quadratus Lumborum (QL) muscle. This study aims to find a physical explanation for this correlation. A mathematical model was used to estimate the forces and moments on the L3 and L4 vertebrae in six postures attained during fast bowling. These forces and moments were used in finite element models to estimate the stresses in the pars interarticularis. Two scenarios were examined per posture: symmetric QL muscles, and right QL muscle volume 30% enlarged. Influence of muscle activation was also investigated. QL asymmetry only correlates with significant stress increases when stress levels are relatively low. When stress levels are high, due to extreme posture or muscle activation, asymmetry only causes small stress changes, suggesting that asymmetry is not the cause of stress fractures in the pars. There are even indications that asymmetry might help to reduce stresses, but more detailed knowledge of the size and activation of the lumbar muscles is needed to confirm this.