Principles of Motor Control and Movement Accuracy

1. Principles of Motor Control and Movement Accuracy Chapter 5

2. Objectives • Understand the concept of invariance in motor control • Explain how the speed and amplitude of a movement can influence movement accuracy • Understand the fundamental causes of inaccuracy in quick movements • Apply the principles of rapid actions to real-world settings

3. Preview Root canal operations take about two hours. Dentists accept the fact that the task is slow and tedious, and to be successful they must execute their movements in a concise and accurate fashion. • Why is slower performance preferable for many tasks? • Does speed of performance always have to be sacrificed for accuracy?

4. Overview • Fundamental timing structure of generalized motor programs • Scientific principles related to the production of movement • Underlying causes of errors in rapid movements and possible ways to maximize speed and accuracy of performance

5. Relative Timing: Invariant Features Components of movements that do not change when the speed, size of the action, force used to produce the action, or trajectory changes

6. Generalized Motor Program • A motor program that defines the pattern of movement rather than a specific movement • Can be adapted by selecting different parameters to produce variations (surface features) of the pattern to meet the environmental demands

7. Surface Features • Easily changeable components of movement (parameters) • Flexible components of movement

8. Examples of Surface Features • Speed of a rapid movement • Size of the action • Forces used to produce the action • Trajectory of the movement

9. Fitts’ Law: Two Concepts • As speed increases, accuracy decreases (speed–accuracy trade-off). • A mathematical formula says that movement time increases linearly with movement difficulty (errors increase with increases in movement distance and decreases in movement time).

10. Motor Programs and Speed and Accuracy • Closed loop—more accurate, more time for correction, slower • Open loop—predetermined, no feedback loop, rapid

11. Sources of Errors in Quick Movements • Scientists believe that the “noisy,” or inconsistent, processes in the central nervous system cause errors in the performance of quick movements. • The central nervous system is responsible for converting nerve impulses into motor unit activation. These processes may not always produce desired actions during muscle contraction.

12. Speed–Accuracy Trade-Offs • Spatial accuracy is the type of accuracy required of aiming movements in which spatial position of the movement’s end point is important to the performance. • Timing accuracy is the type of accuracy required of rapid movements in which accuracy of the movement time is important to the performance.

13. Violation of Speed–Accuracy Trade-Offs • When people were asked to perform a rapid striking task at different times, speeding up the movement resulted in a decrease in timing errors. • For skills in which the goal is to minimize timing error, the main factor seems to be movement time; performers are able to produce shorter movement times more accurately than longer movement times.