Muscle Homology in Coordinated Rhythmic Movements

One of my main experimental tasks is coordinated rhythmic movement. This is a simple lab task in which I ask people to produce rhythmic movements (typically with a joystick) and coordinate those at some mean relative phase. Not all coordinations are equally easy; without training, people can typically only reliably produce 0° (in-phase) and 180° (anti-phase) movements. People can learn other coordinations, however; I typically train the maximally difficult 90° (although my PhD student has just completed a study training people at 60°; more on that awesome data shortly). I use coordination to study the perceptual control of action and learning.

My work is all designed to test and extend Bingham's mechanistic model of coordination dynamics. This model explicitly identifies all the actual components of the perception-action system producing the behaviour, and models them. In particular, it models the perceptual information we use to perceive relative phase; the relative direction of motion. This is an important contributor to coordination stability and this model is a real step up in terms of how we do business in psychology.

There is another factor that affects coordination stability, however, and the model currently only addresses this implicitly. That factor is muscle homology, and it's been repeatedly shown to be an important factor. For a long time, I have avoided worrying about it, because I have had no mechanistic way to talk about it. I think I have the beginnings of a way now, though, and this post is the first of several as I develop my first draft of that analysis.

What Is Muscle Homology?

The original Kelso description of coordinated rhythmic movement was actually all in terms of muscle homology, and it wasn't until the various phenomena also appeared between people, between people and computer displays, and in judgement studies with no action component that perception came to the fore as a factor. 

Homologous muscles are muscles in different limbs that are the functional equivalent of each other; for example, biceps, or triceps. The coordination result is that coordinated rhythmic movements that require the simultaneous use of homologous muscles (in-phase) are more stable than those that require the alternating use of homologous muscles (anti-phase). Hold your arms in an arm-curl position and flex both then extend both at the same time; this is using homologous muscles at the same time (bicep-bicep, tricep-tricep). Alternating them (bicep-tricep, tricep-bicep) is non-homologous muscle use. 

I avoided worrying about this factor for years by studying either proprioceptive or visual judgements of relative phase (no coordination action involved) or by using a unimanual, visual coordination task where the person was only controlling one oscillator and a computer was controlling the other. These tasks gave me lots of ways to study perception of relative phase, but for various reasons, I have moved back into bimanual movements, and muscle homology is now a thing I have to worry about. 

Muscle Homology vs Relative Direction

In the example above, muscle homology and relative direction of motion are the same, and so the muscular-defined in-phase and anti-phase are also the directionally-defined 0° and 180°. (NB: I have adopted this terminology as a way of keeping the factors separate; it emerged as the way most people were talking in the literature.) This is not always the case, and in fact, in my bimanual experiments, it is typically not the case. In my studies, people must move the joysticks side-to-side to get the dots on the screen moving side-to-side; non-homologous muscle activation leads to 0° motion on the screen, and vice versa. It is a common way to separate the two factors, actually; reorient the limbs so that muscle homology and direction are not the same thing.

Swinnen et al (1997) reviewed the literature to date and described the two effects in terms of two different frames of reference. The egocentric frame of reference is defined with respect to the midline of the body which gives you the muscle homology effect. The allocentric frame of reference is externally defined, with respect to motion through space, which gives you the relative direction effects. This terminology is useful, although I will note at this point that it is just a functional description of the data, and not a mechanistic explanation using real parts or operations. My headache is to reformulate the former into the latter.

Some Data

To find out how much I should worry, two MSc students collected data for me using my set-up. We compared my coordination feedback (Wilson et al, 2010) to Lissajous feedback, homologous vs non-homologous muscle activation and 0° vs 180° presented visually on the screen in a fully crossed, 2x2x2 within-subjects design (see Figure 1). I was able to simply alter the mapping from joystick to display in software and so we had people move in the two patterns and had those produce different effects on the screen across blocks. (I am writing this data up now and it should be ready to go out early in the New Year). 

Figure 1. The experimental set up (image credit John Pickavance)

The basic result was three main effects and no interactions*. Coordination feedback was a little more stable than Lissajous feedback; homologous muscle activation was more stable than non-homologous; and 0° was more stable than 180°. A No-Vision control (no display) showed a muscle homology effect. The effect size for homology was about .7, while the other two were about .2; homology is clearly a thing! (See Figure 2.)

Figure 2. Data from the experiment (proportion time-on-target, a measure of stability)

An Interim Summary

At this point in the story, I now know two things

1. Muscle homology is an independent contributor to coordination stability in my experiment set up when I am studying bimanual movements
2. Muscle homology and relative direction are not obviously the same thing, and so right now the Bingham model does not include any information about homology

I now need to answer a couple of questions

1. What is the evidence regarding the proprioceptive perception of relative phase and how does it map onto the visual perception studies? 
2. Are homology and relative direction actually different things, or can I collapse them into one? To put this point another way, does the Bingham model need a new variable added or does it already have all the pieces it needs? 
3. Either way, what the bloody hell is the form of the proprioceptive variable being used to perceive relative phase?

In the next post, I will review the recent paper from Bingham and colleagues which got me thinking about all these questions in the first place.

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*The first run of this experiment actually found visual dominance and no homology effects, an exciting result which we then failed to replicate. The former data was the victim of a faulty joystick, however, and so I don't currently make anything of that data. It does raise the possibility that muscle homology gets swamped when the task is hard, but that will require another experiment in which difficulty is systematically controlled.