Meeting Abstract
The earliest eutherian mammals were small-bodied locomotor generalists with a forelimb morphology that strongly resembles that of extant rats. If form follows function, understanding the kinematics of the humerus, radius, and ulna of extant rats can inform and constrain hypotheses concerning typical posture and mobility in early eutherian forelimbs. Although rodent locomotion, especially that of the Norwegian rat, Rattus norvegicus, has been extensively studied for evolutionary and biomedical research, the three-dimensional (3-D) kinematics of the bones themselves remains under-explored. Here, for the first time, we use markerless XROMM to explore the 3-D kinematics of the humerus, radius, and ulna in three adult male Sprague-Dawley rats (Rattus norvegicus) as they walked across a horizontal platform. Not surprisingly, our data show that rats maintain a crouched forelimb posture throughout the step cycle, with an elbow joint angle never exceeding 130°. Most forelimb posture and movement is dictated proximally at the glenoid through a combination of flexion/extension, abduction/adduction, and long-axis rotation of the humerus that maintains a caudally-facing elbow throughout the step cycle. For all practical purposes, movements of the ulna on the humerus are constrained to flexion and extension only. The radius was shown to be capable of small but significant long axis rotational movements that contributed to pronation and supination. We tentatively suggest, based on qualitative morphological similarities, that early eutherian mammals had forelimb bone kinematics similar to rats.
