Physiological Rigidity

Physiological rigidity may be divided into two classes:

1. that resulting from excess lactic acid production, the most marked instance of which is rigor mortis, and
2. that resulting from simultaneous contraction of antagonistic muscle groups.

In discussing joints and muscles we learned that most movements, at least in their component parts, involved the activity of opposite muscle groups, one of which contracted while the other relaxed. We know that with every impulse to a flexor to contract, goes an impulse to the corresponding extensor to relax. That is to say such is the case if movement (in this case flexion) at the joint be desired.
But if movement at a more distal joint be desired, or some other powerful resistance is to be overcome, flexors and extensors are simultaneously contracted, thus setting the joint firmly against movement, and enabling it to act as a necessary fixed fulcrum. The degree of rigidity depends upon the degree of simultaneous contraction. Mechanically, the particular surfaces of the joint are firmly pressed together because the simultaneous pull of the muscle tendons forces the distal surface firmly against the proximal surface.

Such a condition, since it is necessary in some degree at all joints in order for any movement to occur, involves no undue or unnatural strain upon the organism or any of its parts. If the back of the hand were not held fixed (by appropriate fixation of the wrist) as the finger-tip makes contact with the piano-key, the hand-knuckle (first-phalanx and metacarpal bone: See Diagram1 above) would be pushed up.
In order to permit maximum functioning of the finger-tip, the knuckle must remain fixed during the movement. This rigidity, since it is constantly in operation in all movements, is not felt as rigidity or stiffness, because its degree is favourable for the proper execution of the movement and hence aids, instead of interferes with, the coordination. Only when rigidity reaches an unnecessary degree, or when it is unnecessarily present in some joint not acting as a transmission-point for the force, does it constitute what the piano teacher calls stiffness.
If the tension in a joint exceeds the amount of resistance which the movement is to overcome, this excess tension in the joint is wasted effort and serves no physiological purpose. Unless extreme, it will not make itself quickly felt, nor will it seriously interfere with the execution of the movement so far as this merely overcomes a resistance. On the other hand, if the rigidity is present in a joint where movement would facilitate the total action, the condition seriously interferes with the ease, accuracy, and speed of the movement. It interposes ^[1] an additional, often very considerable resistance, which hastens the onset of fatigue and often directly interferes with the proper movements in related parts. Since, however, the efficient amount of work done by any muscle is not increased, the fatigue, or more accurately strain, is not a condition in the muscle, but in the joint, where the simultaneous contraction of the antagonistic muscles exerts supernormal pressure between the two articular surfaces. Sensations of strain or rigidity, therefore, are referred to the joints . The joints cannot experience fatigue in the same sense of the muscles, because there is no organ at the joint for activating work. Fatigue sensations localized in the joints result not from muscular fatigue but from inappropriate joint-rigidity. They are always, with a few exceptions in extreme performances, undesirable in piano-playing because they tend to destroy the correctness of the movement as a whole, and because they make judgments on extent , degree, and direction of movement more difficult.

[1] interpose verb: interpose; 3rd person present: interposes; past tense: interposed; past participle: interposed; gerund or present participle: interposing
1. place or insert between one thing and another. "he interposed himself between her and the top of the stairs"