The Basics of golf ground mechanics

Introduction

The energy of the golf swing begins at the ground. As the swing starts, the muscles of the legs activate to apply force to the ground. Of course, these forces do not result in the movement of the ground itself, because the ground is stable. Instead, forces are transmitted back into the body and end up creating the necessary movements to produce a golf swing. These forces are called “ground reaction forces.” In the golf swing, the primary movement that is created by the legs interacting with the ground is pelvic rotation.

There are several useful insights to be discovered by studying the forces a golfer applies to the ground. Ground force patterns are correlated with various swing characteristics such as club head speed, club head path, angle of attack and more. The purpose of this article is to focus on the basics of ground mechanics and discuss some of the reasons why a golfer might be interested in studying how they interact with the ground during their swing.

The primary role of the legs in the golf swing is to produce pelvic rotation.

Key Terms

Center of Mass: The center of mass is the average position of all the parts of an object, weighted according to their masses. For the human body, the center of mass lies roughly in the rear third of the pelvis toward the spine.

Human pelvic model. Approximate center of mass is labeled in red.

Lead and Trail: “Lead” refers to the side of the golfer that is more towards the target. “Trail” refers to the side of the golfer that is further away from the target. This is typically used to describe feet, arms, legs, etc. Example: The golfer has more pressure toward the lead side. These designations are more universal than “right/left” because a golfer could be either right or left handed.

Center of Pressure: The average LOCATION of vertical force a golfer is applying to the ground. A golfer applies force to the ground in more than one location simultaneously but the center of pressure is reflecting the average location of them all.

Center of Pressure Trace: A 2 dimensional recording of all the centers of pressure that occur during a golf swing.

Center of Pressure Velocity: The rate of movement of the center of pressure during a golf swing.

Sample of center of pressure recordings of an actual golf swing. This example is for a right handed golfer.

Magnitude of Pressure: This refers to the amount of pressure applied to the ground at a given time during the swing. It is expressed as a percentage of the body weight of the golfer.

Graph representing the magnitudes of pressure each foot applies during a golf swing relative to the body weight of the golfer. The left foot is shown in red. The right foot is shown in green. Total pressure shown in white. This example is for a right handed golfer.

Creating Rotation

The primary job of the legs in the golf swing is to create rotation in the pelvis. When the legs push into the ground, the force is returned into the body. The legs meet at the pelvis where two forces are created. One force is created from the lead leg, and another force is created from the trail leg. These forces have a direction and a magnitude. To understand how the legs create pelvic rotation, it is most important to understand the direction of the forces in the pelvis. Below is an approximate functional representation of how these forces are directed:

The forces created in the pelvis as a reaction to the legs pushing into the ground. This is a downswing example for a right handed golfer.

It is important to remember that the forces shown in the photo are a REACTION to the forces the legs apply to the ground. The legs themselves are applying force into the ground in the OPPOSITE direction to the forces represented in the photo. For example if you wanted to run straight forward, you would push off the ground in a backward direction (behind you) to create this forward motion. So it is with the golf swing but both legs are working simultaneously. The trail leg pushes down, backward, and to the side to create a force on the pelvis while the lead leg pushes down, forward and to the side to create a second force on the pelvis. These forces pass through the pelvis “off axis” meaning they do not pass through the center of mass, therefore their simultaneous action results in a counter clockwise rotation of the pelvis. (For a right handed golfer, on the downswing.)

sample swing

The best way to understand how this is executed is to look at a sample swing. The following sample is taken from a right handed golfer.

Setup:

At initial setup with a driver, ideally pressure is distributed with a bias toward the trail side. About 40% pressure in the lead side, and about 60% pressure in the trail side. A ground force analysis provides objective feedback about setup pressures and can be used to make adjustments in real time. There is also a stance width measurement that can help with consistent setup.

Early Backswing:

During the backswing, the golfer is setting the body in the ideal positions to apply forces that accelerate the club in the downswing. Early in the backswing, pressure begins transferring into the trail leg. With a driver, many of the best golfers are transferring 70-80% of their pressure into their trail leg, quite early in the swing.

Lead Arm Parallel Backswing:

At the point where the lead arm is parallel to the ground in the backswing, the golfer has already begun moving pressure back into the lead leg. This is far earlier than you might suspect. In some golfers this move begins slightly later in the backswing, but ideally it happens before club transition. This characteristic is a major clue that the golfer is initiating the downswing in the proper sequence: lower body first. Another way to think about this is that the lower body begins the downswing while the upper body is still completing the backswing. This disassociation between the upper and lower body is sometimes referred to as “x-factor” or “x-factor stretch.” It is a move that creates a lot of speed, but requires great coordination as well as excellent mobility and strength in the hips, spine and shoulders to pull off.

At this point in the swing, the trail leg is already starting to make its contribution to downswing pelvis rotation by applying pressure to the ground pushing down, back and to the side. This creates the force through the pelvis shown in the picture at the beginning of the article.

Forces acting on the pelvis in the downswing. Right handed golfer.

Club Transition:

From a ground mechanics perspective, the most notable observation at club transition is that the lower body should have already begun the downswing. The pressure trace has already begun moving toward the lead side and is accelerating.

Lead Arm Parallel Downswing:

A very important part of the downswing is the point at which the lead arm reaches parallel to the ground or when the club shaft is vertical. It is the time in the swing where the lead leg is pushing down, forward and to the side to create a pressure spike under the lead leg. This spike in pressure results in a force that goes through the pelvis as shown in the picture at the beginning of the article.

The reason this moment in the swing is so crucial is because the magnitude of pressure the golfer can apply to the ground under their lead leg at this moment is highly correlated to club head speed. The golfer may be losing club head speed if one or several of the following problems are observed:

Location Problem: The golfer does not make a functional transfer in the location of pressure to the lead side.
Timing Problem: The application of pressure is either too early or too late.
Magnitude Problem: The quantity of pressure is too low.

A good magnitude of pressure would be about 1.5x -2.0x (150%-200%) body weight. The golfer in the example is applying about 1.2x (120%) body weight.

Full Swing:

This is what the full swing looks like. It’s not an especially smooth video, but it gives a sense of the entire swing.

Conclusion

This seems like a lot of information, but it is just the basics of a ground mechanics evaluation. There are several other details that can be examined. Although this discussion has outlined some of the basic characteristics that are common to more skilled or powerful players, it is important to note that there is no single trace that is most effective for all golfers. There are variations in the shape of the center of pressure trace even among the best players in the world. There are also variations in pressure trace with driver vs. iron shots or even if the golfer is trying to manipulate club path or angle of attack. Many of these discussions are beyond the scope of this article.

The take home message is that the lower body is responsible for creating pelvic rotation. The timing, location and magnitude of forces applied to the ground are crucial in creating this rotation. If a golfer does not use the ground well, the pelvis will not be rotating quickly or may not be rotating at all but moving in an aberrant way. This makes transferring energy to the upper body very difficult. The chest, lead arm, and club are all dependent upon a fast pelvis for their speed. If the pelvis is slow, the rest of the kinetic chain is likely slow or the golfer will become highly dependent on the upper body as a source of power which creates all kinds of consistency problems and increases the potential for injury.

At Iconic Motion, the goal is to identify physical reasons why a golfer may not be using the ground properly. The ground force analysis is correlated with the physical assessment to develop a targeted exercise program that will result in quantifiably better ground mechanics. If you are interested in such an evaluation but have more questions, I invite you to schedule a complimentary phone consultation to discuss. If you are eager to get started, schedule your initial evaluation here.