Functional Variability in Motor Control Systems

Let’s kick this one off with a video: The sound quality kind of sucks but the content is aces. Here’s my fellow Canadian, Dr. Andreo Spina talking about a few of the things I’m looking to discuss in this article.

So I ended my last post with a promise that I would follow up with commentary about Functional Variability. Purposeful variability more effectively frames what I’m trying to convey though. The moral of the story is that there is invariably variability in movement systems and motor control programs. This is simply due to the complexity of the organisms, tasks, and environments involved. There are literally infinite ways to solve movement problems, and there is never only one solution. There may be a best solution, but there are still hundreds or thousands of permutations that would work just as well.

Presently I am reading Fundamentals of Motor Control by Mark Latash, a brilliant book which blends philosophical perspectives on movement with neuroscience. In chapter 3, there are quality points of consideration regarding Motor Redundancy and Motor Variability. Although this post is about functional/purposeful variability, some background in these two areas is warranted.

Motor Redundancy

Firstly, what is motor redundancy? Well, as discussed in my last post there are plenty of different opportunities for variability within the movement system when it comes to achieving a movement objective. In baseball, a pitcher will have different release points, and forearm/wrist/hand configurations for throwing different pitches. Let’s say for example in throwing a fastball, the objective is to release the ball at a “1 o’clock” release point with the forearm slightly supinated relative to the ground and the shoulder transitioning from external to internal rotation. This is the objective destination, or Point B if you will. How many millions of ways are there to get to this point from the initiation (Point A) of the movement? Even if we know there are seven major axes of rotation in the human arm (three in the shoulder, one in the elbow, two in the wrist, and one between the elbow and wrist) the problem of solving for the number of possibilities in relation to three-dimensional spatial coordinates already makes the task of calculation insurmountable.

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Here is a screenshot from Latash’s Book, detailing one example of kinematic redundancy. Latash, M.,Fundamentals of Motor Control. Academic Press, 2012.

To add to this chaos, Nikolai Bernstein (acclaimed Russian physiologist) considered the problem of motor redundancy to be the central problem of motor control. Specifically “Elimination of redundant degrees-of-freedom” as the problem. Well, any degree of freedom (such as those discussed in the arm earlier) counts as an elemental variable. What if we dig deeper though? The physical degrees of freedom in the arm only details the seven elemental variables at ONE level of analysis. What about the number and size of motor units recruited? Frequency of action potentials? Those are also elemental variables, they just become relevant at another level of analysis. As you can see, the problem is as big or as small as you wish to make it. Whether you are concerned with biomechanics, physiology, or neurology will largely determine your scope and level of analysis.

Latash prefers to use the term abundance in opposition to redundancy. He states that Redundancy refers to excess which we prefer not to use. Abundance refers to excess which we may enjoy to have and use. This leads us to believe that abundance in this case (not redundancy) is a luxury, and indeed it is when it comes to variability. So let’s get to the point……

Motor Variability

“Repetition without repetition” is how Latash quotes Bernstein on the matter of motor variability. Remember how many elemental variables were implicitly proposed in the previous paragraphs? If you consider every possible level of analysis, there are literally countless degrees of freedom in even the simplest movement patterns. No person or people will ever perform perfectly similar repetitions of the same movement no matter how long they have practiced or mastered it. This is known in the literature as inter-subject (you and I do this differently) and inter-trial (I did this differently each time) variability. When you consider neural firing rates and patterns, motor unit size and order of recruitment, organization of joint positions etc. there are infinite ways of “replicating” a movement. Or at least recreating something very similar.

It was previously thought that the variability between reps was simply noise. White, full spectrum, max bandwidth noise, which is basically interference. It turns out however, that the variability (be it via EMG, Joint Angle Displacements, Firing Patterns etc.) is actually colored noise. It is purposeful, and can even give those who possess the ability to exploit this variability an undeniable advantage.

Tying it Together – Functional Variability

In the unlikely event you read my previous post, you would recall that I wrote about great athletes displaying mastery of the cooperation between degrees of freedom and coordinative structures. That is not to say however, that the best athletes are experts in recreating exactly the same movement patterns over and over. Actually, quite the opposite. You may have had a coach at some point tell you that the best baseball players, high jumpers, golfers, and javelin throwers are successful because they practice reproducing identical movements every time. They are successful due to practice in large part because they are successful at reproducing successful attempts. These people (higher level athletes) actually have greater inter-trial (I did this differently each time) variability than poorer athletes. Remember what I said about coordinative segments, worse athletes will have more “fixed” degrees of freedom during execution of the same task over several movement trials. They move stiffly and rigidly in comparison to the athletes who beat them.

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An example in the paper by Davids et. al. which I cited in my previous article, details an example of research on long jumpers. Research identifies a two-phase approach to long-jumping. An initial acceleration phase in which the athlete attempts to achieve optimal horizontal velocity, and a zeroing-in phase in which the athlete attempts to modify stride length to achieve a perfect take-off. During the first phase (horizontal velocity,) there will also be inherent variability in stride length which will cause the zeroing-in phase to occur at unique points during each repetition. It is during this zeroing-in phase that the athlete must make “in-flight” adjustments to foot placement and stride length to achieve optimal takeoff. Since the horizontal velocity achieved in phase one is more important to total displacement in the jump, it takes priority. This makes the zeroing-in phase one of great uncertainty, relatively speaking. This FUNCTIONAL/PURPOSEFUL variability in phase two is an example of the opportunity for truly great athletes to make the difference in performance.

 

 

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