Ground reaction force in the golf swing, measured

Three readings, and a twist between them

A force plate is not a scale with a bigger number on it. It resolves what each foot does to the ground into three directions at once: one vertical, one running toward and away from the target, one running toe to heel. Swing Catalyst's 3D Motion Plate, a pressure plate sitting on a force plate, labels them vertical, right/left and toe/heel.

At address the vertical reading is simply your weight, because nothing is accelerating. David McGhie's Swing Catalyst article uses a bathroom scale: bend your knees on one and the needle dips as you drop, overshoots as you slow the drop, then settles. Every departure from your own weight during a swing is that effect at speed.

The fourth quantity on the readout is not a force. When the trail foot pushes one way along the toe-to-heel axis and the lead foot pushes the other, the pair twists the plate about a vertical axis, and the software calls that torque. McGhie gives the horizontal axes different jobs: toe-to-heel produces the rotation, target-line provides stability.

How big the vertical number gets, by club

1.1 x body weightPeak lead-foot vertical force, 3-iron and 7-iron, 24 golfers

Worsfold, Smith and Dyson put 24 golfers in three shoe designs on two grass-covered force platforms. With a 3-iron or 7-iron, peak vertical force reached roughly 1.1 times body weight under the lead foot and 0.82 under the trail one. With a driver the same golfers peaked at 0.84 and 0.49. The irons beat the club everyone associates with power.

Horizontal force is the component that tracks clubhead speed most closely, and the one nobody discusses. The 2026 Sports Medicine review collecting these studies reports Williams finding peak horizontal force associated with clubhead speed at r = 0.69 across handicaps, and Bourgain and colleagues reaching r = 0.83 once both feet are summed.

Timing separates players more cleanly than magnitude. Queen and colleagues split 28 male golfers at handicap 9 and recorded three-dimensional peaks under both feet. The lower-handicap group moved more vertical force from trail foot to lead foot between the top and contact, and peaked earlier throughout the swing.

The push that takes long hitters off the ground

284% of body weightLargest vertical force Swing Catalyst reports recording

Scott Lynn, research director at Swing Catalyst, published two Justin Rose swings recorded on a motion plate that differed mostly in where the pressure sat. Heels: vertical force peaked at 146 per cent of body weight. Toes: 182 per cent. His demonstration is to stand on your heels and try to jump.

The largest he reports measuring is 284 per cent of body weight, around 500 pounds, from Matthew Wolff as an Oklahoma State freshman carrying drivers past 300 yards. Lynn names Justin Thomas, Bubba Watson and Lexi Thompson as players whose vertical force lifts them off the ground. Those are single captures for coaches; no peer-reviewed tour average exists to compare them against.

The push arrives late. Lynn places the large vertical forces in the late downswing, where Sasho Mackenzie's grip-force work has the golfer pulling almost straight up on the handle. Han, Kwon and colleagues, measuring 63 golfers off handicap three or better, put the point of maximum angular effort at the instant the lead arm reaches parallel with the ground.

Torque, and what a pressure mat cannot see

Rotation is a couple, not a shove. In Lynn's laboratory animation the trail foot's reaction force points forward while the lead foot's points back, and the software reads 143 newton metres of torque through the downswing. Han, Kwon and colleagues found the trail foot supplying more of that pivoting moment and the lead foot more of the forward-backward one.

Most consumer hardware sold as ground-force measurement is a pressure mat, and pressure is force perpendicular to the surface divided by area. That is one dimension. Lynn writes that claims about the linear or rotational parts of a swing made from pressure alone are at best an educated guess. The horizontal and torque figures above are absent from a mat's output.

Why none of it reaches your camera

Video records position. Force is mass times acceleration, and a flat image supplies neither the segment masses nor the depth you would need to compute the accelerations honestly. Every figure on this page came off a plate, which is why they came from laboratories and coaching bays rather than golf courses.

The nearest wearable substitute is not a camera either. Li and colleagues strapped seven inertial sensors to the feet, shanks, thighs and pelvis of 48 professional golfers, then trained deep-learning models on the joint angles to predict all three components. The best network reached an R-squared of 0.94, the vertical component most accurately and the anterior-posterior one least.

Your video shows the consequence, not the cause. Whether the trail heel leaves early, whether the lead leg straightens through the strike, whether the whole body rises: those are in the frames, and Pebble Swing reads joint angles like the lead knee out of them. Read them as evidence that a push happened, at a size the footage cannot give you.