Biomechanical constraints in hindlimb joints during the quadrupedal versus bipedal locomotion of M. fuscata.

The operant-trained Japanese monkey, Macaca fuscata, can walk with both a quadrupedal (Qp) and a bipedal (Bp) gait on the surface of a treadmill belt, which moves at different speeds. The animal can also learn to transform its locomotor pattern from Qp to Bp, and vice versa, without a break in forward walking speed. This nonhuman primate model provides an intriguing opportunity to compare the kinematics of multiple body segments during Qp and Bp walking of the same subject. We found that M. fuscata selects a postural strategy and limb-kinematic parameters appropriate for the execution of both gaits. We propose that the basic locomotor rhythm-generating mechanisms of the brainstem and spinal cord, which are genetically endowed and relatively automatized, are used for the execution of both the Qp and Bp gait. The latter requires in addition, however, some higher-level circuitry which is shaped substantially by motor learning mechanisms.