Compensatory or modifying Reaction
But a free body, as here understood, would be an uncontrolled body. Therefore, toward the end of the movement , a compensatory or modifying reaction must take place in order to lead the movement to the desired goal. And the same muscle that cont racts in order to initiate a movement, will cont ract also to inhibit another movemcnt in the opposite direction. If the pectoralis major contracts to bring the arm rapidly toward the body, it must contract also to stop the rapidly outgoing arm. This contraction is shown in Fig. 34. The movement made was the reverse of that used in Fig. 33. The shift made was from middle C to C3, at a high velocity, and was purposely brought to an abrupt stop; that is to say, the original impetus given to the arm was considerably more than that required to bring the arm up to the stopping point, three lined C.The movement had to be inhibited in order to keep the hand from passing beyond this point .
When this inhibition is
absent, and the "thrown" arm is allowed to travel on freely, until its own length and gravity change the direction of movement to a fall, the contraction of the muscle is absent. It is likewise absent if t he hand or arm be allowed to strike an outside obstacle, such as a cushion. In this case one must be very careful to avoid any inhibit ion which the knowledge of the presence of an obstacle will naturally produce through the fear of pain, and must let the arm fly entirely freely against the obstacle.
Fig. 34a, shows the contraction of the pectoralis major when voluntary inhibition takes place; b, the absence of contraction when the force of the movement is allowed to expend itself freely, or against an obstacle. Eight strokes were made, four with muscular inhibition and four against an outside obstacle, in this case a cushion placed at the point where the movement ended.
The muscular contractions for the four voluntarily inhibited strokes are seen clearly; the arrows point to the four free strokes, where, if there had been muscular contraction it would have deflected the recording point similarly to the four points in a. Under the first arrow a slight deflection is still noticeable, caused by a fear reaction against the imaged pain from striking an obstacle forcibly. The elimination of such a contraction must be assured, and at times requires considerable practice.