Working with chaotic systems: No easy diagnoses in biomechanics.

A few days ago I answered a question by R.B. in this post. R.B. was asking what could be done do to solve a tight hip adductor problem on her dominant side.

I answered that there was a local answer (how to make the symptom better), and a global answer (how to address the underlying cause). The local answer had to do with strengthening the opposing muscles (the hip abductors of her dominant-leg). However, the underlying cause must also be addressed in parallel with the symptom, or the problem will only get worse.

As I was describing how to address the local problem, I pointed out very specific exercises that could be used to take care of it (which I still haven’t posted about). But if we are to extrapolate from there to the global problem (a weakness in the non-dominant leg), we can only have hypotheses, and not conclusions, about what the specifics of the problem are. In other words, we cannot be certain at all of the specifics of the global mechanical problem.

The reason for this is that the body behaves partially as a chaotic system. In layman’s terms, chaotic systems are systems which respond very strongly to very tiny changes in the initial conditions. (Double pendulums are a great example of this). The first time that you let a double pendulum go from a static position, it will exhibit a certain behavior (i.e. spin around in a particular sequence). But the second time you let the double pendulum go from the exact same initial position, the series of spins that it will do will be completely different from the first.

The thing is this: that exact same initial position wasn’t really the same one as before; it was almost the same one. Maybe we would have needed a micrometer to measure the difference, but that’s the thing about chaotic systems—they respond in wildly different ways to very similar conditions.

(The butterfly effect is an example of pop-culture knowledge of the behavior of chaotic systems).

Let’s bring this back to R.B.’s question.

Let’s say that I did indeed properly diagnose R.B’s symptom: Tight hip adductors causing knee pain. But suppose that R.B. had experienced a shoulder injury as a child, which caused tendon damage. Because all of the muscles and tendons of the body are mechanically connected and influence one another (since the entire bone structure shifts as if it were a mobile), that shoulder injury matters both to the global tension pattern in the body and to the brain’s calculations of how it is going to solve the mechanical challenge of keeping R.B. balanced on two feet.

That slight addition to the initial conditions (the addition of a supposed shoulder injury) could make for a wildly different compensation pattern. It’s important to know whether or not that’s the case. The only way to become completely certain is to do an analysis in addition to R.B.’s report of the apparent symptoms. (Medicine practitioners will recognize this as a signs and symptoms assessment).

It’s important to note that because of the brain, the the body a more easily predictable system than a double pendulum, because the brain regulates the body’s behavior. No such regulatory apparatus exists for the pendulum; the pendulum is both chaotic and ballistic; its trajectory cannot be altered from within after it is put into motion. (Hence the saying “he went ballistic”).

The problem with diagnosing R.B. is that it’s necessary that I make a very accurate inventory of the initial conditions (the symptoms) before I extrapolate and ask, “given these conditions, how is the body most likely to solve the global problem of maintaining R.B. vertical?” In fact, even that is irresponsible—which is why I only gave R.B. a set of general exercises that address the whole region of the body that will need to get strong in order for the non-dominant leg to take more of the support.

(Remember that the body is only somewhat chaotic; there are regions of the body designed to perform certain functions). Most of the muscular burden for supporting the body goes to the outside and back of the body. This is especially true for the leg and hip: The largest muscles of the body, the quads and the glutes, are located on the back and sides of these bodily regions.

And it’s not just me that knows this—R.B.’s body also does. In other words, I can depend on R.B.’s brain (that regulatory mechanism), to find a mechanical solution for how to keep R.B. on two feet somewhere within that region. So, by strengthening those muscle groups and muscle chains, we can be reasonably certain that the problem will go addressed.

But for those same reasons, I couldn’t give a specific exercise. There’s no way to know, except by taking that knowledge and making an organized (and hands-on) mechanical diagnosis of the region. Only then can we know what specific effects those particular initial conditions turned out to have in this case.



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