New theories in chiropractic neurology describe a pattern of muscle imbalance called pyramidal non-spastic paresis, sounds like a lot of big words but is really a simple concept. This muscle imbalance is due to spinal reflexes that govern our normal postural muscle tones. This is how it works, a spinal reflex is defined as a loop of circuitry between sensory nerve endings sending information to the spinal cord and the information being processed in the cord and quickly causing a muscular response. This type of loop usually does not interact with the brain. The knee jerk is the simplest reflex.
There are many types of spinal reflexes but the one that is important for this discussion is a little different and more complicated than the knee jerk loop. This reflex has to do with postural reflexes. It is best described as a reflex that causes muscles related to fighting gravity the most to have a higher tone than muscles that do not fight gravity as directly. This reflex also has some influence from postural centers in the brain. Postural centers in the brain are what control are resting muscle tones so we are able to stand, sit, walk etc. The basic loop of this reflex is like a normal reflex but in the area of the ventral horn of the spinal white matter (the front part of the white H shaped area in the diagram above) these postural centers of the brain modulate how the reflex is expressed in the body.
Ok now I will make it simple, this postural reflex causes the muscles in the front of your upper body to increase their tone and the muscles of the back of the upper body to decrease their tone. In the lower body it is the opposite the muscles in the back of the lower body will increase their tone and muscles in the front of the lower body to decrease their tone. So the chest muscles, biceps, finger/wrist flexors, low back muscles, hamstring, and calves tighten. The muscles of the back of the neck, upper back, back of the shoulders, triceps, finger/wrist extensors, abdominals, hip flexors, fronts of the thigh and shin muscles weaken. Sounds complicated but it is really simple, look at your own body you already know that the muscles that have a tendency towards higher tone are naturally stronger (ex. It is easier to push things away from your body with your arms than it is to pull things toward you. It is easier to push with yourself forward while walking than it is to walk backwards.).
So now that you understand a little bit about the spinal reflex I will describe why this can cause harmful muscle imbalances. The reflex in itself if working properly is normal, but when the postural centers in the brain that react with this reflex have stopped regulation it properly the imbalance becomes excessive and results in postural changes that can cause pain. What the new theory in chiropractic describes is spinal motion fixation decreases vital sensory input to these postural centers in the brain resulting in a change in control of the postural reflex loop. A spinal fixation is when the facet joints of the spine have become locked up and our no longer free moving.
These facet joints are located on the backside of the vertebrae as seen in the above picture. These joints guide the motion of the vertebrae as we move our spine. These joints are very important neurologically because they have a lot of receptors in them feeding valuable information to postural centers in the brain. When they are locked up and are not moving this information does not go to the brain and it affects to control of the brain over the postural reflex loop resulting in excessive muscle imbalances through out the body called pyramidal non-spastic paresis. So when a chiropractor is delivering a manipulation to the spine the goal is to open these restricted joints and free the motion so the postural reflex has been restored to normal balance resulting in muscle tone changes in the body. The key to a chiropractic visit is to only manipulate the fixations resulting in this imbalance, not the shot gun approach done by many chiropractors to manipulate all the joints of the spine every visit. This common approach in my field is not only going to be ineffective but can cause the postural imbalances to get worse. A properly trained chiropractor understands how to evaluate the imbalances and the fixations and treat them in a specific manor.