The dynamical systems perspective (DSP) suggests that human movement contains inherently varying patterns, even when the individual intends to repeat the same movement. Rhythmic movements like walking/running and trained discreet movements like a golf swing or racquet swing have subtle variations in the degrees of movement. The DSP in regard to human performance suggests that those with a wide range of movement variability have increased capacity to adapt during a movement than those who have a smaller range of movement variability. Consistent performance outcomes that repeatedly occur with variable patterns of movement suggest that motor control strategies are in place to adapt to these variations.

Small low cost sensors in motion capture technology have resulted in a surge in data collection of biomechanical movement variability. Analysis of the data suggests that seemingly repetitive motion (identical movements as those used in running, biking, pitching) have inherent subtle variations in the relative positions, angles, speeds and muscle tension of body parts, without a change in result.

For example, a golfer may appear to have the ‘same’ swing from the naked eye, but when examined on a micro scale, very small biomechanical changes are taking place within each swing, without a change the club's head speed, arc of the ball or, result of the effort.  How is this possible?*

This theory of human movement implies that the human body is always compensating for the random variations in its movement, (movement variability) due to many different factors- fatigue for example (a pitcher is not the same physiologically in the first inning as he is by the 5th inning.)  These tiny movement compensations are the body’s way of adapting to the inherent movement variability. During a swing, if a golfer's hips are not ‘loaded’ enough, she may speed up the rotation of her upper body.  If her elbow is too stiff, she may loosen her wrist. These minute changes are detectable only with high-speed sensors.

External and Internal Cueing are verbal commands used by movement educators that help an athlete direct their focus of attention.  An external locus of attention is a point in the environment by which  an athlete can visually reference or compare their body - in relation to their actions.  A movement educator can offer external cues with language, by asking a learner to focus their attention on something, an object, or target that is outside their body, during a movement, whereas an internal cue is usually a part of the body or process of moving a body part.

In basketball  free throws, an external cue command could be "focus on the back of the rim", which is an external cue, as opposed to "focus on the wrist motion" - which is an internal cue, in this case - a part of the body.  External cues depend heavily on the visual system, which is the most dominant of the sensory inputs. However the body does not need visual input to know where it is in time and space.  See Visual occlusion training. It is used in training to augment/encourage other sensory inputs -  to help an athlete become more aware of other sensations.

Kinesthetic awareness is also called body awareness. It is a person's knowledge of the location  and movement (in space and time)  in relation to the world around them.  This process to create this continual awareness occurs primarily in the subconscious. The brain creates a model -much like a 3d map - of the body in the world by constantly comparing incoming external and internal information. (See phasic and phasi-tonic receptors)

Neuroplasticity and compensation to change are the benchmarks of motor learning that are revealed in elite performance. A highly skilled quarterback can throw an accurate pass while the ‘usual’ mechanics are not available to him, for example while he is being tackled, or having his arm grabbed. And an elite tennis player can perform very well while playing with a frying pan instead of a racquet. High performance in these examples are demonstrations of advanced motor control strategies-aka adaptability.

It could be related to Entropy: the scientific principle that all organized systems eventually become disorganized. But the better question perhaps?  Is movement variability beneficial to human performance?  Some researchers think it helps to avoid repetitive use injury, aids in conservation of resources (muscle fuel), or helps distribute the physical demands of the tasks across body parts in changing environments.
 

Many pedagogical techniques, especially in sport specificity training, occlude necessary sensory input during the execution a skill. This technique trains the athlete to ‘find a different way’ to get the outcome they want. Researchers sometimes call these ‘task constraints’.  Trainers and coaches sometimes call it compensation training, in accordance with the constraint led approach to motor skill instruction.

There are different physical and temporal (time-related) demands for closed/discreet skills, those with a beginning and end (pitching, basketball, free throws, American football field goal kicks) when compared to open/fluid skills (running route to tackle an opponent in rugby, or a crossover-dribble-head-fake- jump-shot in basketball. These more complex skills are often multiple discreet skills sequenced together in a longer movement. 

Additionally, different situations in all sports demand alternating attention between internal and external cues. Both offense and defense require predicting, and then reacting to external cues. While other continuous repetitive skills (cycling, running, rowing) require sensing more internal movement cues from one’s own body. In general, the role of movement variability for different types of sports, positions, and unique skills is still being explored.  The most useful studies in a human performance would reveal an optimal level of movement variability and adaptation strategies correlated with performance measures and injury rates.

The most robust experiment is the real-world environment  including real world competition, which generally is comprised of practice-then-performance. Instead of training an athlete to achieve automaticity, I suggest training for ‘elite automaticity’  - train to achieve a wider bandwidth in which elite performance occurs, to adapt to environmental changes both internal factors and external factors by training kinesthetic awareness AND adaptation strategies:
 
- Dr. Jo Shattuck