| Despite the existence of numerous investigations on the aerodynamics of avian flight and extensive comparative morphological studies, the traditional viewpoint of the functional morphology of the avian shoulder is surprisingly simplistic. Because of the morphological and kinematic complexity of the many joints in the pectoral girdle including the shoulder, sternocoracoidal, sternoclavicular, and coracoclavicular joints, it is misleading to state, as is typically done, that flight consists of elevation of the wing by the M. supracoracoideus and depression by the M. pectoralis. In this study, radiography, high-speed cinematography, and electromyography were used to correlate details of the anatomy of the shoulder complex with the locomotor movements that occur during take-off and flapping flight in the pigeon. It was revealed that the functions of the smaller and deeper muscles attaching to the proximal end of the humerus must be considered, because they are also active during abduction or adduction. Furthermore, some of the smaller muscles have different functions during different parts of the flapping cycle. For example, the M. subcoracoideus helps to adduct the humerus during the downstroke and also posteriorly rotates the humerus during the flick phase of the upstroke. Moreover, osteological and ligamentous features that restrict the movements of the joints of the pectoral girdle must be considered. For instance, the Mm. pectoralis and supracoracoideus are two-joint muscles that extend from the sternum to the dorsal and ventral surfaces of the humerus, respectively, and thus bony displacement of the components in one joint affects the potential movement in the other joint. It is concluded that in order to understand the functional morphology of the avian shoulder complex, careful descriptive and experimental work must be done. |
