Force vs Pressure

There has been great interest lately in the forces created between the golfer’s feet and the ground. This is mainly due to the fact that products like Swing Catalyst now give golfers and golf teachers the ability to measure and visualize these forces.

Previously, these forces could only be quantified in high tech biomechanics labs; therefore, information related to the effective use of ground forces was limited to a selected few who had access to these facilities.  I run one of these biomechanics labs in the United States at California State University, Fullerton in Orange County, California.  I have always had a great interest in learning how golfers leverage the ground appropriately to produce powerful and accurate golf shots and I have published extensively on this topic.

In reading and watching a lot of the information that is being posted online related to ground forces in the golf swing, I have seen many things that may be slightly misleading.  I feel it is extremely important that those who are trying to implement this information into their teaching and/or their own swings understand the technology they are using and have adequate knowledge to interpret this information.  Therefore, this article will summarize the science behind the ground forces in golf and give an overview of the technology used to collect this information.

Force Change MotionIn order to understand the ground forces in golf, we must start first by examining Newton’s Laws of Motion.  These laws apply just as much today as they did back in 1687 when Isaac Newton first published them.  The most important laws to understand when it comes to the ground forces in golf that will be discussed in this article are Newton’s 1st and 3rd Laws.  Newton’s 1st law states that a body at rest or in motion will maintain its current state of motion unless acted upon by an EXTERNAL force.

A golf swing without ground reaction forces

This means that in order for an object to start moving/accelerate or stop moving/decelerate, there must be forces acting on that object from a source external to that object.  When we talk about the accelerations and decelerations of our body segments that are critical to producing an effective golf swing, the only object external to the golfer’s body that they can push off of is the ground.  Without these essential forces between the golfer’s feet and the ground, an effective and powerful golf swing is impossible.

Newton’s 3rd Law states that for every action, there is an equal and opposite reaction.  So when a golfer produces an action force by pushing into the ground, the ground then pushes back up into them in an opposite direction. These forces that push back on you are called the Ground Reaction Forces (GRFs). It is the effective control of these GRFs acting on the golfer that produce the body and club motions that lead to powerful and effective golf shots.  3D motion tracking systems have shown us that every golf swing involves a combination of motions happening in all three dimensions.  This would obviously lead to the conclusion that these GRFs must also have to act in all three dimensions in order to produce these motions of the golf swing.

This is where much of the confusion originates when discussing many of the products on the market that claim to measure ground forces.  Most of these products are similar to the Swing Catalyst Balance Plate in that they can only measure pressure between the feet and surface.  Pressure is defined as the forces perpendicular to the surface divided by the area over which they are acting.  Since these pressure measuring technologies can only measure forces perpendicular to the surface, they are only capable of measuring force in one dimension (1D).  These perpendicular or vertical forces give us a lot of information and can be very useful in helping to improve the performance of our golfers; however, making claims about the linear (towards/away from the target) or rotational components of the swing based on pressure measurements are at best an educated guess and must be interpreted with GREAT CAUTION.

In order to measure 3D ground reaction forces, a force plate is needed.  The Swing Catalyst 3D Motion Plate provides all of this information as it has a pressure plate mounted on top of a force plate.

This gives golfers and golf teachers the ability to measure 3D ground reaction forces as well as the pressures between the golfer’s feet and the ground.  The force plate part of the Swing Catalyst 3D Motion Plate allows for the measurement of the 3D forces used to produce linear motions towards and away from the target (called Right/Left Forces by the Motion Plate).  These are shown below in animations produced in my biomechanics lab and with the data measured by the Swing Catalyst 3D Motion Plate.

A golfer producing ground reaction forces directed towards the target at the top of the backswing and away from the target braking forces near impact in a 3D biomechanics lab animation. This is measured by the Swing Catalyst Motion plate as the white circle (towards the target GRFs) and red circle (away from the target braking forces).

The Swing Catalyst 3D Motion Plate also allows us to measure how each foot pushes towards and away from the ball in opposite directions in order to produce the torques needed to create the rotational motions of the body segments.  This is shown below in the animation during the downswing as captured in my Biomechanics Lab and as measured by the Swing Catalyst 3D Motion plate.

Animation showing the anterior reaction force created by the back foot and the posterior reaction force created by the front foot during the downswing. This creates the large positive torque (143 Nm) shown in the Swing Catalyst software.

These 3D forces are discussed in much more detail in the Level 2 Swing Catalyst Certification; so if they are very foreign to you, they are explained in much more detail in that seminar.   However, since many golf teachers are now using pressure measurement devices, it is important to understand the technology so that more informed decisions can be made about the data provided.  Most commercially available products that measure the ground forces in golfers are merely pressure plates and only measure forces in 1 dimension (perpendicular to the surface).  These data, while still very useful, must be interpreted with caution as many have attempted to relate the information they provide to the 3D motions produced in the golf swing.  In order to get information about how golfers use the ground to produce the 3D motions required of the golf swing, a force plate such as the Swing Catalyst 3D Motion plate is required.

Scott K. Lynn, PHD
Swing Catalyst Research and Education Director