WALTER , Rebecca M: Kinematics of 90 Degree Running Turns in Wild Mice

To gain insight on the turning kinematics of small mammals, I videotaped six adult wild mice from below at 250 Hz as they performed ninety-degree running turns. Five markings on the sagittal axis were digitized to allow observation of lateral bending and body rotation throughout the turn. Ground contact of the fore and hindlimbs was also noted for each frame. When the six mice turned, rotation of the body about a vertical axis occurred slightly before the deflection of the center of mass heading. This is the opposite of what has been found in cockroaches (Jindrich and Full, 1999). The more sprawled limb posture of cockroaches may facilitate application of ground forces perpendicular to the direction of heading, whereas the parasagittal limb posture of a mouse leaves it better equipped to accelerate after it has rotated its body to face the new direction. The mice also bent laterally during the turns, rotating the upper body slightly before the lower body, which was swung around in a “roundoff” type fashion during the end of forelimb ground contact. Thus, body rotation and heading deflection occurred in bouts synchronous with the stride. Most of the body rotation occurring during the second half of forelimb ground contact and the first half of hindlimb ground contact when the mouse�s body was sagittally flexed and rotational inertia minimized. By rotating the body at this point in the stride cycle rather that when the limbs and trunk are extended, mice can achieve similar angular accelerations with much lower applied torques. This style of turning in which the hindquarters are swung around while the forelimbs are planted, and the body is rotated while it is sagittally flexed, likely improved turning performance in the lineage of derived therapsids that gave rise to mammals.