RESEARCH DIGEST The Dynamical-Systems Approach to Studying Athletic Injury

Figure 1 Self-organization of the sensorimotor system. RADITIONAL MEASURES of movement in sports-medicine research have viewed increases in variability as extraneous noise, as error, or as being representative of pathological dysfunction.1,2 In the past 20 years, however, motor-control researchers have developed a theoretical framework for studying movement variability using nonlinear dynamics that might be applicable to sports medicine. This has led to the theory that movement variability is essential for the stability and function of the sensorimotor system.3 The model that considers this type of inherent movement variability as essential is known as the dynamical-systems approach to motor control.1-5 Variability in movement patterns has traditionally been viewed as nonoptimal deviation from invariant, repeatable patterns of movement for particular skills.2 Any variability in these patterns was considered either unnecessary noise or as representing impairment in pattern-generated movement. There is a growing consensus, however, that there is a functional and essential role of variability in maintaining health and homeostasis.1-4,6 This approach has been used in research on pathological conditions related to severe neuromuscular dysfunction such as Parkinson’s disease, but its role in sports-medicine research is in its infancy. From the dynamical-systems perspective, there is a great deal of redundancy within the biomechanical degrees of freedom (DOF) of multiple joint segments, affording the sensorimotor system numerous options in executing specific movement tasks.1,2,4,5 An example of this process is an upper extremity reaching task. The upper extremity has multiple DOF (three at the shoulder, one at the elbow, and three at the wrist), and a reaching task requires far fewer DOF than are available in the entire extremity. By alterT ing the contributions of the available DOF, there are multiple combinations of joint movements possible to enable task execution. If one of the components in the extremity fails to perform, the other components can alter their contributions to accomplish the same result.2 This results in the ability to use various combinations of DOF to accomplish the same task. These DOF are organized into coordinative structures, or muscle synergies spanning multiple joints, through the process of self-organization based on task, environmental, and organismic constraints (Figure 1).2 The sensorimotor system will organize itself based on the complexity of a specific task, the environment in which the task is being performed, and the ability (or disability) of the organism to execute the task. The functional role of variability might be protective. By having multiple possible ways to accomplish a particular task, variability serves to reduce the repeated stress on tissues and might allow for greater flexibility in dealing with unexpected perturbations.