Vertical Force in the Golf Swing Part 1

Welcome back to the AE Golf Performance newsletter. This week I want to touch on a topic that I posted about recently on social media (see full post here). That is the topic of vertical forces in the golf swing.

But first, we need a basic understanding of ground reaction forces. Bear with me if you’re not into physics, I hope to keep it fairly simple. But I believe this is an important concept to understand, and I’ll be diving into some of the practical aspects of it in future posts!

Intro to Ground Reaction Forces (GRF)

When force is applied to something (like the ground), there is a simultaneous force applied of equal magnitude but opposite direction (Newton’s 3rd Law of Motion). In other words, as we apply force into the ground, there is an equally strong force directed back towards us in the opposite direction, which we refer to as a ground reaction force (GRF). 

Interacting with the ground (and leveraging the resulting GRFs) is fundamental to many movements, and we intuitively pick up on this from a young age. For example, we jump by pushing into the ground, knowing that this will help propel our body upwards if we push hard and fast enough.

But why do we move when the ground doesn’t? The effects of a force applied to something depends on its mass. Something with greater mass requires a larger force to move than something with smaller mass. For example, you can kick a balloon with a soft kick and it will go flying, but that same amount of force would not move a bowling ball (but may leave you with a sore foot).

Because we have considerably less mass than the ground we push into, the reaction force, if large enough, can cause us to move (i.e., jump into the air) while the ground is relatively unaffected (you would need a VERY strong push to cause the mass of the Earth to accelerate). But if you were to jump from on top of a chair, especially at a slight angle (not recommended), the chair may launch back away from you, since the force you apply to it would be enough to move its mass.

While GRFs are not the only forces that matter within the swing, they are a key component since our interaction with the ground plays a critical role in how we move and eventually transfer energy out to the club.

GRFs in the Golf Swing

When we swing a club, we are pushing into the ground in different directions:

1. Vertical: Pushing down into the ground.

2. Lateral: Pushing either towards or away from the target.

3. Anterior-Posterior (AP): Pushing towards the toes or towards the heels of the feet.

Actions in all three directions are important throughout the swing, but verticals often get the most love. This is not particularly surprising, since vertical forces are the largest in magnitude (a skilled golfer can push down into the ground with >2x their body weight of force during the downswing), and also happen to be fairly easy to measure with a variety of technologies.

I will discuss the other directions another time, but for now I want to briefly introduce vertical force in the golf swing.

Example of Vertical Force in the Golf Swing

Here is an example of vertical forces from a low handicap golfer measured from force plates.

The black line represents the combined force (sum of vertical force from both feet), while the blue line is the force from the lead foot (left foot for a right handed golfer). You can see a few key points in the force-time trace:

1. Around the -700 to -480 ms time point, the black line increases slightly while the blue line is quite low. This represents the golfer loading into the trail side during the backswing.

2. There is then a drop in the force, which represents “unweighting.” This occurs as the golfer begins to drop a few cm down towards the ground and prepare to load hard and fast into the lead side. This is a similar situation as a vertical jump, where you unweight, and then push hard and fast as you begin to drop towards the floor.

3. There is then a big peak in the vertical force just before impact, representing when the golfer is pushing through the lead foot as they accelerate the body and club.

This just gives you a basic idea of the pattern of how we push vertically into the ground, and I’ll go into some practical considerations in Part 2. But for now, let’s talk about why vertical force matters.

Why are Vertical Forces Important in the Golf Swing?

When used effectively, well-timed vertical forces can help perform a fast and effective swing for several reasons (alongside forces in the other directions):

Reason 1: Frontal Plane Torque

Torque is a force that causes rotation, which is obviously important for rotational actions like the golf swing. Torque can be increased by producing greater force, and/or by increasing the “moment arm”, which is the distance from the axis of rotation that the force is produced.

A classic example is using a wrench. We apply force to the end of the wrench, which creates a torque and causes it to rotate. It is easier to use a longer wrench than a very short one, because we can produce the same amount of force, but create a larger torque due to the longer moment arm. This is also why a door knob is on the opposite side from the hinge. It would be inefficient to rotate the door open if the knob was located on the same side.

Torque in the frontal plane (think side-to-side movements) also help the body rotate during the golf swing. Skilled golfers take advantage of this by “re-centering” their hips during late backswing or transition, and then pushing hard and fast through the lead foot while the rest of the body remains slightly behind. This is a similar situation to the longer wrench, where a similar amount of force can produce a larger torque due to the greater moment arm.

In contrast, many recreational golfers will not “lead with the hips” during transition and end up producing their vertical force with the body more stacked up. This means they have a very small moment arm, and would need to produce a very large vertical force to get the same frontal plane torque. In essence, they’re trying to use a very short wrench!

That being said, getting the most out of frontal plane torque also requires a large magnitude of vertical force, not just a long moment arm. Especially if a golfer is already skilled at creating a large moment arm via a well-sequenced action.

If you want to dive deeper into this concept, I highly recommend checking out this YouTube video by Dr. Kwon. It’s fairly technical, but does a great job introducing torque in the golf swing.

Reason 2: Lead Foot Verticals Act as a Brake and “Post-up” Mechanism

During the transition and downswing, skilled golfers will rapidly shift their pressure towards the lead side. The lead foot will push in several directions to help brake against this momentum, and then post-up on the lead foot. This helps to transfer momentum up the kinetic chain while also providing support for the body to rotate around. Notably, this lead foot action is when the largest magnitude of vertical force is produced. But if you looked through my social media post, you’ll know that I believe it’s more than just how much force you produce, but also how fast and when (more to come on this).

Reason 3: Side Tilt

Pushing into the ground can cause our body to move in the opposite direction if the force is great enough to outweigh other forces that oppose it (like with the vertical jump example).

One of the direct effects of pushing through the lead foot and extending the lead leg is that it helps tilt the body to the side (lead shoulder tilts up and trail shoulder down towards the ground). This can help shallow out the club and increase its angle of attack as it approaches the ball. In essence, this is helping carve out the club’s path into an advantageous impact position with the ball, particularly when swinging a driver.

Reason 4: Centripetal Force

A fundamental law of motion (Newton’s First Law of Motion) is that an object will only change its current motion if acted upon by an unbalanced force. This applies to both getting something to start moving (e.g., to get a golf ball moving, we need to strike it with a force from the golf club), but also to change how something is moving (slow down, speed up, and/or change direction).

I’ll save a more thorough discussion of centripetal force for another post. But the basic idea is that a moving object wants to continue moving in a straight line at its current velocity unless another force comes into play. The club does not just rotate around your body on its own. There is force involved with maintaining its semi-circular path. And the force required to do this can be quite high (>100 lbs) if you swing the club fast.

If this force was not applied, the club would simply keep moving in a straight line. For example, if you pulled the club down from the top of the backswing, and then just completely relaxed your body, the club’s momentum would simply send it flying straight down into the ground.

What does this have to do with vertical force? As the club rotates into impact, it is usually moving at or close to its fastest speed. This requires a very large centripetal force to carve its path into the ball. By loading into the lead foot, you put yourself into a strong position to control this fast moving rotation using the whole body, rather than just pulling against the club with the hands/arms alone.

Conclusion

Hopefully this Part 1 provides some basic foundational knowledge on GRFs and why vertical force is often a focus in both golf coaching and fitness contexts. But now that we’re through this background info, Part 2 will touch on some practical applications and what we can do to better leverage vertical forces in the golf swing!

Until next time!

Alex

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