Do human bipeds use quadrupedal coordination?

Tackling the question of whether control of human gait is based on that of a quadrupedal locomotion system is of basic and practical relevance. During evolution, the increased influence of a direct cortical-motoneuronal system in parallel with 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 similar way to that in the cat. Hence, it is hypothesized that 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 thoraco-lumbar propriospinal circuits controlling leg and arm movements during human locomotor activities. The persistence of such movement control has consequences for rehabilitation and the applicability of animal research to human patients with spinal cord injury.