BAIER, David B.; GATESY, Stephen M.; Brown University; Brown University: Shoulder Mechanics in Gliding Birds: Implications for the Evolution of Forelimb Aerodynamics

The avian shoulder joint is highly mobile to permit flapping, yet also stable enough to prevent dislocation. Bony and soft tissues making up the joint must accommodate loads to balance muscular, aerodynamic, gravitational, and inertial forces on the wing. Steady-speed gliding is the simplest scenario for understanding this force balance. Classical models hypothesize that the upward aerodynamic force on the outstretched wing is balanced by a downward pectoralis force. These opposing torques require the coracoid to be loaded by the humeral head as a compressive strut. However, the saddle-shaped glenoid provides no ventral articular surface to prevent dislocation by the massive pectoralis. A ligament spanning from the acrocoracoid process to the humerus (the acrocoracohumeral ligament) is well positioned to act as a tensile brace and counter dislocation. We propose that this ligament is a critical structural component in maintaining the integrity of the shoulder joint. On the line to birds, the glenoid reoriented from its ancestral archosaurian condition and aerodynamic forces on the forelimb became increasingly important. Understanding the relative magnitudes and balance among internal and external forces will help constrain scenarios for the origin of flight.(Supported by NSF grant DBI 9974424 to SMG).