For this exercise I was to look for the line of balance or the centre of gravity in a standing figure, it begins at the top of the skull and passes through the middle of the nose straight down the middle of the chest cavity. With a back view the line starts from the back of the neck on the spinal column.
From a side view this line of balance starts at the back of the ear and travels down to the weight bearing foot.
The line indicating the central axis also helps indicate where the body mass or majority of the body weight is placed. If the figure moves or if the model sits the weight or mass changes to a different area of the body.
Move around the model before you begin to draw to get a sense of where the figure is in its allotted space and to identify its centre of gravity and gesture. Mark the central axis in your initial sketches of the standing figure. Ask the model to change poses every two to five minutes. Draw as many quick poses as you can.
I had just treated myself to a book by Andrew Loomis, Figure Drawing For All Its Worth. He teaches that the main wight is distributed within a rectangle if on two feet, or triangle if on one. I tried this out as can be seen in the pictures above and it seemed to be a good way of seeing where the main body weight is placed, in certain poses it does follow the central axis and divides the body in half.
I did this from photographs as my model wasnt available. I must say I have decided to go on a diet after seeing them. What I took from this is the center of gravity will be more important if you were drawing from your own imagination. It will give a good guide as to whether the figure can balance and the stance is actually possible.