The Science of Smooth Movement
A new paper quantifies an essential element of coordination
“Smoothness” is widely recognized as a defining aesthetic quality of expert movement. When we see Roger Federer hit a backhand, his movement is fluid and graceful, and this tells us he is doing something very right. And just as surely, we know that tennis novices are doing somehow very wrong as they hack away at the ball with jagged strokes. The quality of expert movement is easy to see, and seems to have something to do with the fact that it is smooth. But is there any way to objectively measure this elusive quality?
As someone trained in the Feldenkrais Method, I know there are ways to learn how to move with more ease and fluidity, and this can improve your physical comfort and performance. But the Feldenkrais Method doesn’t offer any way to define or measure movement quality with objective tests. Instead, it is acknowledged that movement quality is probablytoo individual, context dependent and complex to measure with simple assessments.
By contrast, many physical therapists, such as Gray Cook and Shirley Sahrmann, have argued they can assess movement quality with objective tests that seek to identify “dysfunctions” in coordination. However, the research has generally not been kind to these efforts, leading many people to dismiss the hope of scientifically measuring general coordinative skills.
Personally, I have never doubted that movement quality is a very real thing. To prove that it exists, you need only look at the flowing movement of an elite ballerina. But can this quality be described in a way that is objective, and generalizable outside of a specific technique?
A recent paper argues that “smoothness” may be an essential quality all good movements, and that this quality can be measured. Here’s a link to the paper by John Keily, Craig Pickering and David Collins called Smoothness: An Unexplored Window into Coordinated Running Proficiency. Following is a brief summary.
Defining smoothness: the minimum jerk model
Smooth movement is best understood as the opposite of movement that is “jerky.” While smooth movement creates an impression of fluid continuity “non-smooth movements, in contrast, leave an impression of abruptness, of erratic discordance and of disjointed, unpredictable control.”
This distinction is useful because jerky movement can be defined in terms of the rate and/or number of accelerations of moving body parts. Thus, smoothness involves a minimum of “abrupt, intermittent, discontinuous changes in accelerations, relative joint positions and/or movement trajectories.” These accelerations can be measured, and the measurements are now more practical with the widespread availability of wearable technology. But there has been little surprisingly little research examining this kind of data, even though preliminary findings are very interesting.
Research on smooth movement
Previous research has shown that smoothness of movement improves along with development of movement skill in a variety of contexts, including infant motor development, walking, writing, rock climbing, golfing, and throwing. For example, the club head trajectories of skilled golfers are smoother than those of novices. Further, declines in function are correlated with increased jerkiness, as seen in aging, injury, neurological damage, or distraction during driving. Tests of smoothness in activities like sit to stand or postural sway can predict injury risk in sports or activities of daily life.
Why is smoothness so good?
Smooth movement is predictable movement, and predictable movement is easy to control. This creates a positive feedback loop: increased predictability makes control easier, which makes movement smoother, which enhances predicability and so forth. This is why smooth movers make hard tasks look easy, and jerky movers may easy things look hard.
Smoothness and complexity
The authors refer to the “loss of complexity” hypothesis, which notes that: “reductions in complexity are indicative of declining neurophysiological responsiveness and adaptive range.”
Why does good movement need to be complex? Because smooth movement arises from an ability to correct deviations from an optimal trajectory or rhythm. Such corrections require variability in movement, and more variability implies greater complexity.
Look at this classic picture of an expert blacksmith repeatedly hitting a target with variable swing paths. As soon as the swing is commenced, there will be minor deviations from the intended swing path, and this requires some form of correction. These variations require further adjustments, so that each swing must be subtly different from the previous one. Thus, an essential aspect of any skill is the ability to use different coordination patterns to get the same result, and to adjust them on the fly.
As applied to running, this idea implies that loss of variability in stride may reduce efficiency and increase injury risk. Thus, athletes should be mindful of conditions that can reduce the range of available movement options, and therefore complexity, including pain, injury, fatigue, and weakness. Conversely, athletes can expect to improve by having more ways to solve movement problems, through increasing fitness, skill, strength, comfort, etc.
Thanks very much to the authors for the excellent new paper. I hope someone follows up!