112.6 Saturday, Jan. 7 Walking With a Twist KAMBIC, Robert E*; GATESY, Stephen M; ROBERTS, Thomas R; Brown University; Brown University; Brown University email@example.com
Studies of tetrapod limb movement and muscle activity during steady locomotion often record data from only one side. This approach assumes that both members of a limb pair act symmetrically when the animal holds stable on a treadmill or moves overground along a linear path at a constant speed. But what if this assumption is violated? What happens when the body is yawed relative to the direction of travel? When the yaw and heading are not aligned, right and left limbs will need to move asymmetrically relative to the body. Our 3-D analysis of skeletal motion in walking helmeted guineafowl (Numida meleagris) reveals that asymmetry is quite common. Although birds move steadily on the treadmill, marker-based XROMM reveals pelvic yaw up to 15°. Such small deviations would not be detectable in lateral view by an external camera, yet have significant effect on joint kinematics. Long-axis rotation of the femur and tibiotarsus increases with yaw angle. By combining hip, knee, and ankle flexion/extension with these long-axis rotations, the feet translate diagonally with respect to the yawed pelvis. When the body yaws to the right, for example, the limb is set down more to the left. During stance, internal femoral and external tibiotarsal rotation shift the foot to the right. Simultaneously, the left limb is lifted more to the right and then shifts left using internal femoral and external tibiotarsal rotation during swing. As a result, long axis rotations of the striding limbs are in-phase while flexion/extension is out-of-phase. This complex pattern suggests that even during steady walking, the limbs require more than a simple pattern of alternating activation. In-phase long-axis rotations reveal a flexible motor control strategy that decouples this degree of freedom from out-of-phase fore-aft movements. Our next step is to use as inverse dynamics approach to contrast the kinetics of yawed and non-yawed locomotion.