Motor Skills

Four Problems in the Study of Motor Skills

File photo | Credit: Aussie Child Care NetworkOpens in new window

In the study of motor skills, four problems occupy a central place (Rosenbaum, 2010). None of the characteristics among motor skillsOpens in new window presented in previous article, for instance, can be adequately addressed without some explanation concerning these four persistent problems.

Indeed, the major theories that guide research concerning motor behavior are largely attempts to provide satisfactory answers to these central questions. The fact that there are different theories is testimony to the complexity of these problems, and to the ongoing lack of a consensus in how they should be answered. These four persistent problems include:

1. the degrees of freedom problem,
2. the perceptual-motor integration problem,
3. the serial-order (timing) problem, and
4. the skill acquisition problem.

The Degrees of Freedom Problem

One of the greatest challenges facing movement scientists concerns answering the question of how various units of action (e.g., joints, muscles, motor units, cells) are organized and controlled in order to accomplish skill goals.

Specifically, how does a person, in response to his or her environment, coordinate and control a complex system of bony segments, linked by joints and layers of musculature, that is capable of moving in a variety of different ways.

Recall that in the performance of any motor skill, there are many ways the motor system might be organized in order to produce movements capable of achieving skillful goals (i.e., motor equivalence).

Only one of many alternative means of achieving a desired outcome must be selected, however. The question is: How is this accomplished?

Stated more precisely, it is the number of dimensions in which a system can independently vary. The term independent is critical to our understanding here.

Simply, it means that regardless of the value one thing takes on (e.g., muscle A contracts a certain amount), another thing within the same system is still free to take on any of the values of which it is capable (e.g., muscle B is free to contract to any value, or to not contract at all — it is not limited by the value to which muscle A has contracted).

We can consider the degrees of freedom available to an individual as all of the possible choices for organizing desired motor skills. We can further specify these available degrees of freedom at various levels of analysis. If what we control during movement are the joints, then to move the arm, for example, would require seven degrees of freedom.

Three degrees of freedom are available at the shoulder (it can move up and down, side to side, and rotate), two at the elbow (it can flex and extend and it can rotate), and two at the wrist joint (it can move from side to side and it can flex and extend). If we go a step further and consider muscles as the unit that is controlling movement, the number of degrees of freedom rises dramatically.

In order to move the same arm successfully, we must now regulate a minimum of 26 degrees of freedom: 10 muscles at the shoulder point, 10 more at the elbow joint, and 6 controlling the different movements of the wrist joint. If this analysis of an arm movement is extended to the unit of action of motor units, the estimated number for the degrees of freedom available rises exponentially— into the thousands. The greater the number of degrees of freedom that must be controlled, it should be pointed out, the greater the complexity of the problem that must be solved by the motor system.

Over half a century ago, the Russian physiologist A. N. Bernstein recognized that human coordination emerges from the accumulated involvement of many redundant degrees of freedom that underlie multijoint movements.

The many different ways in which movements might be organized in order to accomplish skill goals is made possible by a large surplus of degrees of freedom available. How one pattern of organizing these redundant degrees of freedom is selected rather than another has come to be referred to as the degrees of freedom problem.

In initially formulating this problem, Bernstein argued that the computational demands placed upon the central nervous system were too great for it to be considered the sole executor of movement control. The degrees of freedom problem first formulated by Bernstein remains at the core of today’s contemporary theorizing concerning the control of human movement.

The Perceptual-Motor Integration Problem

Skills do not occur in a perceptual vacuum. Perception is the process of acquiring, selecting, interpreting, and organizing sensory information. Skill movements are guided by sensory information arising from both the environment and performer’s body.

How perceptual information is coupled with bodily movement in order to achieve skillful actions is one aspect of the perceptual motor integration problem. For example, in order to catch a tossed object, the trajectory of the object must be determined and related to the current position of the hand that will be used to catch it.

The position of the fingers, as well as the “firmness” with which the catching limb is maintained, must also be adjusted depending upon the perceived size, shape, and velocity of the tossed object. Moreover, these adjustments must be initiated prior to contact with the tossed object. Thus, the relevant aspects of a person’s environment must be perceived and effectively coupled with movements of the catching limb and hand to successfully accomplish the goal of intercepting and grasping the tossed object.

A persistent problem in the study of motor skills is to understand how the highly coordinated movements of limbs and bodily segments are effectively coupled with sensory information to produce the exquisitely executed skills of which humans are capable. This is referred to as the perceptual-motor integration problem.

Although perception guides movement, movement also influences perception. One obvious reason for this is that movement transports sensory preceptors to new locations, thus benefiting perception.

