Kenneth's Clark's The Nude is a classic study of this classical theme. Here is an extended passage in which he explains the role of Apollo and the significance of the Kritios Boy.
The Greeks had no doubt that the god Apollo was like a perfectly beautiful man. He was beautiful because his body conformed to certain laws of proportion and so partook of the divine beauty of mathematics. The first great philosopher of mathematical harmony had called himself Pythagoras, son of the Pythian Apollo. So in the embodiment of Apollo everything must be calm and clear; clear as daylight, for Apollo is the god of light. Since justice can exist only when facts are measured in the light of reason, Apollo is the god of justice; sol justitiae. But the sun is also fierce; neither graceful athlete nor geometrician’s dummy, nor an artful combination of the two, will embody Apollo, the python slayer, the vanquisher of darkness. The god of reason and light superintended the flaying of Marsyas.
But the earliest nudes in Greek art, traditionally known as Apollos, are not beautiful. They are alert and confident, members of a conquering race, “the young, lighthearted masters of the waves.” But they are stiff, with a kind of ritual stiffness; the transitions between their members are abrupt and awkward, and they have a curious flatness, as if the sculptor could think only of one plane at a time. They are notably less natural and less easy than the Egyptian figures upon which, to a large extent, they are modeled and which, over a thousand years earlier, had achieved a limited perfection. Stage by stage, in less than a century they grew into models that were to satisfy your western notion of beauty till the present day. They have two characteristics, and only two, that foreshadow this momentous evolution. They are clear and they are ideal. The shapes they present are neither pleasant in themselves nor comfortably related to one another, but each one is firmly delineated and aspires to a shape that the measuring eye can grasp. Historians who have written in the belief that all art consists in a striving for realism have sometimes expressed surprise that the Greeks, with their vivid curiosity, should have approached nature so reluctantly; that in the fifty years between the Moscophoros and the funeral stele of Aristion, there should have been so little ‘progress.’ This is to misconceive the basis of Greek art. It is fundamentally idea. It starts from the concept of a perfect shape and only gradually feels able to modify that shape in the interests of imitation. And the character of the shapes chosen is expressed in the word used to describe the earliest form of Greek art, geometric; a dreary, monotonous style and at first ill adapted to realization in the round. But the head yields easily to geometric treatment, and already in the most archaic heads of Apollo we see how geometry can be combined with plastic vitality. In a century the same unifying power will subordinate the dispersed and intractable forms of the body.
So Apollo is clear and ideal before he is beautiful. How and when did the transformation take place? Ranging in a hypothetical order of time the kourai—the nude male figures—of the sixth century, we see the transitions from shape to shape becoming smoother, and absorbing, in the process, details that had been left as decorative notations. Then, quite suddenly, in about the year 480, there appears before us the perfect human body, the marble figure from the Akropolis known as the Ephebe of Kritios. Of course we was not really a sudden, isolated creation. We have only a slender reason to attribute him to the sculptor Kritios, and we have even less reason to suppose that Kritios was the initiator of so momentous a change. Literary sources give the name of Pythagoras of Rhegium as the sculptor who ‘first gave rhythm and proportion to his statues.’ All the evidence suggests that the new concept of form would have been first expressed in bronze and not in marble; and the Apollo of Piombino, although slightly earlier and stiffer, may give some notion of what had been going on in the first twenty years of the fifth century. But since almost every bronze statue made in Greece in classic times has been melted down, the Ephebe of Kritos remains the first beautiful nude in art. Here for the first time we feel the passionate pleasure in the human body familiar to all readers of Greek literature, for the delicate eagerness with which the sculptor’s eye has followed every muscle or watched the skin stretch and relax as it passes over a bone could not have been achieved without a heightened sensuality... [T]wo powerful emotions ... dominated the Greek games and are largely absent from our own: religious dedication and love. These gave to the cult of physical perfection a solemnity and a rapture that have not been experienced since. Greek athletes competed in somewhat the same poetical and chivalrous spirit as knights, before the eyes of their loves, jousted in the lists; but all that pride and devotion which medieval contestants expressed through the flashing symbolism of heraldry was, in the games of antiquity, concentrated in one object, the naked body. No wonder that it has never again been looked at with such a keen sense of its qualities, its proportion, symmetry, elasticity, and aplomb; and when we consider that this passionate scrutiny of the individual was united to the intellectual need for geometric form, we can estimate what a rare coincidence brought the male nude to perfection.
Perfection hangs by a thread and is weighted in the jeweler’s balance. We must therefore submit the classic nude, at its first appearance, to an examination that may seem fastidious, until we remember how the rhythmic organization of this form was still dominating sculpture 2300 years after its invention. When, a page ago, I used the Apollo of Piombino as an example of bronze casting, how strikingly it brought out the classic character of the Kritios youth! In twenty years a basic alteration of style has taken place. It can be illustrated by examining the lower part of the torso—to be precise, the junction of the hips, abdomen, and thorax. One of the most peculiar features of the early kouroi—for example, the Apollo of Tenea—is their thin, flat, stomachs. They conform to a sharp, ogival rhythm, which we may describe as Gothic. The chief areas—thighs and stomach—are inscribed within elongated ovals. Gothic nudes, dominated by the pointed-arch form, do in fact display very much the same characteristics, and one of the earliest nude studies that have come down to us, a drawing in the Ufizzi from the circle of Uccello, combines Gothic and naturalistic forms with a remarkable likeness to a sixth century kouros. In the Apollo of Piombino these Gothic forms are less marked. The thorax is of classic rectangularity, but it bears an uneasy relationship to the flat triangle of the stomach. Like Perrault’s facade of the Louvre, we feel that a richly classical upper story is resting on a base too stiff and thin to support it. In the Kritios youth this uneasiness has vanished altogether. The legs and divisions of the torso flow together with the same full and fruitful rhythm. How is this achieved? To begin with, the hips are not parallel, but since he rests his weight on his left leg, that hip is slightly higher. The full implications of this pose are more easily seen from behind, for, as usual in early Greek sculpture, the back is more naturalistic and more plastically developed than the front. But even from the front we can perceive, for the first time, that subtle equilibrium of outline and axis which is to be the basis of classical art. This delicate balance of movement gives the torso its unity of rhythm. It also allows the sculptor to solve the problem of the abdomen by realizing it as a dominant, as opposed to a recessive form: and this has involved an anatomical emphasis that was to be exaggerated to the point of distortion in the next fifty years. I refer to the muscles that lie above the pelvis and mark the junction of the thighs and the torso. They are largely absent from archaic sculpture, and since it seems unlikely that between the years 500 and 450 Greek athletes really did develop these muscles to such an unequaled extent, we must reckon them chiefly a device by which the rhythmic structure of the torso might be set in motion, and its lower half supported by two buttresses, before descending to the arc of the abdomen. They were elements in the classic architecture of the human body, and as such they lasted as long as metopes and triglyphs.
All this we discover in the youth of Kritios when we compare him to the figures that precede him. But it is not obtruded. He is so straight-forwardly beautiful that we do not willingly use him to demonstrate the mechanics of form or the rules of an aesthetic theory. To the sculptors of the next generation this grace and naturalness was a defect, or at least a danger. It is as if they foresaw the frivolous beauty of Hellenistic art, and wished to defend themselves against it as long as possible” (Kenneth Clark, The Nude: A Study in Ideal Form, Doubleday Anchor Books, Garden City, New York, 1959, pp 55, 57, 61-3).