Locomotor kinematics and EMG activity during quadrupedal vs. bipedal gait in the Japanese macaque.

Abstract

Several qualitative features distinguish bipedal from quadrupedal locomotion in mammals. Here we show quantitative differences between quadrupedal and bipedal gait in the Japanese monkey in terms of gait patterns, trunk/hindlimb kinematics and electromyographic (EMG) activity, obtained from three macaques during treadmill walking. We predicted that, as a consequence of an almost upright body axis, bipedal gait would show properties consistent with temporal and spatial optimization countering higher trunk/hindlimb loads and a less stable center of mass (CoM). A comparatively larger step width, a ~9% longer duty cycle and ~20% increased relative duration of the double support phase were all in line with such a strategy. Bipedal joint kinematics showed strongest differences in proximal, and least in distal hindlimb joint excursions compared to quadrupedal gait. Hindlimb joint coordination (cyclograms) revealed more periods of single-joint rotations during bipedal gait, and predominance of proximal joints during single-support. The CoM described a symmetrical, quasi-sinusoidal left/right path during bipedal gait, with an alternating shift toward the weight-supporting limb during stance. Trunk/hindlimb EMG activity was non- uniformally increased during bipedal gait: most prominently in proximal antigravity muscles during stance (up to 10-fold). Non-antigravity hindlimb EMG showed altered temporal profiles during lift-off or touchdown. Muscle co-activation was more, but muscle synergies less frequent during bipedal gait. Together, this shows that behavioral and EMG properties of bipedal vs. quadrupedal gait are quantitatively distinct, and suggests that the neural control of bipedal primate locomotion underwent specific adaptations to generate these particular behavioral features to counteract increased load and instability.