Fig. 1. Diagrammatic illustrations of alternative models to explain increases in femoral loading during upright locomotion in alligators (adapted from Blob, 1998, 2001; Blob and Biewener, 1999, 2001). For visual clarity of the forces and moments bearing on the model, the views are presented from an oblique posterolateral perspective (this causes the femur to appear not to project perpendicular to the vertebral axis as it does at mid-stance). For each arrow depicting a force or moment, a difference in thickness (not length) between the left and right sides of a panel indicates a difference in force or moment magnitude between the postures illustrated in those panels (with thicker arrows indicating larger forces or moments). Note that panels A and B are drawn to different scales (A is magnified for clarity), and that comparisons of force and moment magnitudes are not intended between A and B. Because the ground reaction force (GRF) does not differ in magnitude between sprawling and upright steps (Blob and Biewener, 2001), both models are based on changes in the action of hindlimb muscles between these postures. (A) Bending induced by the ground reaction force (red arrow) places the dorsal femur in compression (-), and the ventral femur in tension (+). In sprawling posture (left), the GRF might have a longer moment arm about the hip than in upright posture (right), resulting in a larger abductor moment that would tend to rotate the femur dorsally. To keep the hip joint in equilibrium, the hip adductors might exert a larger force in sprawling posture (left) and a smaller force in upright posture (right). Because the hip adductors bend the femur in the opposite direction from the GRF, larger adductor forces during sprawling steps could more effectively mitigate strains induced by the GRF, resulting in lower dorsal and ventral stresses and strains during sprawling steps. (B) As limb posture becomes more upright, the center of pressure of the GRF shifts away from the ankle, increasing the moment arm of the GRF at the ankle (R_GRF). Consequently, ankle extensors (e.g. gastrocnemius) must exert higher forces during upright steps in order to counter the larger ankle flexor moment and maintain joint equilibrium. Because gastrocnemius also spans the knee, it makes a greater contribution to the flexor moment at the knee during more upright steps, and knee extensors (femorotibialis and iliotibialis, on the dorsal aspect of the femur) must exert greater force to counter this moment and maintain equilibrium at the knee. Increases in knee extensor forces could then raise dorsal and ventral femoral strains and stresses as alligators use more upright posture. Data from previous force platform studies (Blob and Biewener, 2001) are consistent with the model proposed in B, but changes in muscle activity patterns have not been tested prior to this study. r_ankext, moment arm of ankle extensor muscles at ankle (no change between sprawling and upright stance).

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