The Ground Realities of Ground Reaction Force … Special Report By Kiran Kanwar

by Kiran Kanwar

So much has been said about Ground Reaction Force (GRF) in golf that it has been subjected to very strange interpretations by different members of the world’s golfing fraternity.

What is GRF?  How can a golfer use it to increase club speed (in theory)?  CAN a golfer use it to increase club speed (in practice)?

There is much confusion about certain aspects of GRF and even more about how to beneficially harness it. The advantage of being a golf-instructor/golf-swing-researcher is that one can access previous research on the subject, then discover how best golfers can actually do what research says is happening.

In simple terms, according to Sir Isaac Newton, when two bodies are in contact, they push against one another with an equal amount of force, and in a direction perpendicular to their contact surface. If you’re leaning your body against a wall, it is pushing back at you with an equal amount of force.

Similarly, while standing, you are pushing down on the ground, simply with the weight of your body. Therefore, the ground is pushing back upwards, with an amount of force equal to your body weight (BW).

Ground Force Reaction

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In order to create movement, or more specifically, acceleration, GRF must increase to an amount greater than BW. Incidentally, GRF acts in three directions – vertically up from the ground, as well as side-to-side and front-to-back. These latter two components of GRF are termed friction.

One research study ( defines GRF in simple terms:

ANY force, not just ground reaction force = mass x acceleration. As a golfer’s mass does not change, the ground will experience a force from the golfer which depends on the acceleration of the golfer’s body.

If GRF is less than body weight, the weight of the body is not being supported by the ground. That implies a downward acceleration, such as when a person squats. When the body is being thrust upwards, as when a golfer’s trunk and lead shoulder rise during the downswing, more force, and thus acceleration is required to move the body upwards, and that force will be experienced by the ground upon which the golfer stands. When GRF is measured, it is actually a measure of the body pushing down on a set of force-plates.

So what creates a reaction force? The ground or the golfer? Is GRF an active force or a passive one?

The best explanation for this comes from a 2004 study titled, “The effect of passive and active impulse on the performance of drop jumps” by Lin, Tsai and Liu. It is worth a read, and is easily available on the internet). [The drop jump, incidentally, is a movement where a person jumps off a platform a few centimeters above the ground, then immediately jumps up as high as possible).

Basically, any upward GRF, is ACTIVE because it is created solely by muscles under the volitional control of the golfer. PASSIVE GRF (such as when a runner’s foot first hits the ground or a drop-jumper first lands after the drop) is a time when the muscles have not had time to react (muscle latency), and the body must respond passively with eccentric passive deformation of body tissue.

While the above two papers discussed general movement, a lot of research has been conducted  relating GRF to clubhead speed production in golf. It has been noted that maximum vertical force for a driver is 1.6 times body weight (see “How has biomechanics contributed to the understanding of the golf swing” by Dillman and Lange, 1994). Vertical force increases as a golfer is able to transfer more vertical force from the trail to the lead foot, and at a faster rate.

Another study, (“Ground reaction forces and torques of professional and amateur golfers by Barrentine, Fleisig and Johnson, 1994) found that for all their skill-levels of golfers (PGA, low handicap and high handicap), just 0.2 seconds after the top of the backswing, the trail foot pushes backwards and the lead foot pushes forwards.

As the golfers spiked shoes are locked into the ground, a force couple is created in reaction to the foot pressures, allowing the golfer’s body to rotate towards target.

Recent research by one of the world’s leading golf biomechanists (not yet published) has correlated greater ground reaction force moment (GRF-M) as well as ground reaction moment (GRM) to greater club speed.

What do these terms mean?  Have you ever wondered why the handle of a door is as far away as possible from the hinges around which the door rotates? That is simply because a force which creates a rotation of the door can be increased either by applying more pushing force into the door-handle OR by having a greater distance between the place where the force is being applied, to the axis (hinges) around which the door rotates.

So, the GRF-M concept is telling us that if the balance point of the body (its center of mass) is far from the vertical GRF (created by the downward pressure of the feet), more club speed can be generated. GRM, on the other hand, refers mainly to the horizontal-plane force-couple created, as a result of the anterior-posterior forces through the feet, already described above.

From the studies described above, it is obvious that a golfer must move the body forwards towards target, move the lead side upwards, and rotate the torso, all as fast as possible. If ACTIVE, by-the-muscle force is to be used to create greater body acceleration, to achieve all this movement quickly, then more muscle activation is required. What can a golfer who does not have the muscle strength or speed of a more skilled golfer do to harness greater GRF?

The solutions which have been offered typically include pushing down on the ground more and shifting weight to the lead foot faster. One solution involves lifting the trail side of the trunk up during the backswing; purposefully dropping the trail side down; and shifting body weight forwards in a stepping-forward fashion.

These solutions ignore the fact that a less skilled golfer simply cannot move so much so quickly, especially given the limited time span within which all downswings take place.

The Barrentine et al. paper found that golfers of all skill levels take about the same time (0.28 sec PGA, 0.28 sec low handicap, 0.33 sec high handicap) to make the downswing, regardless of the club used. In that tiny time-frame it takes a lot of power (strength and speed) to move side-to-side or up-and-down or around.

One study (“A review of biomechanical differences between golfers of varied skill levels” by Lindsay, Mantrop and Vandervoort, 2008) has found that skilled golfers move their “weight”, or, in scientific terms, their center of mass, an average of 8.2 cm away from target during the backswing, and 12.3 cm towards target during the forward swing.

How many golfers can move through these quite large distances in the fraction of a second during which a downswing occurs? The same thing applies to a golfer (such as Tiger Woods in recent years) who tries to drop the body down to push up off the ground against.

The golfer runs out of time to use the ground to push off to against to achieve good lead-side lift. [Incidentally, the higher the lead shoulder is, the longer the arm-plus-club lever will be at impact, which will increase the linear velocity imparted to the ball.] Just as a golfer runs out of time to move back-to-front and down-and-up, he or she also runs out of time to rotate fast enough, especially when the trail hip is bent forwards or flexed (which is the case with most golf swings). The golfer must now extend or straighten out the trail hip before rotation can begin, once again a temporal issue.

So, what can a golfer who is older or has less athletic ability do to use the ground  efficiently to push off against, in order to use both the vertical and rotary components of GRF? Position the body so that the weight is centered throughout the back- and downswings, and position the lead shoulder and trunk higher, and the trail shoulder and trunk lower, from address to impact.

When the golfer is already closer to the front foot and to the “up” position of the lead shoulder, it requires less movement and less time to reach the desired positions. Only one golf swing in the world currently allows the harnessing of more GRF in a manner conducive to all skill levels, ages and muscle capabilities  – The Minimalist Golf Swing.

Kiran Kanwar, 

  • Developer of The Minimalist Golf Swing System -100% scientific, simple and specific
  • BS (physics, math); MS (sports science, nutrition); PhD (biomechanics – student)
  • Class A Member: the LPGA, The PGA (GB&I), The NGA of India, The PGA of India

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