Mechanical properties of the avian acrocoracohumeral ligament and its role in shoulder stabilization in flight

Control of movement in the avian shoulder joint is fundamental to understanding the avian wingstroke. The acrocoracohumeral ligament (AHL) is thought to play a key role in stabilizing the glenoid and balancing the pectoralis in gliding flight. If the AHL has to be taut to balance the pectoralis, then it must constrain glenohumeral motion during flapping flight as well. However, birds vary wing kinematics depending on flight speed and behavior. How can a passive ligament accommodate such varying joint movements? Herein, mechanical testing and 3-D modeling are used to link the mechanical properties and morphology of the AHL to its functional role during flapping flight. The bone-ligament-bone complex of the pigeon (Columba livia) fails at a tensile loading of 141 ± 18 N (± s .D., n = 10) or 39 times body weight, which corresponds to a failure stress of 51 MPa, well above expected loads during flight. Simulated AHL length changes, comparisons to glenohumeral kinematics from the literature, and manipulations of partially dissected pigeon specimens all support the hypothesis that the AHL remains taut through downstroke and most of upstroke while becoming slack during the downstroke/upstroke transition. The digital AHL model provides a mechanism for explaining how the AHL can stabilize the shoulder joint under a broad array of humeral paths by constraining the coordination of glenohumeral degrees of freedom.