Abstract & Commentary
Synopsis: This study describes a potential solution for the problem of motor control variability. The hypothesis addresses the question of why an athlete cannot, for example, control the trajectory of a ball in exactly the same way, every time he or she performs the task.
Source: Latash ML, et al. Exercise and Sports Science Reviews. 2002;30(1):26-31.
Latash and colleagues reviewed the uncontrolled manifold hypothesis of motor control. Motor Control Variability is the term that describes the phenomenon that several attempts at the same task always lead to different performance patterns, including differences in kinematics, kinetics, and muscle activation during the movement. For example, when you attempt to sink a 10-foot putt from the exact same spot on the green, you often perform this putt in slightly different ways and most concerning, with slightly different results. The question is why? Latash et al believe that motor control variability is not bad (unless you want to sink 10 free throws in a row). At the very least, it gives us a way to look at motor control patterns.
Motor redundancy likely underlies motor control variability. Motor redundancy is present when many more elements contribute to the performance of a task than are necessary to complete the task. Earlier hypotheses to explain motor redundancy are the Principle of Minimal Interaction and the Principle of Abundance. The Principle of Minimal Interaction states that the controller (the brain) organizes elements and dictates the task. If one element errors, the other elements correct the error without consulting the controller. The Principle of Abundance dictates that all elements participate to assure stability and flexibility of performance. This principle renders redundancy irrelevant because no degrees of freedom are ever frozen or eliminated.
The new hypothesis, the Uncontrolled Manifold Hypothesis, states that the controller (Brain) attempts to stabilize a particular variable by setting a subspace in which the control components are allowed to function. The Hypothesis predicts the relation between the controller and the allowed variability in the subspace. Following practice of the desired task, variability may increase or decrease, but must remain within the subspace designated by the controller. The presence of a multi-element system may stabilize several variables simultaneously within the subspace.
Comment by Timothy E. Hewett, PhD
Latash et al should be commended for taking on a complex topic and making it understandable with the use of a few salient examples. They reviewed a few studies involving whole-body motion, bimanual task coordination, and joint and finger coordination. Much of it was Latash’s work. The sit-to-stand studies showed differences in the horizontal and vertical motion of the center of mass and suggest an envelope of acceptable variability. The bimanual pointing study provided support for the hypothesis that the joints of the 2 arms were united in bimanual synergy, again within a subspace of variability. Latash et al state "the hypothesis and associated computational apparatus have great potential for application in the areas of motor rehabilitation and motor skill acquisition." They may be correct in their conclusion. However, the uncontrolled manifold hypothesis remains just that, a hypothesis.
In summary, this hypothesis is similar to Principal Component Analysis, except it tests hypotheses about variability within a subspace. As of yet, the hypothesis has only been tested on some simple motor tasks. Athletic maneuvers can be much more complex. Questions that remain include, how do we use this hypothesis to help athletes address dynamic joint control for both the pursuit of athletic perfection and prevention of joint injury? In addition, how does this hypothesis fit with theories of muscle agonist-antagonist synergy for the attainment of technique perfection and dynamic joint control? Until our athletes can readily exceed the performance of a Michael Jordan on the basketball court or a Tiger Woods on the golf course, this hypothesis and new ones will continue to be tested in order to better our understanding of motor control variability and motor redundancy.
Dr. Hewett, Director, The Sports Medicine Biodynamics Center, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, is Associate Editor of Sports Medicine Reports.
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