Mechanics of the avian propatagium: Flexion‐extension mechanism of the avian wing

The supporting elements of the avian propatagium were examined in intact birds and as isolated components, using static force‐length measurements, calculated models, and airflow observations. The propatagial surface supported between Lig. propatagiale (LP) and brachium‐antebrachium is equally resistant to distortion over the range of wing extension used in flight. The lengths LP assumes in flight occur across a nearly linear, low‐stiffness portion of the force‐length curve of its extensible pars elastica. In an artificial airflow, intact wings automatically extend; their degree of extension is roughly correlated with the airflow velocity. Comparisons between geometric models of the wing and the passive force‐length properties of LPs suggest that the stress along LP blances the drag forces acting to extend the elbow. The mechanical properties (stiffness) of the LP vary and appear to be tuned for flight‐type characteristics, e.g., changes in wing extension during flight and drag. Lig. limitants cubiti and LP combine to limit elbow extension at its maximum, a safety device in flight preventing hyperextension of the elbow and reduction of the propatagium's cambered flight surface. Calculations using muscle and ligament lengths suggest that M. deltoideus, pars propatagialis, via its insertions onto both the propatagial ligaments, controls and coordinates propatagial deployment, leading edge tenseness, and elbow/wing extension across the range of wing extensions used in flight. The propatagial ligaments and M. deltoideus, pars propatagialis, along with skeleto‐ligamentous elbow/carpus apparatus, are integral components of the wing's extension control mechanism. © 1995 Wiley‐Liss, Inc.