Erik Dalton, Ph.D., in Structural Integration, June 2005, has done an exemplary job of tracking down the references on the short-leg controversy and explaining skeletal distortions in terms of the central nervous system. While I concur with much of what he has to say, I want to add to the discussion with some alternate points of view.
His article points up the role of the central nervous system in the control of postural functions and dysfunction. He correctly identifies the role of the dominant cortical hemisphere in the anterior rotation of the ipsi lateral ilium. However, perhaps in an effort to sketch the outlines of a neurological explanation more simply, he has unfortunately made some errors which detract both from the explanatory and therapeutic power of the model.
I have some observations about the short leg controversy near the end of this paper, but I am more interested in the way the nervous system is placed so prominently on the Rolfing® table, so to speak. While I offer critical comments, I am keenly aware that there may be information more recent than mine. I will be questioning Dr. Dalton s exposition from two perspectives: i) I think the relationship between cortical dominance and vestibular dominance is unclear and perhaps mistaken and ii) the notion of “functional short leg” is, I think, incomplete.
In what follows, “dominance” is an important concept. But in clinical settings, dominance is less important than its opposite. Moreover dominance is both relative (side to-side) and not necessarily permanent. It is possible in the clinical setting to reverse the dominant side by providing sufficient neural input (via environmental stimuli – sound, light, smell or proprioceptive input) to the non dominant side. Thus, a right anterior ilium, right anterior T-12 and left side contracted lumbo-pelvic extensors, with a few seconds of sound in the right ear, will flip to the mirror image. It won’t stay that way without continued input, but the phenomenon itself is fascinating.
Dr. Dalton writes:
“Since motor cortex dominance crosses over contra-laterally, the right-sided extremities …perform most motor dominant tasks… The postural muscles … react to right motor dominance by tightening and shortening. Tightening of these postural muscles and their synergistic stabilizers typically increases lumbar lordosis and pelvic tilt on the client’s right side anterior/inferior direction by the iliopsoas, rectus femoris, etc.”
Unfortunately, it is not quite so straightforward. Dr. Dalton has the importance of right cortical dominance correctly, but other observations are less accurate. There is also an apparent contradiction in the first sentence. If the right cortex is dominant, and if, as he states, “motor cortex dominance crosses over contra laterally,” we would expect the dominant right-side cortex to control the left side extremities, not the right sided ones.
The actual relations are more complicated and involve the inhibitory, as well as the excitatory, functions of the cortex. Inhibition serves an important role in the grading of movements. There are loops throughout the CNS that speed up or slow down firing rates to produce the desired movement. Understanding the principles of inhibition makes working with the neurological patterns and the structural patterns that they produce that much easier.
Dalton writes, “Eighty per cent of people presenting in the most common fetal position … are left vestibular and right motor dominant. Vestibular dominance controls balance and tends to travel ipsi laterally up and down the spine.”
I am not sure what he intends to convey with this concept of left vestibular dominance. He uses “vestibular dominance” as an apparent conceptual equivalent to “motor dominance.” An analogy might be to say that my car’s engine governs the left side and the steering mechanism controls the right side. It adds a layer of confusion but no clarity.
The motor system and vestibular system are not in competition. In fact the vestibular nuclei are a part of the motor system. There seems to be a misconception about how the parts of the brain interrelate. The left inner ear vestibular apparatus fires into the right side of the neuraxis and summates in the vestibular nuclei of the brain stem. These nuclei accept input from cortical pathways, from the cerebellar centers that transmit proprioceptive information, and from the extra-ocular muscles that enable the brain to track a visual target when the head is in motion. The centers that control pursuit of the eyes are carried down the spinal cord and contribute to righting reflexes of the spine.
The activity of the cerebral cortex is a function of the frequency of firing (FOF) of presynaptic neurons that terminate in the cortex. This means that all the neuronal activity – but in particular the output of the spindle cells of the right side of the body terminates in the left cortex, and vice versa. The side with the higher activity is termed the dominant cortex. Thus it is a loose measurement of the difference in electrical activity of the brain between the two hemispheres. It is not a permanent state, though people tend to maintain one side or the other as a preferential side. In people who are fairly close to balance, this sidedness changes from time to time. I have several (cortical) switch hitters in my practice.
What is measured in neurological testing is not so much the dominant side but the relationship between the slower firing side the non dominant or “hemispheristic” side and the dominant. This non dominance shows up as the inability to perform certain sensory or motor tests. It is the side that shows the slower response or the faster fatigue rate compared to the opposite that is of interest.
The vestibular apparatus in the inner ear is a collection of receptor cells cells that transduce environmental stimuli (in this case mechanical) into electrical charge. Their frequency of firing is determined by the activity of the stimuli. They summate in vestibular nuclei in the brainstem just anterior to the cerebellum. Their job is to integrate the input from the inner ear, which relays information about the position of the head, with information about the position of the body in space coming from the cortex and cerebellum. One primary job is to integrate the tracking of eye movement with movements of the head. My understanding of the vestibular system in the larger sense that is, including proprioceptive and ocular information is that the fibers from the inner ear and extra ocular muscles summate in the vestibular nuclei just anterior to the cerebellum in an area known as the tectum.
