Biomechanical analysis of the human head‐neck‐shoulder complex: Asymmetry as a natural experiment

Abstract

The unique human mastoid processes and clavicles are a consequence of the shoulder girdle being suspended from the skull. When one arm is used, the upper trapezius and sternocleidomastoid muscles contract to counterbalance the added weight on that shoulder. Core muscles acting on the atlanto‐occipital and cervical joints synchronously contract to prevent retroflexion of the head. Our hypothesis was tested by a natural experiment based on the asymmetry of biomechanically relevant paired skeletal features in right‐handed males. Our biomechanical free‐body diagram analyses show that asymmetrical muscle forces act on the mastoid processes, skull, and clavicles when one arm is favored and predict that these forces, over time, stimulate different bone growth at their attachment sites. Quantitatively, our data show that at least 70% of the individuals had a wider right and a longer left mastoid process; a higher and/or more robust right superior nuchal line; a broader left scapula; and/or a greater right humerus diameter. Qualitatively, however, each individual displayed a unique combination of at least two of these features indicative of right‐handedness. Individual differences are probably caused by long‐term force regimes due to idiosyncratic movements and athletic or professional activities.Funding: LSU Robert C. West Fund to MLO; LSU Foundation to DGHGrant Funding Source: LSU Foundation