On the Movement of Animals, Part Two

Chap. 1. Concerning the modes, and mechanical operations, by which the contraction of muscles can occur.

But such a force of impediment is internal and is exercised by the animal itself, and it acts throughout the course of the whole life, except for brief pauses when, naturally, large weights are lifted by order of the will or the appetitive faculty; the animal would therefore be compelled to be fatigued by huge and assiduous exercise through the course of its whole life, not so that it might perform a useful work for the animal—that is, so that it might lift weights—but only so that it might impede the action of the muscle machines through continuous conflict; namely, it would labor so that it might do nothing, or so that it might induce the rest of the animal; which ridiculous and imprudent action seems entirely absurd and contrary to the most artful economy by which the actions of an Animal are exercised.

Moreover, just as in the tension of a bow a sense of lassitude and languor is not perceived by the material machine of the bow, but by the hands if they were compelled to keep it continuously tense, so a huge lassitude ought to be borne by the internal force and organ of the animal impeding the action of the muscle, while, namely, it held its machines violently distracted. Wherefore, during the whole course of life while we rest and are recreated by sleep, we would then be afflicted by assiduous and insane labor and lassitude; and on the contrary, just as the restitution of a bow to its natural contraction is not laborious—nay, is conformable to its nature—so the natural restriction of the small machines of the muscles, by which huge weights are lifted, ought not to be laborious and create languor, but rather should be pleasant, just as rest from labor and from violent distraction and restitution to the natural state is highly enjoyable. Since, however, this is false, and lassitude is perceived when we sustain large weights, not when we rest, it must be admitted that the contractive force of the muscles