An hypothesis for the role of the spine in human locomotion: a challenge to current thinking.

Locomotion was first achieved by the motion of the spine. The limbs came after, as an improvement, not as a substitute; and yet, analysis of bipedal gait concentrates almost exclusively on the motion of the limbs. The requirements for land locomotion are examined from a general point of view and the evolution of the vertebrate spine is presented as a mechanism designed to move the animal. The necessary spinal movements are also analysed; the role of the musculoskeletal system is discussed and it is shown that the lumbar spine is a key structure in land locomotion, the pelvis being driven by the spine. The optimum control of motion demands that the stress at all the intervertebral joints should be minimized and equalized. This theory of locomotion requires the central nervous system to control the torque at those intervertebral joints and suggests that a breakdown of the control system would result in torsional failure of the spine. The theory is supported by EMG, force and torque data collected from several sources.