Of human movement

When you wish to represent a man in the act of moving some weight reflect that these movements are to be represented in different directions. A man may stoop to lift a weight with the intention of lifting it as he straightens himself; this is a simple movement from below upwards; or he may wish to pull something backward, or push it forward or draw it down with a rope that passes over a pulley. Here you should remember that a man's weight drags in proportion as the centre of his gravity is distant from that of his support, and you must add to this the force exerted by his legs and bent spine as he straightens himself.
The sinew which guides the leg, and which is connected with the patella of the knee, feels it a greater labour to carry the man upwards in proportion as the leg is more bent; the muscle which acts upon the angle made by the thigh where it joins the body has less difficulty and less weight to lift, because it has not the weight of the thigh itself. And besides this its muscles are stronger being those which form the buttock.
The first thing that the man does when he ascends by steps is to free the leg which he wishes to raise from the weight of the trunk which is resting upon this leg, and at the same time he loads the other leg with his entire weight including that of the raised leg. Then he raises the leg and places the foot on the step where he wishes to mount; having done this he conveys to the higher foot all the weight of the trunk and of the leg and leaning his hand upon his thigh, thrusts the head forward and moves towards the point of the higher foot, while raising swiftly the heel of the lower foot; and with the impetus thus acquired he raises himself up; and at the same time by extending the arm which was resting upon his knee he pushes the trunk and head upwards and thus straightens the curve of his back.
Man and every animal undergoes more fatigue in going upwards than downwards, for as he ascends he bears his weight with him and as he descends he simply lets it go.
A man, in running, throws less of his weight on his legs than when he is standing still. In like manner the horse, when running, is less conscious of the weight of the man whom it is carrying; consequently many consider it marvellous that a horse in a race can support itself on one foot only. Therefore we may say regarding weight in transverse movement that the swifter the movement, the less the weight towards the centre of the earth.…
It is impossible that any memory can hold all the aspects and mutations of any limb of any animal. We shall demonstrate this by taking the hand for an example. Since every continuous quantity is divisible in infinitum the movement of the eye, which observes the hand, travels through a space, which is also a continuous quantity and there divisible in infinitum. And in every stage of the movement the aspect and shape of the hand varies when it is seen, and will continue to vary as the eye moves in a complete circle. And the hand in turn will act in a similar way as it is raised in its movement, that is to say it will travel through space which is a continuous quantity.
There are [four] principal simple movements in the flexion performed by the joint of the shoulder, namely when the arm attached to the same moves upward or downwards or forward or backward. One might say, though, that such movements are infinite. For if we turn our shoulder towards a wall and describe a circular figure with our arm we shall have performed all the movements contained in the said shoulder. And, since [every circle is] a continuous quantity, the movement of the arm [has produced] a continuous quantity. This movement would not produce a continuous quantity were it not guided by the principle of continuation. Therefore, the movement of that arm has been through all the parts of the circle. And as the circle is divisible in infinitum the variations of the shoulder have been infinite.
source: encarta encyclopedia