Hind limb movements of the American alligator (Alligator mississippiensis) and postural grades

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Abstract
The hind limb kinematics of American alligators (Alligator mississippiensis) executing the high walk were analysed using cineradiography. The stance phase is characterized by hip extension of 60–80d̀, knee extension of 30–40d̀ and a flexion (30–40d̀)—extension (80–90d̀) sequence of the ankle. At initial foot contact with the substratum, the femur is positioned 20–25d̀ from the sagittal plane of the sacrum. During retraction the femur is abducted to a maximum angle of 40–50d̀ near foot lift‐off. A slight medial rotation about the femoral axis takes place to maintain the orientation of the femoral condyles. The sacral region undergoes rotation about a vertical axis of 20–30d̀ and transverse roll of 10–12d̀ with each stride. One individual used an atypical limb posture in which the femur was adducted to 20d̀ rather than abducted during the stance phase.Although the crocodilian high walk appears to be a good example of a ‘semi‐erect’ posture that is intermediate between ‘sprawling’ and ‘fully erect’, the current system of postural grades hampers our understanding of locomotor variability and diversity. Grades must be considered categorical abstractions until enough data can be acquired to demonstrate that postural divisions are based on well‐defined kinematic parameters and functional mechanisms. The indiscriminate use of postural grades, particularly in fossil taxa, blurs differences between members of each grade and artificially broadens distinctions between members of different grades.