On Garde
Below is a picture of what a fencer looks like in the "on garde" position. As you can see, the larger arrow shows the pull of gravity. The smaller arrows show where gravity is pulling the limbs. Red being for the legs and yellow for the arms.
Thrust Position
The next image shows the fencer in the thrust position. In this position, the effects of gravity are the same in the legs, but the force of gravity on the weapon arm is greater,because of the distance from the body.
This is described more effectively in the next two images. These next two show with the green lines where the weight of the arm is being held. As you can see,in thrust, there is more weight parallel with the ground, and therefore is harder to hold up.
Lunge
Next, we will discuss the lunge. Below you will see two more images. First it is from the thrust position. Then the next image shows the lunge position. The lunge picture shows the forward momentum in the blue arrow. As in the previous pictures, the same colors are used for the same gravity applications, but the major difference is the effects of gravity on the legs. In this position, the effects of gravity are more severe because the legs are again further from the center mass of the body and therefore, more of the force of gravity is "pushing down" on the legs.
So that will conclude the discussion of basic stances and physics of them. Next, we will discuss the more advanced moves and how fencers use physics to their advantage with them.