The Skeleton as an Armature
Most of the structure in the body is due to the skeleton, not the muscles. The skeleton without the muscles retains its familiar human shape. The muscles without the skeleton would be an unrecognizable pile of limp meat.
Artists can take advantage of this fact by using the skeleton as an armature for the forms of the body in their drawings, which will be demonstrated here with a group of three standing figures in a classic contrapposto pose. Begin by drawing a line indicating the spine, which is the origin of movement in the body.
Next, draw ovals indicating the pelvis, ribcage, and skull, preferably in that order. (Especially in a standing figure, the pelvis is like a bowl holding the body's center of gravity. Therefore the position of the pelvis influences the rest of the torso and the legs, more so than the other way around.) It is worth noting that the thoracic vertebrae are only slightly mobile, so that action on the spine takes place between the pelvis and ribcage, and between the ribcage and the skull. These three forms, in themselves, do not move.
Next, draw axes for the shoulder girdle and the pelvis. There are many ways of doing this, but the idea is to choose opposite symmetrical points on the body. Artists will commonly use the acromion process of the scapula on each side, the anterior superior iliac spines of the pelvis, and the great trochanters of the femurs. On a rear view, one can indicate the sacral triangle.
Draw a line indicating the weight-bearing leg. This line would begin on a lower, outer edge on the pelvis-oval (particularly from the line through the great trochanters), run straight to the knee (where one would leve a tick mark), and go from there to the point of contact between the heel and the ground.
Draw a triangular paddle shape for the foot (really, the footprint). This line would start at the heel-point, go to the base of the big toe, from there to the base of the little toe, and back to the heel.
From there draw a line to the heel-point on the opposite foot. This is a useful construction line that establishes perspective in the drawing and assures that the standing figure appears convincingly balanced. Then draw a triangular foot-paddle in the same manner as the first.
Draw up the non-weight-bearing leg from the heel-point, to the knee, to the pelvis.
Next indicate the arms. Beginning at the axis of the shoulder girdle, draw to the elbow (leaving a tick mark) and to the joint of the wrist. Draw a square-ish paddle shape indicating the palm and metacarpal bones. The four fingers come from the distal edge of the square, while the thumb comes from the appropriate side; one can draw a line for each digit, or use a single line for the fingertips.
Draw a line from the little-finger side of the square back to the elbow, which effectively locates the ulna. Also, locate the lower angle, medial angle, and spine of the scapula on each side, and draw them as well.
Returning to the center of the body, draw the downward arch of the front of the pelvis. Then draw a line for the sternum, which drops halfway down the ribcage, and a thoracic arch. This arch would correspond to the rounded shape of the top of rectus abdominis, not the pointed arch on the skeleton. Draw a frontal midline down the center of rectus abdominis all the way to the pelvis.
Lastly, check if the oval of the head is in a good place (it often needs to be moved a bit at this point) . If so, draw a line through the brow ridges on the skull, indicating the lower edge of the brow, and through the midline of the face.
One can elaborate from here with confidence, perhaps drawing along the control lines, and proceeding to the muscles. Clearly, the steps described above must be changed according to circumstances, but the general ideas of starting at the center of the body, working downward to the points of contact with the ground, and leaving the arms and head for last are good principles in most cases.