Hip rotation speed: the pelvis peaks and fades

The measured numbers, and how far apart they sit

415 deg/sPeak pelvis rotation in the downswing, eleven professionals

Steele and colleagues ran eleven professional golfers through three-dimensional motion analysis and recorded peak pelvic rotational velocity in the downswing at 415.2 degrees per second, give or take 32.9. That is a shade over one revolution per second, and the pelvis never turns anywhere near that far, so read it as a rate reached briefly rather than a distance covered.

A second laboratory gets a different answer. Liu and colleagues filmed ten college golfers off a mean handicap of 1.29 and put the pelvis at 515.2 degrees per second with a driver, 474.4 with a 7-iron. The less decorated group turned exactly a hundred degrees a second faster than the professionals, and the mid-iron cost forty of that. A pelvis speed quoted without its club and its laboratory is not a number you can stand next to.

The peak lands before the ball does

289 deg/sPelvis rotation at the instant of impact, same eleven golfers

In Steele's professionals, peak pelvic velocity arrived just before impact and peak upper-torso velocity arrived just after it. By the instant the clubhead reached the ball the pelvis was down to 288.8 degrees per second, roughly thirty percent below its own maximum, and it never recovers. The follow-through peak for the pelvis is 309.8, where the upper torso reaches 929.2.

So the pelvis is losing speed while the clubhead is still gaining it. The handover between segments, each peaking later and faster than the one below, is the kinematic sequence and has a guide of its own. The size of the drop belongs here, and it is the part golfers find hardest to believe. At the moment of contact the hips are giving speed back, not adding it.

Where the number stops telling good from bad

Twenty male amateurs hit a 7-iron at three shaft weights for Yang and colleagues. Ten of them played off 4.3, ten off 29.1. Maximum upper-torso angular velocity separated the groups in the downswing at p = 0.017, trail-wrist velocity at p = 0.026. Maximum pelvic rotation angular velocity did not: after correction there was no significant difference between the groups at any of the three shaft weights.

The raw means show why. With the lightest shaft the low handicappers averaged 484.3 degrees per second against 431.9 for the high handicappers, and with the middle shaft it was 470.2 against 465.8. Ten golfers a side is a small study and one result is not a law. Still, a twenty-five-shot spread in handicap that the pelvis cannot see beats another anecdote about clearing.

Clear your hips faster asks you to raise a figure the twenty-nine handicappers in that study already matched. What the low handicappers had more of sat higher up the body, at the torso and the trail wrist. The instruction names the pelvis and the measurements point somewhere else.

What the pelvis number does carry

The maximum is the least useful part of it. Steele found pelvic velocity at impact lower in the amateurs than in the professionals, at p = 0.019, and found the professionals' downswing pelvis peak among the most repeatable measures in the whole study, with a coefficient of variation of 0.079. What marked the professionals out was how little the figure moved between swings.

Zhou and colleagues built a single score from three figures: peak pelvic rotational velocity before impact, pelvic rotational velocity at impact, and peak upper-torso velocity after impact. It told eleven professionals from five amateurs with an area under the curve of 0.97. Two of the three describe the pelvis, and the maximum earns its place there in company rather than alone.

What the measurement can and cannot see

A wearable sensor checked against laboratory motion capture on thirty-six golfers, eighteen professional and eighteen amateur, agreed almost perfectly by correlation: Kim and colleagues report an intraclass coefficient of 0.98 for pelvic rotational velocity. The Bland-Altman limits of agreement were still minus 63.1 to plus 66.4 degrees per second. Two instruments both working correctly can disagree by more than the 52 degrees a second between Yang's two handicap groups.

The methods disagree as well. Bourgain and colleagues screened 517 papers down to 92 and found rotational speeds computed several different ways, from time derivatives of Euler parameters to differentiation of rotation matrices to an instantaneous screw axis, and advised caution until the field settles on one. A pelvis figure is a reading of one segment definition by one method.

None of this comes off a phone. Pelvis rotation happens largely toward and away from a single lens, which is the axis a flat image has to infer rather than see. Pebble Swing reads joint angles out of the frames and the backswing-to-downswing tempo ratio out of the motion, and neither of those is a degrees-per-second figure for your pelvis.