The same passive condition holds when external forces stop a physiological movement. If the arm be moved rapidly in a lateral direction- as for example, in a leap from middle Cto C' on the keyboards, it will normally be brought to a stop by appropriate cont raction of the large chest -muscle (pectoralis major). If, on the other hand, a cushion be placed at the end of the movement, and the arm be thrown freely against it , letting the cushion do all the stopping, the need for muscular contraction vanishes, and the muscle take s no part in the movement.

The muscles yielding in stretch to the increase in distance involved do so without any increase over their normal tonicity. The muscles yielding to a decrease in distance likewise do so without an alteration of their normal tone. This leaves for consideration the muscular reaction when the distance relationships remain constant and the muscular contraction changes.

1. Question 1: What happens, for example, when the finger-tip meets a fixed obstacle, such as the fully depressed piano-key-yet the muscles controlling finger descent continue to contract?
2. Question 2: Do the antagonistic muscles continue to relax proportionately or do they cease when external movement ceases?

Since all such contraction is voluntary, and since the compensatory relaxation of the antagonistic muscle group is associated with this contraction, in the sense, of course, that it contributes to the coordination of the movement, we may expect the relaxat ion to parallel the contraction. When movement has stopped, therefore, but contraction continues, the other muscles will reach the state of hyper-relaxation. The tendon will become slack in proportion to the degree of excess contraction of the other muscles. If this results, let us say, in ten units of excess pressure on the key-bed, there will be ten units of " slack " (hyper-relaxation) in the antagonistic muscular group.
The presence of this excess looseness is illustrated in the chapter on Coordination, where the experiment of pushing the tendon to either side, to show the slack, is described. The relationship concerns the act ive and passive phases of movement ouly in so far as the excess relaxation bears a close mechanical analogy to the passive state of movement, although I do not feel that they are entirely the same. They both are forms of moderate incoordination, since, in order to become effective, the hyper-relaxed muscles must first take up the slack in the tendons.