[continuing] made by your figures, that you disclose those muscles which alone are engaged in the movement and action of your figure; and let the muscle that in such a case is most used be shown most, and the one that is less used be expressed less, and the one that does nothing remain slack and soft and with little display. For this reason I persuade you to understand the anatomy of the muscles, cords (tendons), and bones, without which knowledge you will do little. And if you portray from life, perhaps the model you choose will lack good muscles in the very action you wish it to perform; but you will not always have good nudes at hand, nor will you always be able to draw them; it is better and more useful for you to have such variety in practice and in mind.

[304] On the movements of man and other animals.
The motions of animals are of two kinds, that is, local motion and actional motion. Local motion is when the animal moves from place to place; and actional motion is the motion the animal makes within itself without changing place. And local motion is of three kinds, that is, ascending, descending, and going along level ground. To these three are added two, namely slow and fast, and two others, namely straight motion and winding motion, and one more besides, namely leaping. But actional motion is infinite, together with the infinite operations which man, not without harm to himself, often pursues.
The motions are of three kinds, that is, local, simple actional, and the third is motion compounded of actional with local.
Slowness and speed ought not to be counted among the local motions themselves, but among the accidents of those motions.