Human Bipeds Use Quadrupedal Coordination during Locomotion

During evolution, the increased influence of a direct cortical-motoneuronal system in parallel with a more specialized hand function might have replaced phylogenetically older systems that organized locomotor movements. However, recent research indicates that interlimb coordination during human locomotion is organized in a way similar to that in the cat. During locomotion, corticospinal excitation of upper-limb motoneurons is mediated indirectly, via propriospinal neurons in the cervical spinal cord. This allows a task-dependent neuronal linkage of cervical and thoracolumbar propriospinal circuits controlling leg and arm movements during human locomotor activities. During obstacle avoidance steps, an anticipatory quadrupedal limb coordination is up-regulated, with an involvement of proximal arm muscles during the acquisition and performance of this precision locomotor task.