The Science Behind a Short Leg and an Upper Cervical Vertebral Subluxation.
I’ve described the connection between a short leg and the upper cervical spine in a previous blog, but I wanted to explain this topic in more detail for those who are interested in the science behind it.
How and why a leg length discrepancy is associated with an upper cervical vertebral subluxation, is based on biomechanical and neurophysiological compensations.
While an anatomical short leg is possible, the majority of short legs observed are physiological in nature. Some functional cause is creating the leg length discrepancy. In Chiropractic, our focus is on the neurophysiology and biomechanics of the spine and how it can effect all other areas of the body in some way, shape or form.
A vertebral subluxation occurs when a spinal bone loses its normal juxtaposition to one or both of the corresponding segments and interferes with nerve system communication.
When a cervical subluxation occurs, either the atlas bone, the axis bone or both; it effects all the structures that attach and associate with it. In general, when the upper cervical spine subluxates, it will laterally shift left or right, deviate superior or inferior and rotate anterior or posterior. This causes the head to tilt out of balance, making the eyes unlevel.
This head tilt negatively effects proprioception. Proprioception is how the body communicates its position in space with the brain. A vertebral subluxation disrupts four sensory structures of proprioception found in the spine.
The first structure effected is the extrafusal fibers found in the muscles connected to the upper cervical bones. A vertebral subluxation will stretch the extrafusal fibers. This stretch causes the intrafusal fibers of the muscle spindle to detect the change. It distorts both nuclear chain type II afferent fibers and nuclear bag type Ia afferent fibers of intrafusal fibers. Both static and dynamic positional detection is altered.
Secondly, the stretch causes the Golgi Tendon Organ found in the muscle’s tendon to detect a change in proprioception. The Golgi Tendon Organ, or GTO for short, senses that there is tension in the muscle. This is detected by type Ib afferent fibers.
Thirdly, a subluxation creates a stretch change in the facet joint. In the facet joint, you’ll find mechanoreceptors. Regarding the spinal column, the greatest amount of mechanoreceptors are found in the upper cervical spine. The facet plays a major role in proprioception sensory function. The three types of receptors are Ruffini, Pacinian and Golgi-type.
The Ruffini receptors found in the capsule are the most common and signal mechanical stress. Pacinian receptors respond to sudden changes and are found in the ligament articulation. Golgi-type only responds to an extreme range of motion and are found in the ligament articulation.
The fourth structure of proprioception effected is found in the intervertebral disc. Ruffini and Golgi-type receptors are found predominantly in the annulus and longitudinal ligaments. A subluxation creates a malformation that is detected by the receptors found in the disc’s fibrocartilage.
To summarize thus far, a vertebral subluxation creates a disruption in muscle, tendon, facet joint and intervertebral disc proprioception.
Type Ia, Type 1b and Type II afferent fibers send the message to the cerebellum that there is a disruption occurring in the upper cervical spine. This information travels up the dorsal horns through two main ascending tracts: the ventral and dorsal spinocerebellar tracts.
The cerebellum processes the stretch information because it is primarily responsible for proprioception and motor control information. This information is then sent to the thalamus so the brain can integrate it. In general, the cerebellum tells the brain what is occurring in the body.
The afferent information from the thalamus is then sent to the sensory cortex. The sensory cortex contains information about what should be happening in the body. At this point, the brain compares what should be happening in the body to what is actually happening in the body. If the information is the same, the brain considers this normal and doesn’t send a compensated response to the body.
A subluxation creates a disruption between what is occurring in the body and what should be happening in the body. The brain must send a compensatory response through the motor cortex to “right the wrong.” The motor cortex sends a response through four primary descending tracts that travel through the brainstem.
The tecto-spinal tract is responsible for head and eye movements and responds to sight and sound stimulus. The rubro-spinal tract is responsible for sending out information from the cerebellum and red nucleus. The reticulo-spinal tract is responsible for intraspinal loops. The vestibulo-spinal tract is responsible for distal muscle effects, such as the short leg. The vestibulo-spinal tract facilitates the contraction of postural muscles primarily located in the low back, pelvic and lower limb region.
The descending tracts are all part of the extrapyramidal system. The tracts receive constant messages from both the cerebellum and cortex to help control the regulation of muscle tone, smoothness of movement, posture and balance.
So when the brain interprets the upper cervical subluxation, it sends efferent information through the vestibulo-spinal tract to activate an interneuron in the spinal cord. When the interneuron is activated, it initiates an alpha motor neuron of the contracted or agonist muscle. This causes a physiological shortening of the extrafusal fibers of the postural muscles. The result is a short leg.
The extrafusal muscle fiber also causes the intrafusal muscle fibers to coil. A Gamma motor neuron causes a contraction of the poles of the intrafusal fibers, creating a lengthening of the center portion of the fibers. The intrafusal fibers become taut and sensitive to further changes in the muscle. This is called alpha-gamma coactivation. The activated interneuron also inhibits the antagonistic muscle group
Finally, with a subluxation present, cortical inhibitory tracts are inhibited. Under normal situations, the inhibitory tracts provide an inhibitory response to balance facilitatory tracts. But with a subluxation, the cortex inhibits this tract. This phenomenon is called “dis-inhibition” because when you inhibit an inhibitory tract, you actually activate it. Dis-inhibition encourages the contracted leg to stay short.
When we correct a upper cervical subluxation through a specific chiropractic adjustment, the leg lengths can balance very quickly. This is because we are decreasing local muscle tension which is sensed by the type Ia, type Ib and type II afferent nerve fibers.
These afferent fibers are the fastest nerve fibers in the body, they can send neurological information between 80-100 meters per second. That is roughly the speed of traveling a football field in 1 second. The Chiropractic adjustment helps turn compensations into proper neurophysiological and biomechanics adaptations.
Jarek Esarco, DC, CACCP is a pediatric, family wellness and upper cervical specific Chiropractor. He is an active member of the International Chiropractic Pediatric Association (ICPA). Dr. Jarek has postgraduate certification in Pediatric Chiropractic through the ICPA. Dr. Jarek also has postgraduate certification in the HIO Specific Brain Stem technique through The TIC Institute. Dr. Jarek is happily married to his wife Regina. They live in Youngstown, Ohio with their daughter Ruby.
