Skeletal muscle shape influences joint torque exertion through the mechanical advantages

Skeletal muscle morphology is linked to its function. Extensive literature demonstrates that muscle volume is crucial for determining joint torque exertion, a primary function of muscle. However, whether muscle shape also influences torque exertion capacity remains unclear. This study illustrates that the three-dimensional shape of muscles independently determines joint torque exertion, irrespective of muscle volume, using a statistical shape model designed to quantify muscle shape features. The statistical shape model was developed from magnetic resonance images of the quadriceps femoris muscles in 33 healthy young adults (26 ± 5 yr; 18 males). We investigated the association between the shape components of each quadriceps femoris head and isometric knee extensor torque. The findings reveal that the mediolateral curvatures of the rectus femoris (<i>R</i>² = 0.60) and the bulging in the distal region of the vastus medialis (<i>R</i>² = 0.65) were associated with increased knee extensor torque despite muscle volumes. Moreover, the rectus femoris and vastus medialis shapes were correlated with the medial-directed line-of-action (<i>r</i> = -0.42 and ρ = -0.36). The vastus medialis shape was correlated with the moment arm of the patellar lateral spin (ρ = 0.45). Therefore, the three-dimensional muscle shape determines the joint torque exertion by forming the mechanical advantages that balance the force/torque output optimally. Our findings demonstrate that muscle shape is crucial in the mechanical output of skeletal muscle and provides a framework for enhancing the understanding of muscle morphology and its functionality.<b>NEW & NOTEWORTHY</b> Here, we developed a statistical shape model, a geometric model that can quantify muscle morphology, particularly the quadriceps femoris muscle, to determine the influence of three-dimensional muscle shape on its force-generating capacity in young adults. The results revealed that curvature of the rectus femoris and bulging of vastus medialis were determinants of isometric knee extension strength, coupled with their muscle volumes. This morphological functionality relies on the critical relationship between muscle shape and mechanical advantage.