Comparative anatomy of the postural mechanisms of the forelimbs of birds and mammals

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One of the many differences between quadrupedal mammals and birds is that during standing, the forelimbs in mammals are involved in locomotion and support of the body, whereas the forelimbs of birds are involved in locomotion but not in body support. This dichotomy is reflected in the morphology and fiber types of the forelimb muscles. In mammals, the forelimb musculature forms a “pectoral sling” that suspends the trunk. Since mammals use the same muscles for postural and dynamic movements, their muscles possess a combination of slow and fast-contracting muscle fiber types that function in postural and dynamic activities. In a specific muscle, slow muscle fibers can perform postural actions via isometric contraction with minimal fatigue, whereas fast fibers can elicit dynamic, rapid, and forceful actions for locomotion. In contrast, the forelimb muscles of birds need only hold the wing folded while not in use. The muscles specialized for avian forelimb posture are comparatively small and consist entirely of slow fibers, and likely do not function in locomotion. In soaring birds, muscles with slow fibers also function isometrically and maintain the outstretched wing for extended lengths of time. Thus birds and mammals have evolved different strategies to deal with their forelimb postures and have different muscles specialized for posture.