The pose of Michelangelo's David is unlike that of earlier Renaissance depictions of David.
The bronze statues by Donatello and Verrocchio represented the hero standing victorious over the head of Goliath, and the painter Andrea del Castagno had shown the boy in mid-swing, even as Goliath's head rested between his feet, but no earlier Florentine artist had omitted the giant altogether.
According to Helen Gardner and other scholars, David is depicted before his battle with Goliath.
Instead of being shown victorious over a foe much larger than he, David looks tense and ready for battle after he has made the decision to fight Goliath but before the battle has actually taken place.
His brow is drawn, his neck tense, and the veins bulge out of his lowered right hand.
His left hand holds a sling that is draped over his shoulder and down to his right hand, which holds the handle of the sling.

However, John Vedder Edwards, in his "Michelangelo / Revelations" suggests that the David is in fact shown after the defeat of Goliath, and that David is looking in consternation at the Philistine army, concerned as to whether or not they will honor the agreement to settle the dispute on the terms of single-combat previously agreed to.
Alternatively, Edwards considers that the David is pictured not at a battle at all but can be understood as the Biblical Prophet as stated in the Book of Acts 2: 29–30, long before his call to duty, struck with a divine vision of his future, the future of Israel, and the coming of the Messiah.
Edwards compares the expression of the David to the ecstatic expression of the Delphic Sybil that Michelangelo painted on the Sistine ceiling.

The twist of his body effectively conveys to the viewer the feeling that he is in motion, an impression heightened with contrapposto.
The statue is a Renaissance interpretation of a common ancient Greek theme of the standing heroic male nude.
In the High Renaissance, contrapposto poses were thought of as a distinctive feature of antique sculpture.
This is typified in David, as the figure stands with one leg holding its full weight and the other leg forward.
This classic pose causes the figure's hips and shoulders to rest at opposing angles, giving a slight s-curve to the entire torso.
The contrapposto is emphasized by the turn of the head to the left, and by the contrasting positions of the arms.

Michelangelo's David has become one of the most recognized works of Renaissance sculpture, a symbol of strength and youthful beauty.

The colossal size of the statue alone impressed Michelangelo's contemporaries.
Vasari described it as "certainly a miracle that of Michelangelo, to restore to life one who was dead," and then listed all of the largest and most grand of the ancient statues that he had ever seen, concluding that Michelangelo's work surpassed "all ancient and modern statues, whether Greek or Latin, that have ever existed."

The proportions of the David are atypical of Michelangelo's work; the figure has an unusually large head and hands (particularly apparent in the right hand).
The small size of the genitals, though, is in line with his other works and with Renaissance conventions in general, perhaps referencing the ancient Greek ideal of pre-pubescent male nudity.
These enlargements may be due to the fact that the statue was originally intended to be placed on the cathedral roofline, where the important parts of the sculpture may have been accentuated in order to be visible from below.
The statue is unusually slender (front to back) in comparison to its height, which may be a result of the work done on the block before Michelangelo began carving it.

It is possible that the David was conceived as a political statue before Michelangelo began to work on it.
Certainly David the giant-killer had long been seen as a political figure in Florence, and images of the Biblical hero already carried political implications there.
Donatello's bronze David, made for the Medici family, perhaps c. 1440, had been appropriated by the Signoria in 1494, when the Medici were exiled from Florence
The pose of Michelangelo's David is unlike that of earlier Renaissance depictions of David. The bronze statues by Donatello and Verrocchio represented the hero standing victorious over the head of Goliath, and the painter Andrea del Castagno had shown the boy in mid-swing, even as Goliath's head rested between his feet, but no earlier Florentine artist had omitted the giant altogether. According to Helen Gardner and other scholars, David is depicted before his battle with Goliath. Instead of being shown victorious over a foe much larger than he, David looks tense and ready for battle after he has made the decision to fight Goliath but before the battle has actually taken place. His brow is drawn, his neck tense, and the veins bulge out of his lowered right hand. His left hand holds a sling that is draped over his shoulder and down to his right hand, which holds the handle of the sling. 
However, John Vedder Edwards, in his "Michelangelo / Revelations" suggests that the David is in fact shown after the defeat of Goliath, and that David is looking in consternation at the Philistine army, concerned as to whether or not they will honor the agreement to settle the dispute on the terms of single-combat previously agreed to. Alternatively, Edwards considers that the David is pictured not at a battle at all but can be understood as the Biblical Prophet as stated in the Book of Acts 2: 29–30, long before his call to duty, struck with a divine vision of his future, the future of Israel, and the coming of the Messiah. Edwards compares the expression of the David to the ecstatic expression of the Delphic Sybil that Michelangelo painted on the Sistine ceiling. 
The twist of his body effectively conveys to the viewer the feeling that he is in motion, an impression heightened with contrapposto. The statue is a Renaissance interpretation of a common ancient Greek theme of the standing heroic male nude. In the High Renaissance, contrapposto poses were thought of as a distinctive feature of antique sculpture. This is typified in David, as the figure stands with one leg holding its full weight and the other leg forward. This classic pose causes the figure's hips and shoulders to rest at opposing angles, giving a slight s-curve to the entire torso. The contrapposto is emphasized by the turn of the head to the left, and by the contrasting positions of the arms.
Michelangelo's David has become one of the most recognized works of Renaissance sculpture, a symbol of strength and youthful beauty.
The colossal size of the statue alone impressed Michelangelo's contemporaries. Vasari described it as "certainly a miracle that of Michelangelo, to restore to life one who was dead," and then listed all of the largest and most grand of the ancient statues that he had ever seen, concluding that Michelangelo's work surpassed "all ancient and modern statues, whether Greek or Latin, that have ever existed."
The proportions of the David are atypical of Michelangelo's work; the figure has an unusually large head and hands (particularly apparent in the right hand). The small size of the genitals, though, is in line with his other works and with Renaissance conventions in general, perhaps referencing the ancient Greek ideal of pre-pubescent male nudity. These enlargements may be due to the fact that the statue was originally intended to be placed on the cathedral roofline, where the important parts of the sculpture may have been accentuated in order to be visible from below. The statue is unusually slender (front to back) in comparison to its height, which may be a result of the work done on the block before Michelangelo began carving it.
It is possible that the David was conceived as a political statue before Michelangelo began to work on it. Certainly David the giant-killer had long been seen as a political figure in Florence, and images of the Biblical hero already carried political implications there. Donatello's bronze David, made for the Medici family, perhaps c. 1440, had been appropriated by the Signoria in 1494, when the Medici were exiled from Florence
David (Michelangelo)