Axis lines in the figure

                          

Here are a couple examples of axis lines in gesture drawings. When referring to the back of the body, the spine is the most reliable center line.  From the front view, the central axis line runs from the sternum bone, through the naval, to the center of the pubic bone between the legs. This line often curves, depending on the model's pose. Try to determine the initial attitude of the posture and how the body shows this charge--then emphasize or exaggerate this in the drawing. In gesture drawing there is only time to capture the most important truths!  In the first drawing you can see axis lines running down the spine (S curve)and the back of the legs.  In the third drawing there are horizontal axis lines determining the angle of the hips and shoulder space and one main vertical axis determining the gravity line of the body.