Turning your head supplies additional visual information concerning the environment, thus enriching and benefiting perception. Walking to a new location allows you to see and hear more of what is present in the environment. Exploring objects with your hands allows you to better perceive their shapes and surface textures.

Movement affects perception in even more subtle ways, however. There is growing evidence that perception is altered by the movements that humans are capable of producing (Proctor and Dutta, 1995; Vickers, 2007).

When people view a cluttered environment through which they must navigate, for instance, they are most likely to perceive those environmental features that afford the greatest opportunity for passage given their movement capabilities (such opportunities are called affordances).

Individuals with poor or limited movements skills are most likely to perceive features within the environment that provide for the greatest ease of passage, whereas those with a greater repertorie of movement skills are the most likely to perceive features indicating a more difficult but shorter path across the cluttered space, for example. In other words, a person’s movement capabilities play a role in what he or she perceives in the environment, supposedly providing the perceptual cues that will be of the greatest benefit.

Although perception and movement are intricately linked and mutually dependent upon one another, there is considerable debate concerning how they are linked. Some theorists posit that higher-order cognitive representations of skills mediate between perception and movement, viewing perception as “information” used to plan and carry out movements. Other theorists emphasize a direct link between perception and action, seeing little need for additional mediation (Gibson, 1966, 1979).

Regardless of which theoretical perspective is assumed, however, the coupling between perception and movement remains one of the most challenging problems facing motor skill theorists and researchers.

The Serial-Order (Timing) Problem

Whenever skills require the sequencing of discrete movement elements, some means of organizing their execution is required (Proctor and Dutta, 1995). A dancer rhythmically linking dozens of individual movements into delicate routines of choreographed precision and grace, a typist whose fingers appear to fly from letter to letter on a keyboard without error, the competitive swimmer perfectly coordinating patterns of arm circles and leg kicks while racing through the water — all illustrate the human capacity for stitching individual threads of movement into the many patterns representing the fabric of motor skills.

We need not look only to expertly performed skill examples such as those of the dancer, typist, or swimmer to observe the intricate timing involved in most motor skills, however. Daily activities such as walking, speech, and dressing are also comprised of the linkage of many precisely timed and ordered submovements.

The serial-order problem consists in how the ordering and timing of the various subelements that comprise motor skills are controlled. One possible explanation is that skills rely on some sort of stimulus-response mechanism so that sensory feedback from one response acts to initiate the following response in a sequence. This notion is referred to as linear chaining.

A major weakness in the linear chain notion, however, is that no single motor element acts to elicit the same response in every skill situation, but may indeed be followed by many different responses depending upon the particular situation.

This weakness, first pointed out many years ago by Lashley (1951), has lead to the search for alternative explanations, for how motor acts are timed. Two major alternatives have been advanced, with both a hierarchical model based upon cognitive control and a constraints-led model based upon the interaction of the actor and environment proposed.

Although theoretically different, both models have provided revealing insights and continue to advance our understanding of how the timing and sequencing components of human motor skills are controlled.

The Skill Acquisiion Problem

In presenting the characteristics common to all motor skills, we have stated a seeming contradiction; that is, skills exhibit motor variability—even when performed in highly stereotypical fashion, they are never organized in exactly the same way twice.

On the other hand, skills exhibit motor consistency. Through repeated experiences, people become more capable of meeting the goals of a skill—they become more skilled. How does such learning take place given the variability inherent in skilled behaviours? This question represents the skill acquisition problem.

To the movement practitioner, this is probably the most critical problem in the study of motor skills. Certainly, an adequate understanding of how motor skills are acquired is critical to the design of effective practice experiences. To the movement scientist also, though, the question of how motor skills are acquired presents many research and theoretical challenges. The general problem of skill acquisition is really expressed as a series of subproblems. Among the most intriguing and widely researched are the following:

1. What are the underlying processes responsible for skill learning?
2. How are skills represented in the nervous system? Or are they?
3. Are some skills innate, or are all skills acquired through experience or practice?
4. Do people pass through identifiable stages when learning skills? If so, are they the same for everyone?
5. As motor skills are acquired, what changes occur within individuals?
6. What kind of instructions prove of the greatest benefit to learners? Of what information are learners in the most need?
7. Is there an upper limit to how proficient an individual can become in a given skill?
8. Why do individuals differ in their capacity for learning different skills?
9. How does the learning of one skill influence the learning of other skills?

Answers to these questions, as well as the broader question concerning the primary mechanisms responsible for skill acquisition generally, remain among the most challenging to movement scientists.