Let us take the normal arm-hand position shown in Fig. 166 as an example. The weight of the arm is supposed to rest upon the keyboard through the finger-tips. If normal arm-weight be active, this will suffice not only to depress the keys upon which the fingers are placed but also to exert a considerable part of its weight upon the elbow and wrist joints. If these joints were completely relaxed, no resistance would be offered to the vertical action of the arm-weight, and for this reason the fulcrums must be firm. As a result, the wrist would be forced down and forward until the hand rested against the wooden casing beneath the keyboard of the piano. Another scenario which might occur is, the upper arm would seek a vertical position and pull the hand and forearm from the keys. Such an arm-position is useless for piano-playing. As a matter of fact, it is mechanically impossible to rest the entire weight of the arm on the keys. Some of this weight is necessarily supported at the shoulder. The additional remaining weight can be transmitted to the finger-tips and through them to the keyboard, only when every joint between the shoulder and the finger-tips is held sufficiently firm to support, without movement, whatever weight is to be so transmitted.
The mechanics of this leverage system are discussed later in connection with the various types of pianistic touch.

The shoulder, in such a case, is the only fully relaxed joint involved in the arm-rest position.
All other joints are firm beyond the resistance point, or at least, up to the resistance point necessary to support the arm-weight. Since the aim of the movement is just the support of the arm, the movement is fully coordinated. Here coordination and relatxation are actually opposed. When such joint-fixation exceeds the amount demanded by the force of "finger-key impact", this will result in an incoordinated movement.
The stiff wrist of pupils is an example. This stiffness is not without a physiological basis. High finger-lift (which is unnecessary and to be avoided) is normally associated with contraction of those wrist muscles which lower the hand at the wrist. The down-stroke of the finger is associated with the contraction of the muscles pulling back the hand at the wrist.

Slow finger-stroke against (with little or no resistance will eliminate the stiffness). This is the physiological explanation as to why you should move your fingers slowly into the keys. This association between finger and wrist muscles is a mechanical necessity, if the force is to act at the finger tip. Otherwise, instead of moving the key, the wrist will move in the opposite direction.

Whenever the tendon of a muscle passes over more than one joint, relative fixation of all joints over which it passes is necessary to permit the force to act at the desired point.

Since practically all tendons pass over two joints, some even over four, isolation and relaxation in their strict sense, does not exist in the physiological organism as applied in piano-playing.

If instead of a fixed position, we consider the process during the movement itself, we find similar conditions. If the arm is permitted to drop freely, its speed will vary with the time, since gravity is a constant. A speed slower or faster than the natural fall according to Newton's law of graviation shows muscular control. Muscles acting against gravity will slow the downward acceleration of the arm.

1. If the fall of the arm is faster, it shows contraction of the muscles pulling downward on the arm,
2. if the fall of the arm is slower, it shows contraction of the muscles pulling upward.

Since the center of gravity of the arm is just below the elbow, a perfectly relaxed arm would descend with this point in a straight line.

As the key-level is reached the hand would be bent back (since the various finger-joints and the wrist-joint are supposed to be fully relaxed) and the arm would assume the position described in the preceding paragraphs, so that here again complete relaxation would defeat the purpose of the movement .