Ground Reaction Forces in Golf

Dr. Scott K. Lynn, PhD

Anyone who plays the game of golf knows how difficult this game truly is.  It can be incredibly frustrating for golfers of all skill levels, from the raw beginner to the seasoned professional.  If the goal is to get and keep more people playing this game and enjoying their experiences, improved scientifically validated instructional techniques and feedback tools need to be developed.

Traditionally, golf teachers and coaches have relied on one basic form of feedback to analyze the swings of their students and determine the appropriate corrective drills and/or cues, a two dimensional video camera.  This is a very valuable tool, and with recent technological advances, many now have the ability to view these two dimensional images at very high speeds.  Since the human eye cannot pick up images at the speeds produced during   the golf swing, this gives the coach the ability to clearly see these extremely fast moving components of the swing.  This is demonstrated in the picture below as the club head/club face is just a blur in the video on the left, while it can clearly be seen in the video on the right.

Also, three dimensional motion tracking systems have recently been developed so that the coach is not limited to the planar views provided by standard video cameras.  Now coaches can rotate the image/animation of the golfer to view positions in multiple planes that may have not previously been easily accessible.  This also allows for researchers to analyze movements of body segments in planes of motion that are extremely difficult to quantify with a simple two dimensional view (see picture below) .

However, with all of this technology from the regular video to the new high speed three dimensional motion tracking systems, the coach is only able to see the movement that is the result of various other processes that must occur first.

Isaac Newton taught us that in order to change the movement state of an object (either accelerate it/speed it up/start it into motion or decelerate it/slow it down/stop it from moving) there must be an external  force applied to that object.  So without these external forces, we would not be able to get parts of our body to start moving or attempt to stop these parts from moving, both essential elements required in the production of an effective golf swing.  The external forces needed to produce or stop movement require that we have something to push off of, and in golf the only object we have to push off of is the ground.

Newton also taught us that if we push on something with a certain amount of force, that object will push right back on us with a force that is of equal magnitude, but in the opposite direction.  Therefore, when we push down into the ground with our feet, the ground will push right back up on us and create something called a Ground Reaction Force (GRF).  How we push into the ground will help determine the resulting movements that occur; therefore, control of these forces is essential in producing an effective golf swing.

Recent technology now allows us to record and view these GRFs in three dimensions using a piece of equipment called a force plate.   This piece of equipment is like a large weight scale that can continuously record the forces that are being applied to it.  However, unlike a simple weight scale, these force plates not only record how hard you push straight down on the plate, they can also record how hard you are pushing forward/backward (or  in the anterior-posterior (AP) direction) and side-to-side  (or in the medial-lateral (ML) direction) as well.  It is the combination of the vertical force and these shearing forces in the AP and ML directions that determine the direction of the GRF during movement.

I recently published a study in the International Journal of Golf Science (IJGS) that examined the differences in these GRFS between highly skilled and beginner golfers and found some very interesting results.  During this study we had golfers swinging the club with each foot on a separate force plate so that we could determine the forces on both the lead and trail leg during the entire swing.  All of our subjects were right handed golfers so to lead leg was the left leg and the trail leg was the right leg.

For the forces in the vertical direction, we found several interesting results:

(a)    The skilled players kept much more weight on their lead foot during the backswing and didn’t unload it as much as the beginners.   Some of our beginners actually shifted close to 100% of their weight onto their back foot and were right up on their front toe at the top of the backswing (similar to the driver swing of Bubba Watson).  This is obviously something you would want to correct in a beginner golfer, as it requires tremendous skill and timing to get the club back to the ball from this position.

(b)   The skilled golfers transferred their weight off of their trail foot and onto their lead foot much earlier in the swing than the beginners.  Most of the experts actually transferred their weight onto their lead foot very early in the downswing, while many of the beginners still had not fully transferred their weight to the lead foot as they were well into their downswings.  It takes time for the energy generated from the ground to be transferred up through the body to the club; therefore, transferring the weight earlier in the downswing gives more time for this to happen.

One common swing key given by golf coaches involves trying to teach beginners to transfer all their weight to their front foot.  The results of our study suggest that this is not as important as the timing of weight transfer and restricting too much weight transfer to the trail foot during the backswing.

Although the differences found in the vertical direction are important, many do not realize that GRFs occur in three dimensions.  The AP and ML components of the GRF are also extremely important in the production of an effective golf swing.  For the forces in the AP direction, it was determined that lead foot of skilled golfers produce much anteriorly directed shear force early in the backswing and much more posteriorly directed force in the early part of the downswing.  The AP shear forces on the ground are essential to producing the rotary motions required during the golf swing.  As can be seen in the picture below, during the early part of the backswing these forces cross with the lead foot force pointing anteriorly and trail foot force pointing slightly posteriorly.  These GRFs create something we call a force couple that causes the pelvis to spin to the right during the backswing (see below – picture on the left).  These forces then reverse their directions at the top of the backswing (lead foot pointing posteriorly, trail foot pointing anteriorly) causing the pelvis to being to rotate to the left (see below – picture on the right).  The more angle that is created between these two forces, the more rotation force is being developed from the ground during the swing.

In the ML direction, there were several differences in GRFs between the skilled and beginner golfers.  Since the goal of golf is to propel the ball in this direction, these forces become extremely important in the production of an effective golf swing.  It was found that the skilled golfers had both of these vectors pointed towards that target at the transition/top of the backswing (see below – picture on the left).  This creates some linear momentum of the body towards the target that must be decelerated in order for that energy to travel up the body to the club.  If this momentum is created and not decelerated, too much lateral slide of the body towards the target would occur (some golf coaches call this a “sway”) and much of this momentum would not get transferred up to the club.  Therefore, before ball contact these forces (mainly in the front foot) must reverse and point away from the target to decelerate this momentum.

For the ML forces, the beginner golfers tended not to have both of these forces pointed towards the target at transition.  If one or both of those forces are pointing backwards, they are not creating sufficient momentum in the desired direction.  Many would refer to this as a “reverse pivot” type of swing.  Some form of “walk-through” drill may help teach golfers to get both feet producing this momentum towards the target as the club changes direction.  However, this would only address one common issue, as golfers would then need to be taught how to decelerate this momentum so that they don’t “sway” and the momentum can be passed up the body to the club.  These decelerations would require a large amount of muscular strength/power if the speed of the swing is high.  If golfers are incapable of decelerating this momentum, simply working on their golf swing may not be appropriate.  They would need to develop this muscular strength/power and activation strategies that produce deceleration with appropriate exercises in the gym.

3D GRF patterns have great potential to provide feedback to the golfers and coaches about the external forces that create the golf swing motion.  Designing drills/exercises to modify these GRFs during the swing, and how these changes then affect the swing motion and resultant ball flight requires further investigation, as very little work has been done in this area.  The feedback provided by 3D GRFs may have the ability to revolutionize how the golf swing is taught and could help us understand much better how the motions of the swing are produced.

2014-15 PGA Tour Dates


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