What is a good X-factor number?

There isn't a portable one. Change the upper line from the thorax to the shoulders and the same swing reads about 19 degrees higher, in one published case. A target only means something inside one measuring system. Judge the turn by whether your hips can start forward freely.

Does more X-factor mean more distance?

Partly. In a 100-golfer study, separation at the top was one of eight variables with a moderate link to ball speed, r of 0.50 or higher. A 308-golfer study found it mattered alongside release timing, trunk tilting and weight shifting. One contributor, not the lever.

Should I hold my hips still in the backswing to create separation?

Usually not. Restricting the hips tends to shorten the whole turn and reduce the time you have to build speed.

Can a phone camera measure X-factor accurately?

It can estimate it, with a wide band. The shoulder line and the hip line foreshorten by different amounts in a flat view, and the top of the swing often falls between frames. Treat the figure as a range.

What the numbers mean

X-factor in the golf swing: what it's worth

X-factor is one subtraction: how far your shoulders have turned at the top, minus how far your hips have turned. By the end of this you will know where the famous number came from, how much of your distance it really explains, and why any single figure for it carries a wide error bar.

A golfer photographed from behind at the top of the backswing, shoulders turned well past the hips, club above the trail shoulder. Kristin Hardwick, CC0

The number came from ten swings in 1992

38° vs 24°McLean's 1992 sample: 19th vs 161st in tour driving distance

Jim McLean put the X-factor on the cover of Golf Magazine in 1992. He measured five professionals ranked 19th in tour driving distance against five ranked 161st. The first group averaged about 38 degrees of separation between shoulder turn and hip turn at the top. The second averaged about 24.

That 14-degree gap, from ten swings, has been a teaching target ever since. It is a real observation, and a very small sample. It was never a prescription for your own number.

What the research supports, and what it does not

Myers and colleagues ran 100 recreational golfers through motion capture in 2008. They reported moderate relationships, r of 0.50 or higher, between ball speed and eight variables, separation at the top among them.

Chu and colleagues analysed 308 golfers in 2010. Their models explained 44 to 74 percent of the variation in ball speed. X-factor was one of the significant variables. So were a delayed release of the arms and wrists, trunk tilting, and weight shifting during the swing.

The result that gets left out: Cheetham's group compared 10 highly skilled golfers with 9 less skilled ones and found no significant difference in X-factor at the top. What separated them showed up just after the top, in the change of direction, which has its own guide.

Move the lines and the same swing reads 19 degrees higher

A 2020 case study by Bourgain and colleagues recomputed one golfer's X-factor several ways. Drawing the upper line through both shoulders instead of through the thorax raised the reading by an average of 19 degrees in the horizontal plane and 14 degrees in the swing plane. Same swing, same lab.

Joyce and colleagues list 61.8 ± 7.8 degrees and 55 ± 10 degrees for fast swings, the first from that same Myers study. Both sit well above McLean's 38. A separation target means nothing until someone says which body lines were used and which plane they were projected onto.

Hear it

Three clicks: takeaway, top, impact. Start at the tour beat and move to whichever one you can actually swing to.

What chasing it does to an amateur swing

The usual way a golfer adds separation is by holding the hips still. Mike Carroll, a strength coach who works with tour players, points out the cost: resisting hip rotation shortens the total turn and cuts the time available to build speed, the opposite of the intended effect.

The injury argument is weaker than it sounds. A 2020 review by Edwards, Dickin and Wang found the research on rotational load and golfers' low back pain conflicting, not settled. The honest case against forcing a bigger number is that it costs speed.

What Pebble Swing measures here, and the error bar on it

Pebble Swing runs pose tracking in the browser on 24 frames cut from your video and reads joint angles from those landmarks. A shoulder line and a hip line seen by one lens is a flat read of a three-dimensional rotation. Depth is inferred, not seen.

Two things widen the band. Seen flat, the two lines foreshorten by different amounts, which shifts the figure. And the true top of the backswing usually falls between two frames. That is why the backswing score is built on turn shape and tempo ratio, which can be timed from the motion itself.

Questions

What is a good X-factor number?: There isn't a portable one. Change the upper line from the thorax to the shoulders and the same swing reads about 19 degrees higher, in one published case. A target only means something inside one measuring system. Judge the turn by whether your hips can start forward freely.
Does more X-factor mean more distance?: Partly. In a 100-golfer study, separation at the top was one of eight variables with a moderate link to ball speed, r of 0.50 or higher. A 308-golfer study found it mattered alongside release timing, trunk tilting and weight shifting. One contributor, not the lever.
Should I hold my hips still in the backswing to create separation?: Usually not. Restricting the hips tends to shorten the whole turn and reduce the time you have to build speed.
Can a phone camera measure X-factor accurately?: It can estimate it, with a wide band. The shoulder line and the hip line foreshorten by different amounts in a flat view, and the top of the swing often falls between frames. Treat the figure as a range.

Sources

Every number on this page comes from one of these.