Coordination of Movement Amplitude and Duration

In general, goal-directed movements have one accelerating phase and one declerating phase if their trajectory is approximately along a straight line. This is not true any longer if an obstacle requires that the movement is curved or if the movement requires a specific curved trajectory. In these conditions the movement is segmented and consists of a sequence of trajectories, each with an accelerating and declerating phase.

Corresponding to the accelerating and decelerating phase, the activation of muscles has two components: one burst of electromyographic (EMG) activity for the muscles that start the movement (agonist) and a burst of EMG activity in the muscles that brake the movement (antagonist). For movements made as fast as possible, a third burst of EMG activity may be observed in the agonist muscle. It is generally accepted that this third burst contributes to stabilization of the limb at the final position. The few data available suggest that the third burst in the triphasic activation is not simply due to reflex components, but that it has, at least partially, a central origin. Its presence and amplitude are to some extent under voluntary control and can be changed independent of the amplitude and duration of the first and second bursts.

Remarkably, all goal-directed movements have a similar shape more or less independent of the amplitude and duration. Each velocity profile is bell-shaped; velocity profiles of movements with different amplitude and duration can be superimposed after proper scaling in time and/or amplitude. If the amplitude is varied, keeping the movement duration constant, the velocity profiles are scaled in amplitude proportionally to movement amplitude. After appropriate scaling, all corresponding EMG bursts in the agonist muscles superimpose. This is not true for the EMG burst in the antagonist muscles, indicating a more complex activation of the antagonist muscles for braking movements as a function of movement duration and amplitude.

When movement duration is varied, keeping movement amplitude constant, the duration of the accelerating and decelerating phase of the movement is scaled by the same amount. However, the duration of the two phases varies with the inverse of the square of that scaling factor. This is not true for EMG in the antagonist muscle, presumably owing to muscle nonlinearities. These observations can be generalized to both single- and multijoint movements.

The fact that the shape of the velocity profiles is more or less the same for single- and multijoint movements despite the large variability in duration and amplitude has led to the idea of the generalized motor program. This idea implies that the activation pattern of the muscles is based on a general and abstract representation of a motor program, which acts as an elementary unit for any type of action and which can adjust movements for different task conditions by simply scaling a basic velocity profile. However, in movements like grasping and for movements aiming at targets of variable size, the velocity profiles were distinctly different from velocity profiles of fast movements to large targets. These data clearly indicate that planning and control of trajectories are task dependent and cannot always be reduced to a generalized motor program. A coherent model, which describes and explains the movement trajectories for different conditions, is still lacking.