Soft tissue constraints on joint mobility in the avian shoulder

Joints and their surrounding soft tissues facilitate and restrict vertebrate skeletal motion. Measures of maximal joint mobility provide insight into articular function and its limits on potential joint motion and thereby behaviour. In extinct vertebrates the reconstruction of joint mobility permits us to decipher shifts in locomotor evolution. Such measurements are generally limited to studies of osteological joint mobility. However, only a subset of osteologically feasible poses are biologically feasible because true joint mobility is limited by soft tissues, such as ligaments, that are seldom preserved in the fossil record. To address this issue, we implemented an in silico model to simulate avian glenohumeral (shoulder) movement and the constraints imposed by six ligaments on its joint mobility. We evaluated our in silico model of the partridge shoulder joint with measured ex vivo shoulder mobility using X-ray reconstruction of moving morphology (XROMM). Our results indicate that modelling ligamentous constraints is integral to accurately quantifying shoulder function owing to the role of ligaments in maintaining articular contact during complex glenohumeral motion. Our approach enables more confident estimates of functional joint mobility in both extant and extinct vertebrates and thereby stands to improve inferences of behaviour and musculoskeletal function in the vertebrate fossil record.