One of the trickiest things about the effort to measure movement quality is that there is no single “right” way to accomplish a physical task, even for the same person. It’s not as easy as creating a strict template for what a squat or throwing a baseball should look like and declaring that every single repetition should precisely follow this pattern.
In fact, as athletes become more proficient, they display more variability in how they move while improving the outcomes of the movements they produce.
A novice athlete’s movement will be more rigid from rep to rep. You’ll see less variability, both in terms of how joints are moving as well as in how much tension or force is expressed throughout the motion. Their outcomes - whether that’s making a free throw, jumping over a hurdle, or climbing a rock wall – will vary substantially. They’ll use rigid and imprecise motions and make large and frequent errors.
An expert, on the other hand, has highly variable movement execution. They can reflexively adapt to an external disturbance– an opponent on the field, a change in terrain on a ski mountain, or a sudden shift in the way a wave is breaking beneath a surfboard – while still attaining the outcome that they’re after: a touchdown, a clean turn through a section of moguls, or a cutback on a rapidly breaking wave.
If you break down the joint-by-joint movements of any of these athletes, you’ll never see a single fixed path like a pendulum swinging back and forth. Instead, what you’d find would be more like if you were to roll a marble around in a bowl. The movement of the marble would always go in the same general direction, toward the same low area of the bowl, but would take a slightly different path each time.
In this analogy, the bottom of the bowl where the marble is always returning is an attractor. It’s a point toward which variable movement tends to draw itself, over and over.
To measure movement quality in a meaningful way, one of our first steps is to identify how these attractor states occur, and how they vary between expert movement to injurious or amateur movement. There are no simple, convenient straight lines, but there are patterns. We simply have to find them.