Effects of limb cycle speed on stability during quadrupedal arboreal locomotion

Locomotion on narrow branches presents challenges that make falling more likely. Quadrupedal mammals traveling on arboreal supports augment stability via many behavioral traits. However the effects of movement itself contributing to stability is rarely examined. We examine the effects of limb cycle speed on stability. Seven laboratory rats ran on a “rope-mill” (arboreal treadmill). The device simulated locomotion at 28, 45, and 57 cm/s. From two 210Hz camera footage, we selected 15 consecutive strides, and  measured duty factors of the right-side limbs. We digitized the skin overlying the glenohumeral joint, hip joint, and distal extremes of hands and feet. From these digitized coordinates, we calculated the effective limb length throughout the strides. These kinematic variables are ways that animals can augment static stability – presumably if an individual shows decreased static stability at a particular speed, the movement of the limb might be contributing to stability via some dynamic means. Preliminary data show that effective limb length is longer at the slow and fast speeds compared with the middle-range speed. The shorter limb length at middle speed suggests crouching and lack of stability. However, duty factor vs. speed residuals showed the opposite pattern, where the middle speed had the lowest duty factor, suggesting more stability. With additional animals and kinematic variables, we hope to determine if the speed of limb rotations impacts stability.