The Swing Catalyst Balance Plate consists of more than 2000 high-resolution sensors that measure the exact amount of pressure applied to the ground. In short, pressure is force working over an area. In our case we are talking about the force the golfer is exerting on the ground and the area of the two golf shoes touching the ground (shown as the footprints in the Swing Catalyst software). The Balance Plate measures the exact size of the two footprints and the pressure that is applied to these areas. From this we get the pressure distribution, which is simply how much the golfer is pushing down on one foot compared to the other. In other words, it shows where the golfer is exerting force on the ground. When the pressure distribution between the feet changes throughout the swing, it indicates a shift in pressure, which is also shown in the center of pressure (CoP) trace. This is why we use pressure shift to describe the movement of the CoP. Most good players will perform a rapid, continuous pressure shift during the downswing. In the screenshot of PGA Tour player Kevin Streelman, this is illustrated by the smooth and straight line in the CoP trace from Kevin's trail foot (right) to his lead foot (left).
When analyzing a golf swing, weight shift is commonly used to describe what is actually pressure shift. We have even used weight shift ourselves since most golfers are familiar with this term. However, it is primarily a coaching term. By definition, weight shift refers to the movement of body mass, not the pressure measured by the Swing Catalyst Balance Plate. It's important to note that the golfer does perform a weight shift during the swing by moving the body mass, but this is not what is measured by pressure distribution. Nevertheless, pressure shift, or movement of the CoP, is usually a good indicator of weight shift for most parts of the swing, the potential exception being certain transition points.
We will discuss the relationship between pressure shift and weight shift in more detail later. For now, the distinction between the two is most important. The Swing Catalyst Balance Plate measures pressure shift, although this is often referred to as weight shift.
Dr. David McGhie (Swing Catalyst Head of R&D)