Shoulder and forelimb orientations and loading in sitting cats: implications for head and shoulder movement.

A three-dimensional static analysis was carried out to characterize the effects of ground reaction forces propagated through different joints in the forelimb and shoulder of quietly sitting cats. Stereofluoroscopy, used to identify the orientations of bones, showed that the scapula is held in a parasagittal plane, with its vertebral border located dorsally to the spines of thoracic vertebrae. The forelimb is held with the elbow flexed. Loading on the elbow tended to flex and adduct the ulna, whereas loading on the glenohumeral joint tended to extend, adduct and internally rotate the humerus. Loading on the scapula was confined primarily to the sagittal plane and tended to rotate the vertebral border of the scapula caudally around the head of the humerus. This caudally-directed moment suggests that the static equilibrium of the feline scapula depends upon muscular forces directed cranially, presumably by way of attachments on the skull and cervical column. The differing arrangement of the shoulder-to-neck transition in cats and humans suggests caution in the use of feline models to represent some aspects of human head movement.