Move over, Rory McIlroy–Stanford researchers have identified some of the biomechanical factors that allow professional golfers to excel in comparison to amateurs.

The study used advanced movement capturing technology at the Motion and Gate Analysis Lab of the Lucille Packard Children’s Hospital to analyze 10 professional and five amateur golfer’s swings in three dimensions over the course of the entire swing. Of the five amateurs, one was an advanced college-level amateur, two were intermediate amateurs and two were novices.

Researchers identified what professional swings had in common and correlated these factors to a measurement of the club’s speed at the moment of impact (CSI). The most significant factors that emerged were measurements called “x-factor,” “s-factor” and “free moment.”

X-factor was described by Stanford golf coach Conrad Ray ’97, a co-author of the study, as “the difference between the turn of the hips and the shoulders.”

Peak x-factor was highly consistent among the professional golfers, with a variation of only 7.4 percent. Both peak x-factor and x-factor at the moment of impact were highly correlated with a high CSI.

“X-factor has been quantified before, and this study confirmed its importance both by how consistent the pros were and its correlation to club-head speed,” said Jessica Rose A.M. ’82, P.D. ’91, Ph.D. ’92, lead investigator and associate professor of orthopedic surgery.

The researchers also took measures of upper torso rotation and pelvic rotation. Upper torso rotation was found to have a much higher correlation with CSI than pelvic rotation.

Free moment, a measurement taken by force plates during the swing that reflects rotational movement, had the least variation, at 6.8 percent, and the highest correlation with CSI. S-factor, a measurement of the tilt of the shoulders when compared to parallel, was almost the same among the professionals, but was not as strongly linked to CSI as x-factor or free moment.

Overall, the consistency of some of the movements among the professionals was surprising to the researchers.

“Typical, normal walking motion varies at about 30 percent,” Rose said. “Some of these movements varied at only about 7 percent.”

Among the amateurs, there was much less consistency in the swings; the less experience the player had, the more their data deviated from the averages of the professionals. Mean club-head speed at impact was 35.4 meters per second for the professionals, around 34 for the more advanced amateurs and not more than 30.2 for the beginners.

The study also plotted the swing data in the context of the timing of the swing.

“A lot of this underscores that efficiency is king,” said Amy Ladd, co-author and professor of orthopedic surgery. “The amateurs had erratic movements and their timing was off, but a really good golf swing looks effortless.”

There are plans to move the research to Stanford’s Human Performance Lab and offer a commercial service for golfers. Ray hopes to be able to use these advancements to improve the instruction of golf, especially for the Stanford varsity team.

“I would really like to provide great coaching and the data to support that coaching…without so much guesswork,” he said.