Range of Movement

The shift of muscular activity varies in proportion to the range of the movement. The extensors of the fingers, when finger extension has reached its maximum, bend the wrist backward (dorsal flexion). The extensor of the elbow is followed by the rear adductors of the humerus, when the movement is extensive. The deep flexor of the finger (flexor digitorum profundus) flexes first the third phalanx, and then as the range of movement increases it flexes in turn, the second and the first phalanx, and finally the wrist. The first phalanx and metacarpal bone is the fulcrum at which the finger should be lowered to set the keys in motion.
This spread of movement results from the passage of the tendon over the intervening joints and is a mechanical necessity. The mechanical arm described in the module on Relaxation illustrates the muscular action very positively. If the string corresponding to the deep flexor of the finger be pulled slightly, it flexes the nail joint. If the pull continues it flexes the middle finger joint , then the hand-knuckle and finally the wrist.

Moreover, the multiplicity of function, as a result of which one muscle acts in various ways, affects the range of movement. Observations made upon the biceps, which flexes the elbow and supinates the fore-arm seem to indicate that when ouly one of these motions is made the other is not simply eliminated, but is prevented by appropriate contraction of the antagonistic muscles.
That is to say, if only flexion at the elbow be desired supination will be prevented by contract ion of the pronating muscles, in this case the pronator teres. If the movement be extended to cover both supination and flexion, the cont ract ion of the pronator is absent . If this conclusion be correct , it seems to support the theory of all-or-none action of muscle. The entire biceps contracts in either case, but the effect of its supinating part is neutralized by antagonistic cont raction elsewhere. This part of the biceps, accordingly, is not in a state of relaxation.
The line of action of a muscle is determined not by the muscle itself but by the position of the points of origin and insertion . Since these for any one muscle are fixed, it is scarcely possible that variations in the line of action can result from the contraction of only a part of the muscle.

The neutralization of the undesirable part of the action by contract ion of the antagonistic muscles natur ally results in a certain amount of fixation, and is furth er evidence that a coordinated movement is by no means necessarily a relaxed one. Whatever contraction of antagonistic muscle-groups is present is that much hypertension, but this, as a whole) remains coordinated. A similar condition exists in the relationship between the finger extensors and the wrist flexors; and between the finger flexors and wrist extensors. In a lorcelul finger-stroke, lor example, the ascent of the wrist is prevented by the cont ract ion of the muscles that lift the hand backward at the wrist. The tendons of the fingerflexors likewise pass the wrist , and, as a result, fixation of th e wrist occurs, commensurate with the force desired at the finger-tip .

Finally we have to consider what happens to a coordinat ion when a moving part of the body strikes an obstacle. A simple case of this kind is offered by the finger-key impact , and in order to simplify the mechanical analysis, wl! shall consider the key immovable. This unnatural condition, as we shall see, in no way interferes with the mechanical principle involved.
If the analysis of tonus and relaxation be correct, and if, when the descending finger strikes the obstacle, additional force be acting upon the latter, there must be corresponding relaxation in the antagonistic muscles in any coordinated movement . As an illustration : If the finger strike the key with a force of 70 (assuming the total tonus value at 100) then the antagonists will be acting with a force of 30. If, after key impact an additional force of lObe exerted upon the key, there must be a corresponding decrease in force in the antagonistic muscles, which will then act with a force of 20. And since there has been no movement for the additional 10 units of force, the other 10 will represent so much slack orlooseness, When the 10 units of downward act ing force are released, the antagonist ic muscles will take up th eir 10 units of slack, but there will be no actual finger-lift. This begins only after the 10 units of cont ract ion have been covered. At this point the resistance of the finger-weight is introduced and results in a momentary jerk in the muscular contraction. Mter thi s point the contraction is smooth, since no new and sudden resistance is met during finger ascent.