what happens when you walk?

The walking gait is a very complex mechanism using various components of the musculoskeletal system.

To most people it seems that the lower limbs are the main contributors to walking, but it is important to know where most of the movement originates from to produce the walking gait.

The major problem of locomotion is the need to rotate the pelvis in the horizontal plane as it transmits the movement into the lower extremities. As most muscles of the human body lay in the vertical plane it is not possible to induce pure horizontal rotation of the pelvis. The solution therefore lies in the mechanism by which these muscles are used to efficiently rotate the spine, and therefore the pelvis.

The so called “spinal engine” is comprised of the three sections of the spine: the cervical, thoracic and lumbar spine. Not to go into too Light stepsmuch detail, however it is important to understand the mechanics of the spine. When a spinal segment bends to one side, a rotation also occurs at that segment. This is called a coupled motion.

In terms of gait, when the lumbar spine bends to the side, it will cause a rotation of the pelvis through the coupled motion which swings the leg forward. Thus, the lumbar spine is constantly side-bending from one side to another producing an axial torque which drives the pelvis and therefore the lower limb.

To avoid the upper body bending from side to side with every step, the thoracic spine will counteract this motion by bending and rotating to the opposite side.  All these movements contribute to the counter-rotation of the shoulders and pelvis which provides the axial torque needed to rotate the pelvis in the opposite direction and therefore swing the leg forward.

Remembering that the head needs to be stabilised during locomotion the cervical spine needs to be able to de-rotate the rotation of the shoulders. This also occurs through a coupled side-bending motion, but to the opposite side compared to the thoracic spine.

Therefore, the “spinal engine” has the ability to counter-rotate pelvis and shoulders while stabilising the head at the same time.

The musculature also plays a major role in locomotion. The muscles of the trunk can be divided into the inner and outer unit. The inner unit is made of primarily tonic muscles, and is responsible for stability. The outer unit consists primarily of phasic muscles which serve mobility. The outer unit can be subdivided into different muscular chains which are all responsible for creating the torque that drives the spinal engine. These muscular chains are design to create tension in the thoraco-lumbar fascia for example by two synergistic muscle contractions. The thoraco-lumbar fascia gets wound up and will be release in a pulse of energy assisting the muscles of locomotions and therefor reducing energy expenditure.

The muscles therefore are the power of the engine which has to accelerate and decelerate the trunk musculature and other tissues. During deceleration the kinetic energy is stored and later used for re-acceleration. As seen elastic energy is a not only stored in muscle, but also in ligaments, such as the thoraco-lumbar fascia.

The integrity of each muscular system is of individual importance. If one muscle is damaged it puts an increased stain on every other muscle in the same system and also in other systems as they need to compensate for the lack of strength and mobility of the damaged system.

There are many conditions such as a fractured vertebrae, scoliosis, and other instability conditions which are often treated with spinal fusion.  Spinal fusion is a procedure by which two or more vertebrae are fused together to completely restrict the movements at these segments at is sometimes believed that the pain can originate from spinal motion.  This procedure may or may not eliminate the pain, but it has a major effect on gait as a fused spine is not able to produce its natural movements and therefore is not able to generate the torque needed to rotate the pelvis. The musculature has to take over the work of the spine to try and create this torque which leads to an increased energy expenditure and a gait modification. This is just one obvious example that shows the relevance of the biomechanics of the spine and pelvis with regards to the musculature. Any joint restriction of the spine, pelvis and legs, or any muscular overuse will also cause biomechanical changes, which in turn may lead joint pain, muscle spasms or ligamentous strains.

As you can see, the legs are not the primary drivers with the trunk being only a passenger as most people believe, but that the spine and pelvis play the major role in locomotion.



Natalie PatemanNatalie Pateman, BSc. is a U.K. trained, and qualified osteopathic manual therapist and cranio-sacral therapist. She has completed a 4-year full-time course in osteopathy at the British College of Osteopathic Medicine in London (U.K.) followed by additional courses in cranial osteopathy, pediatric osteopathy and animal osteopathy.





  • Middleditch, A. & J. Oliver (2005) Functional Anatomy of the Spine, London: Elsevier Limited
  • Gracovetsky, S. (1988)  The Spinal Engine, Wien: Springer-Verlag
  • Tortora, J.G. & Grabowski, S.R. (2003) Principles of Anatomy & Physiology, USA: John Wiley & Sons, Inc.

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