THE UNDERLYING PRINCIPLES
The nervous system is driven by afferentation. (Dr. Dalton has already alluded to this principle in his discussion of the left-vestibular/right-cortical relationship.) This means that the activity of the entire system relies entirely on the firing of various sensory receptors and, in particular, by gravity-driven sensory receptors (important for Rolfers). Neurons require stimulation by pre-synaptic neurons to survive. The beginnings of the chain of firing are the receptor cells which transduce light, sound, etc., or joint movement into action potentials. What this means is that the entire nervous system would fail if the receptor cells of the body stopped firing.
2) Inhibition of Inhibition
The cortex is responsible for the “inhibition of inhibition” of motor commands. When you go to move your right leg, the left cortex initiates the motor signal (contra-laterally) down the neuraxis to the ventral horn cell in the spinal cord of the lumbar area. This signal excites the motor neuron to your leg. It simultaneously excites an inhibitory cell which circles back and basically cancels the action it just initiated. It takes the ipsilateral brain (the right brain in this example) to inhibit the inhibitory cell so the action can take place.
This is even more important on the non dominant (left) side. Here the ipsi lateral cortex (the left brain) is unable to maintain the inhibition of the inhibition of the faster firing right (contralateral) side, and the muscles of the left leg are weaker. The weakness of the non dominant leg is relative to the difference of firing rates of the two hemispheres.
Since the function of inhibition is a difficult concept, think of the grading of response that is required to do finely detailed work. One of the primary jobs of the cortex is to create graded responses so that, for instance, when the vestibular system makes a correction when you lose your balance, the response doesn’t throw you over to the other side.
3) Specific Inhibition
The cerebral cortex selectively inhibits the anterior muscles above T-6 and posterior muscles below T-6 upper body flexors and lower body extensors. The functional reason for this is probably to keep muscle groups that get more use from overpowering their antagonists. This principle helps explain another non dominant phenomenon that the left leg extensors are more contracted than their right leg counterparts because the left brain’s inhibition is diminished.
4) Cross-Cord Reflexes
When a motor neuron is fired to a right hip flexor muscle, whether reflexively (in response to a stretch reflex) or supra segmentally (from the brainstem or cortex) at least seven other neurons are affected. The right hip extensors are inhibited. The left hip flexors are inhibited, and extensors are excited. And the reverse pattern takes place in the upper girdle. So in our model of right-sided hip flexor contraction, the right neck extensors will contract, as will the left neck flexors.
5) Oxidative Capacity
The oxidative capacity of the body, and hence the firing rate of the cerebral cortex, diminishes by about 1% per year after age 20. This doesn’t directly relate to our current discussion but has an interesting correlate with principle 3. When people reach their 50’s they’ve lost 30% of their oxidative capacity and thus 30% of the inhibitory power of the brain. This is about the age when you start to see shoulder problems showing up because the pectorals are no longer well inhibited and the back muscles are losing ground. By their 70’s the extensors of the legs have lost their cortical inhibition and have contracted so much you start to get the typical older person’s posture of shoulders rounded and knees bent.
To return to the article, Dr. Dalton states, “The postural muscles … react to right motor dominance by tightening and shortening.” In my experience (for a long time I have taken complete notes on these patterns), the dominant side extensors are relatively less constricted than the non-dominant – which causes the posterior rotation of the non dominant side. This is supported by both Principles 2 and 3 above.
“Vestibular dominance controls balance and tends to travel ipsi laterally up and down the spine.”
I’m not entirely sure what is intended by this statement, but I don’t think it fits with my perceptions, which predict that muscles will be contracted in the subdominant lumbar region and the dominant thoracic area. This is also what would be expected from Principles 2 and 3. This pattern is also expressed in most scoliotic curves.
Now about short legs. I had enough discussion about short legs, anatomic and functional, in Chiropractic school to last for a while. In Juhl’s study, cited by Dalton, 35% of back pain patients had one leg or the other longer by 10 mm. That’s a bunch of crooked people. That also happens to be the population we see. Do 1/3 of the people you see have one leg half-an-inch longer? Hopefully not when you get through with them.
What could be happening here? I read the Juhl article that Dalton cited carefully. (You can get the full text from JAOA for free.) The assistant in the test made sure the feet were hip-width apart and the buttocks were touching the bucky but not leaning on it. That was it. Then they lined the central ray with the sacral base and took the shot. Apparently nobody bothered to check if the subjects’ weight was evenly distributed between both feet. Apparently, they weren’t supposed to.
People come into my office frequently with an apparent short leg that is, in fact, due to organizing their weight over one leg. That makes the weighted side appear to be longer. Dalton has already said that the weight in a right-dominant pattern tends to be carried over the left. Apparently, this is what is included in the concept of functional short leg. Presumably Juhl was trying to establish some sort of correlation between back pain and short legs, which apparently he did. But we can’t tell if short legs cause back pain or back pain causes short legs.
The background for my comments comes from a 300-hour post-doctoral study of clinical neurophysiology that I did after Chiropractic school. During that course we spent hours learning how to do competent, rather sensitive assessments of central integrative states of the nervous system and techniques for changing those states.
I have taken this study and for the past twelve years integrated some adaptations to some of the more obscure chiropractic techniques to create a way of assessing and interacting with the nervous system and its role in structural issues. So I have a bias. I think the nervous system plays a much larger role in structural issues than we as a community have ever been willing to acknowledge.
Additional reading on topics covered can be found in: Kandell and Schwartz, The Principles of Neural Science, McGraw-Hill, 2000.
www.dhazen.com/Neuro Pages/ Neurology.html