The Utility of Interappendicular Connections in Bipedal Locomotion

Homo sapiens constitute the only currently obligate bipedal mammals and, as it stands, upright bipedal locomotion is a defining characteristic of humans. Indeed, while the evolution to bipedalism has allowed for the upper limbs to be liberated from ground contact during ambulation, their role in locomotion is far from obsolete. Rather, there is reason to believe that arm swing offers important mechanical and neurological advantages to bipedal locomotion. In this short review, we present some compelling findings on the neural connections between the arms and legs during human locomotion. We begin with a description of the importance of arm swing during walking from a mechanical perspective. Then, we examine evidence for the existence of interappendicular connections that converge along with peripheral afferents, descending inputs, and propriospinal projections, onto the neural circuits innervating the muscles of the arms and legs. The varied effects of interappendicular coupling on the neural control of locomotion are also examined in cases of neurological injury. We use the insight gained from these collected works as well as those from our own studies on locomotor training to discuss strategies to use interappendicular connections to rehabilitate walking in individuals experiencing loss of function after debilitating spinal cord injury.