When studying Kevin Streelman's transition from backswing to downswing, why does the Swing Catalyst software show that most of the pressure is on his trail foot while most of the body mass is over his lead foot? The answer lies in the definitions of mass, weight, and pressure.
Weight is often used to describe mass since it corresponds to the notion of how "heavy" something is. You would typically say that your weight is 85 kilograms or 187 pounds, although you would actually be describing your body mass. By traditional definitions, weight (measured in newton) refers to the force of the gravitational pull of the earth, and is defined as mass (in kilograms) multiplied by the acceleration of gravity (9.81 m/s2). To illustrate the difference, consider an astronaut who is weightless in space. His body mass is still 85 kilograms or 187 pounds (he is not considered to be without mass), but his weight has changed because of the change in gravity. In short, weight is the force of your body mass caused by gravity.
The Swing Catalyst Balance Plate measures the pressure applied to the ground by the golfer. A common source of confusion in the Swing Catalyst software is the apparent lack of synchronization between pressure and body mass, illustrated here with PGA Tour player Kevin Streelman. Why does the Swing Catalyst software show that most of the pressure is on the trail foot (R) when the video shows that the golfer has most of his body mass placed over his lead foot (L)?
First, it's important to know that pressure is force working over an area. This means that pressure is the result of the force exerted on the ground by the golfer. When standing still at address, the golfer's weight usually causes the pressure to be distributed fairly evenly between the feet, because the body mass is distributed fairly evenly between the feet (the Tour players we have studied normally favor the lead foot slightly, typically displaying a 55-45 distribution). However, weight only determines the distribution of pressure in a passive situation, such as when standing still. During the swing, this relationship is not as clear. This is because the golfer can also actively push down, increasing pressure through force produced by the muscles. Note that this force is not the same as the weight, the force caused by gravity alone.
Now consider the transition from backswing to downswing, the point where this confusion often arises. A golfer will push down with the trail foot to initiate the downswing. In the picture, this can be seen in the greater pressure on Kevin's right foot (R). The body mass, on the other hand, will start to move toward the lead foot in the transition. Notice that Kevin's body mass is favoring his lead foot (L), even though the pressure is favoring his trail foot (R). This is because he actively increases pressure on the trail foot (R) through muscle force. From this we can see that the distribution of pressure does not always follow the distribution of body mass. It will quickly "catch up" with the body mass in the downswing, though. Most good players shift their pressure rapidly to the lead foot after the transition, illustrated here by the straight grey line in the center of pressure trace from Kevin's trail foot (R) to his lead foot (L).
Dr. David McGhie (Swing Catalyst Head of R&D)