A finer and more convincing proof is found when we record by graphic means, the contraction of a muscle or its non-extensible tendon, when the muscle itself is moving the part, and when the same part is being moved by an outside agency. The following figures are records of the contraction of the third finger tendon of the extensor communis digitorum^[1], the muscle responsible for all finger-lift in the hand-knuckle. The method of recording is shown in Figure 29. A long aluminum lever, with an adjustable setting is so placed that its edge rests upon the tendon, the contraction of which is to be recorded.

Its position in any other plane of movement is fixed by its own axis and the two points resting upon the hand near the tendon. Any contraction of the tendon will then lift the lever, and by making this sufficiently long, and placing the fulcrum sufficiently close to the tendon, any degree of magnification may be secured, thus recording even extremely slight degrees of contraction. These are then transferred upon the smoked surface of a revolving drum, passing beneath the free end of the lever .

The results are seen in Figure 30. In the upper line, the finger was lifted by another person, in the lower, the subject lifted his own finger by voluntary muscular contraction. An ascent in the line indicates a pull on the tendon resulting from the contraction of the muscle, the height of the ascent furnishing a fair index of the degree of contraction. In the passive movements there is complete absence of contraction. In the active movements the marked contraction for each of three successive finger-lifts is clearly shown. The number, extent, and speed of movement were exactly the same for both experiments.

[1] extensor communis: The extensor digitorum muscle (also known as extensor digitorum communis) is a muscle of the posterior forearm present in humans and other animals. It extends the medial four digits of the hand.