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George E. Carvell, PhD, PT   University of Pittsburgh
 
Motor Control Strategies and Task Demands II
See Brooks Fig. 1.3, p. 10
      Slow movements tend to be feedback-assisted or feedback-guided. Motion paths are 'discontinuous'. Velocities tend to be multi-peaked. EMG is often characterized by repeated Agonist 'steps' with little contribution by the antagonist, although the pattern differs according to the stiffness required and/or the action sequence required (cyclic vs. non-cyclic). Slow 'discontinuous' movements may be used: 1. when one is a novice, 2. when one is uncertain how to proceed, 3. when errors creep into previously 'automatic' skilled actions, or 4. when absolute accuracy is a must.
      Moderate to fast movements are thought to be programmed with the potential for but not a requirement of, feedback adjustments. Feedback may be used as 'knowledge of results' on the next trial. Motion paths are 'continuous' smooth transitions from beginning to end. EMG is triphasic: 1st Agonist burst followed by an Antagonist burst followed by a 2nd Agonist burst. This triphasic pattern results in a single velocity peak. The antagonist EMG activity and movement velocity peak simultaneously. These 'continuous' movements are used when skill has been obtained, when one is confident in how to proceed, and environmental factors or task demands have not changed. If speed and accuracy are essential a slightly reduced speed and use of feedback for minor adjustments may be required as the movement proceeds.
      Ballistic movements, by definition, are brief and very fast (movement time <200 msec). Skilled self-terminated ballistic movements are thought to be pre-programmed with no feedback assistance once they are launched. Motion paths are smooth, rapid transitions from start to finish. EMG is biphasic with an Agonist burst followed by an Antagonist burst that corresponds temporally to the velocity peak. Ballistic movements are not the usual choice for most things you do. However, when speed is essential you need to go ballistic. If these movements are not practiced they will be relatively and absolutely  inaccurate. With practice skilled ballistic movements become relatively more accurate. With talent and practice, ballistic motions become relatively & absolutely more error-free.