Stakebake, E.F.; Meyers, R.A.*: The albatross shoulder lock: A biomechanical and immunohistochemical study.

As a postural behavior, gliding and soaring flight in birds requires less energy than flapping flight. Slow tonic and slow twitch muscle fibers are specialized for sustained contraction with high fatigue resistance and are typically found in muscles associated with posture. Albatrosses are the elite of avian gliders; as such we wanted to learn how their musculoskeletal system enables them to maintain spread-wing posture for prolonged gliding bouts. We used dissection and immunohistochemistry to evaluate muscle function for gliding flight in Laysan and Black-footed Albatrosses. Albatrosses possess a locking mechanism at the shoulder; internal and external tendinous sheets extend from origin to insertion throughout the length of the deep layer of the pectoralis muscle. This fascial “strut” passively maintains horizontal wing orientation during gliding and soaring flight. A number of muscles, which likely facilitate gliding posture, were found to be comprised exclusively of slow fibers. These include Mm. coracobrachialis cranialis, extensor metacarpi radialis dorsalis and deep pectoralis. In addition, a number of other muscles, including triceps scapularis, supracoracoideus, and extensor metacarpi radialis ventralis, were found to have populations of slow fibers. We believe that this suite of uniformly slow muscles is associated with sustained gliding and is unique to birds that perform such behaviors. These findings demonstrate that albatrosses utilize a combination of slow muscle fibers and a rigid limiting tendon for maintaining prolonged, gliding posture.