Muscular and Mechanical Isolation

1. The principle of action and reaction illustrates once more the impossibility of muscular or mechanical isolation in the physiological mechanism^[1] of man.
2. The motionless state may result (and usually does) from a multiplicity of forces, the total effect of which is equal and opposite.
3. It is the familiar principle of the parallelepiped of forces.

1. Lack of motion frequently results in the necessary fixation of some point as a fulcrum.
2. The muscle pulling the index finger toward the thumb is joined to the bone which carries also the connection of the thumb adductor.
3. When it is desired to abduct the second finger toward the thumb, the position of the thumb must be fixed to act as a fulcrum.
4. Consequently, the extensors of the thumb are contracted.
5. Without the action of the extensors of the thumb, the thumb would move toward the second the finger.
6. What is observed? Only one finger moves, but the necessary muscular coordination is not restricted to that finger.
7. When we record the position of the various joints or parts of the arm during a movement, we find that what appears to be a motionless state is in reality a minute movement.
8. Each finger has 3 joints
   1. hand knuckle (metacarpophalangeal): The hand knuckle consists of the first phalanx and metacarpal bone.
   2. midjoint : Connection between the first phalanx and second phalanx.
   3. nailjoint. Connection between the second phalanx and third phalanx. Both the midjoint and nail joint are interphalangeal.
9. The method of direct lever recording was used. During this procedure, one aluminum lever being attached closely behind the second finger joint (first interphalangeal) of the index finger.
10. The other lever was attached between the hand-knuckles of the third and fourth fingers.
11. In order to detect very small movements in the hand knuckle, the leer was so adjusted that these appeared on the record magnified three times.
12. Reading from left to right, we find both finger and hand stationary.
13. The first part of finger descent is accompanied by a similar descent in the hand itself.
14. This indicates that the player making this movement is using some forearm or wrist movement.
15. When the finger reaches the key surface the hand knuckle is affected.
16. The retardation of finger descent resulting from the resistance of the key is directly reflected in the ascent of the hand knuckle.
17. When the key is fully depressed the effect upon the hand knuckle is naturally more marked.
18. The significant point is that even the slight resistance introduced by the piano key is not without some effect upon the hand-knuckle.
19. To infer the absence of this reaction would be to misconstrue entirely the underlying mechanical principle.
20. The reaction of finger key-bed impact upon the wrist is seen in Figure 7.1

1. A non-percussive touch was used in this instance, hence there is no retardation in the descent of the finger until the key is fully depressed.
2. At that moment the line recording wrist movement rises, and as the finger subsequently rises, the wrist in reaction falls again to the original level.
3. This reaction goes past the wrist and to the elbow.
4. At the point a, the finger has not met any resistance, and is travelling freely through the air, hence there cannot be impact-reaction at another joint.
5. This fact is conclusively shown by recording the movements of finger and hand when the former is moved without meeting any resistance.