Variations in Coordination

A preliminary survey of coordinated movements discloses that muscular coordination changes with a change in the

1. speed,
2. range, and
3. force of movement.

The muscular action of a rapid movement differs from that of the same movement of extent and force, made slowly. The action of a forceful movement differs from that of a weak movement of the same range.

We speak correctly of

1. coordinated and
2. in-coordinated movements

because the same movement, as far as its external characteristics are concerned, may be made in physiologically different ways. Changes in the speed of a movement, though its range and direction remain unchanged, likewise involve variations in muscular response. These variations are not necessarily uncoordinated movements^[1]. On the contrary, they usually indicate a high degree of coordination.
When considering the effect of speed upon the muscular action in a movement, the most natural inference is that an increase in the speed of the movement is paralleled by an equal increase in the speed of muscular reaction, extending throughout the range of movement. However, this is not true.
The type of muscular reaction changes radically with variations in the speed of movement and it must do so for mechanical reasons. Take the arm for example, we have its weight as a constant and since we are dealing here with a specific movement, the 1) range and 2) general direction will be designated as constants.
Speed is a scalar (represents only magnitude) and velocity is a vector (consists of magnitude and direction). Hence, from theses parameters the only variable is speed. If the muscle contracts more quickly (forcibly) it carries the arm through a given distance in less time. The arm thus attains a greater momentum.. A continuation of this increase in muscular contraction will naturally reach its maximum at the end of the movement.

In order to bring the movement to an end, therefore, a maximal impact with some obstacle must take place, or an "instantaneous relaxation of the contracted muscles" and powerful contraction of the antagonistic muscles in order to overcome the momentum of the moving arm in that instant. Mechanically this interaction of forces, in the case of muscular contraction, may be compared to the sudden application of the airbrakes on rapidly moving railroad train or in the case of impact, to the striking together of two obstacles. The shock, which is the result of maximal force-transformation in minimal time, not only places a considerable strain on most parts of the mechanism but also lessens the control of the action considerably. This undesirable effect of shock on muscular movement, is discussed repeatedly later on, in connection with the analysis of various pianistic movement forms.

In a slow movement, the same arm generates less force, this being the product of the mass and the acceleration (F=ma). The momentum being less, the necessary inhibition can readily occur very close to the end of the movement. If we assume, in order to make this difference clearer, that one-fifth of a second is required to bring the arm to rest, a slowly moving arm may be practically at the end of its movement before the muscles need contract to bring it to a full stop.

The reason you want to execute your motions slowly and smoothly is because of the physics involved with momentum.

A rapidly moving arm on the other hand covers much more space in a fifth of a second. Hence the muscles will have to contract earlier in the course of the movement or more powerfully towards the end of the movement. This is necessary because the rapidly moving arm has a greater momentum, which requires a greater force to overcome it.

[1] Uncoordinated movement: Uncoordinated movement is also known as lack of coordination, coordination impairment, or loss of coordination.