Mechanics of limb bone loading during terrestrial locomotion in the green iguana (Iguana iguana) and American alligator (Alligator mississippiensis)

In vivo measurements of strain in the femur and tibia of Iguana iguana (Lin naeus) and Alligator mississippiensis (Daudin) have indicated three ways in which limb bone loading in these species differs from patterns observed in most birds and mammals: (i) the limb bones of I. iguana and A, mississippi ensis experience substantial torsion, (ii) the limb bones of I. iguana and A. mississippiensis have higher safety factors than those of birds or mamma ls, and (iii) load magnitudes in the limb bones of A. mississippiensis do n ot decrease uniformly with the use of a more upright posture. To verify the se patterns, and to evaluate the ground and muscle forces that produce them , we collected three-dimensional kinematic and ground reaction force data f rom subadult I. iguana and A. mississippiensis using a force platform and h igh-speed video. The results of these force/kinematic studies generally con firm the loading regimes inferred from in vivo strain measurements. The gro und reaction force applies a torsional moment to the femur and tibia in bot h species; for the femur, this moment augments the moment applied by the ca udofemoralis muscle, suggesting large torsional stresses. In most cases, sa fety factors in bending calculated from force/video data are lower than tho se determined from strain data, but are as high or higher than the safety f actors of bird and mammal limb bones in bending, Finally, correlations betw een limb posture and calculated stress magnitudes in the femur of I. iguana confirm patterns observed during direct bone strain recordings from A. mis sissippiensis: in more upright steps, tensile stresses on the anterior cort ex decrease, but peak compressive stresses on the dorsal cortex increase. E quilibrium analyses indicate that bone stress increases as posture becomes more upright in saurians because the ankle and knee extensor muscles exert greater forces during upright locomotion. If this pattern of increased bone stress with the use of a more upright posture is typical of taxa using non -parasagittal kinematics, then similar increases in load magnitudes were pr obably experienced by lineages that underwent evolutionary shifts to a non- sprawling posture. High limb bone safety factors and small body size in the se lineages could have helped to accommodate such increases in limb bone st ress.