All Forms on the Body are Convex
The human figure should be treated as if it had no concavities. Just as one could touch any point on an egg with a pencil and find a curved surface rising out to meet it, the body as a whole and its individual components should be thought of as convex at all points.
The body is composed of masses of bone and flesh, and conceiving the body in terms of masses results in a more accurate conception. Beginners often have a rough time of figure drawing because of their inability to appreciate this point. What comes to their immediate attention are the shadows on the figure, which lurk in the hollows formed by the masses, and they render these instead of the masses themselves. Or they will see an outside contour line on the figure that may be generally concave, but can be seen by a more practiced eye to be formed by a series of convexities.
Anatomical knowledge is consummately helpful in this matter. Being able to identify the masses which form an outside contour allows you to think sculpturally in the midst of drawing, and create a convincingly dimensional form using line alone.
When faced with what seems like a depression on the figure, one should look for the masses on either side that form the depression and draw them instead. For example, consider the flexed forearm below. One can see the deep ridge between brachioradialis and extensor carpi radialis longus. On the left, this depression has been rendered with concave cross-contours. On the right, the two surrounding muscles have been rendered with convex cross-contours, with the depression indicated at their intersection. The right-hand drawing is clearly more effective and convincing.