"Lines" within the Torso

As shown in the drawings, the torso has "lines" in it formed by the bones within.  In profile, these landmarks show up as diagonals.  By stretching or compressing these lines while using the spine as a hinge can help you draw the torso bending forward or back.  You can remember these lines by thinking they look like "Pac-Man's mouth."

In contrast to the profile, the torso when viewed from the front or back has landmarks that show up as parallel lines.  By compressing the lines to the left (as shown above), one can show the shift of the torso as the body moves into a contrapposto pose.

Keep these lines in mind when drawing the figure.  Drawing them in at the start of a sketch can help you capture natural-looking poses and actions.

from The Human Figure by John H. Vanderpoel

Bones and Surface Anatomy

The bottom sketch illustrates how the bones affect and shape surface anatomy.  The shape of the hip on the right-hand side is formed by crest of the pelvis and the great trochanter of the femur.

from The Human Figure by John V. Vanderpoel 1935

Muscles of the Torso

Note that the Latissimus Dorsi (D on top image, B on the bottom) and the Trapezius (A on the bottom) are two back muscles that are visible from the front of the torso.  Likewise the external obliques (C on both the top and bottom) are muscles that are seen both in the front and the back.

from Principles of Figure Drawing by Alexander Dobkin