Vishwas R. Gaitonde in the Prague Review:
In Renaissance Italy, one of the perennial debates was whether sculpture was superior to painting, or the other way round. Many said two dimensional paintings couldn’t hold up against sculpture, which was three dimensional. Michelangelo, an archrival of Leonardo and who regarded himself as a sculptor first and a painter second, used the medium to depict movement; an excellent example was his statue, David. David’s head is slightly turned so that the point he faces is different from the point in front of his body. As a result, the body seems to be in motion. This is heightened by distributing the body weight unevenly between the two legs. The positioning of the face and upper body half-way through a motion was called contrapposto.
Donald Sassoon, a professor of Comparative European History and an authority on the Mona Lisa, describes the process Leonardo went through before he created Mona Lisa. Leonardo firmly believed that contrapposto was not the sole property of sculptors, that it could be employed in two-dimensional forms as well. The trick was to capture a moment in sequence rather than paint a static pose as though one was painting still life. When the face of the person pointed to a different direction from that which the torso was facing, the body would appear to be in movement. Leonardo perfected contrapposto in painting with the Mona Lisa.