Center of Pressure
The center of pressure (COP) is the average location of the vertical force applied by the person on the ground. If a 200 pound person stands perfectly still and evenly on both feet, that person will apply 100 pounds of pressure on each side. The center of pressure would be exactly halfway between the two feet. If the person stands with 75% (150 pounds) on the right and 25% (50 pounds) on the left, then the center of pressure would be 75% of the way from the left foot to the right foot. And if the person was standing still with all of his weight on the right foot, the center of pressure would be under the right foot.
Center of Mass and Center of Pressure
When a person is standing still, the center of pressure lines up directly beneath the center of mass. It is very important to note that this is only true when standing completely still. When a person is not standing still, the center of pressure will move depending on how hard someone is pushing on either side - it does not stay beneath the center of mass.
The feeling of “loading the trail side” during the backswing is the golfer feeling the trail side pushing harder down into the ground, which moves the center of pressure to that side. It does not mean the golfer’s body and center of mass are moving very far. Let's take a look at how center of mass and center of pressure interact in the following video.
Here, we will take a look at the relationship between the center of mass and the center of pressure.
The center of mass is shown by the blue ball in the middle of the golfer, and the center of pressure is located at the base of the yellow combined ground reaction force arrow. When someone stands still, you can see that the center of pressure will be directly beneath the center of mass. As you see here, the golfer was asked to slowly shift his body from side to side. When movements are slow like this, the center of mass and the center of pressure move together pretty well. They separate a little bit as the golfer starts and finishes each shift. But otherwise, the center of pressure basically stays under the center of mass.
In the second example, the golfer was asked to quickly shift how much he was pressing down with each foot but not let the rest of his body shift from side to side. In this case, you can see that the center of pressure moves rapidly from side to side, but the center of mass stays nearly stationary. In dynamic movements like this example and the golf swing, the center of mass and center of pressure very much can move separately.
During the golf swing, a golfer can independently control them both. And using them both correctly not only helps the golfer position his body properly, it also plays an important role in creating good club speed.