Why the Straight Arm Cannot Push, and the Flesh at the Joints
Back and legs drive the pull; the flesh swells inside a bend and stretches outside
Continuing the comparison of pulling and pushing, the text explains that a straight arm can add no thrust: pushing with it is like wedging a piece of wood between the shoulder and the load, so the real power comes from the sinews that straighten the curved back and the bent legs. It then turns to the flesh clothing the joints, which swells on the inner side of a bend and thins and stretches on the outer side, the middle sharing this change in proportion to its nearness to the angle. A final heading opens the question of turning the leg without the thigh. The page is entirely text with no drawing.
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A straight arm adds nothing to a push
When the arm is straight, the muscles that move the elbow can do no more in pushing than a man leaning his shoulder against a load; to push with a straight, motionless arm is like interposing a piece of wood between the shoulder and the thing pushed. The real power in pulling comes from the arms joined to the extension of the back and legs and the weight of the body.
The sinews of the back and legs behind the effort
In pushing, only the sinews that straighten the curved back are used, together with those that straighten the bent leg and lie beneath the thigh and in the calf. So to a pull are added the power of the arms and the powerful extension of the back and legs, with the body's weight in the measure the man's obliquity requires.
Flesh swells inside a bend and stretches outside
The flesh clothing the joints grows and diminishes in thickness with the bending or extension of the members: it grows on the inner side of the angle formed in the bend and thins and stretches on the outer side. The middle part between the convex and concave angle shares this increase or decrease, more or less according to how near it lies to the angle of the bent joint.