Hindlimb function in the alligator: integrating movements, motor patterns, ground reaction forces and bone strain of terrestrial locomotion.

Alligator hindlimbs show high torsional loads during terrestrial locomotion, in sharp contrast to the bending or axial compressive loads that predominate in animals that use parasagittal limb movements. The present study integrates new data on hindlimb muscle function with previously obtained data on hindlimb kinematics, motor patterns, ground reaction forces and bone strain in order to (1) assess mechanisms underlying limb bone torsion during non-parasagittal locomotion in alligators and (2) improve understanding of hindlimb dynamics during terrestrial locomotion. Three dynamic stance phase periods were recognized: limb-loading, support-and-propulsion, and limb-unloading phases. Shear stresses due to torsion were maximized during the limb-loading phase, during which the ground reaction force (GRF) and caudofemoralis (CFL) muscles generated opposing moments about the femur. Hindlimb retraction during the subsequent stance-and-propulsion phase involves substantial medial rotation of the femur, powered largely by coordinated action of the GRF and CFL. Several muscles that actively shorten to flex and extend limb joints during stance phase in sprawling and erect quadrupeds act in isometric or even eccentric contraction in alligators, stabilizing the knee and ankle during the support-and-propulsion phase. Motor patterns in alligators reveal the presence of local and temporal segregation of muscle functions during locomotion with muscles that lie side by side dedicated to performing different functions and only one of 16 muscles showing clear bursts of activity during both stance and swing phases. Data from alligators add to other recent discoveries that homologous muscles across quadrupeds often do not move joints the same way as is commonly assumed. Although alligators are commonly considered models for early semi-erect tetrapod locomotion, many aspects of hindlimb kinematics, muscle activity patterns, and femoral loading patterns in alligators appear to be derived in alligators rather than reflecting an ancestral semi-erect condition.