Back in 2000 my colleagues and I wrote a paper where we showed that an increase in the X-Factor at the beginning of the downswing seemed to be more important that the X-Factor itself. We called this the “X-Factor Stretch”.
Jim McLean is well known for his teaching of the X-Factor and has also mentioned in several articles that the hips should turn into the downswing before the shoulders, but he hadn’t quantified how much and hadn’t given it a name, so I coined the term “X-Factor Stretch” to describe this increase in X-Factor.
The X-Factor is the difference in the amount of turn between the hips and the shoulders at the top of backswing. The X-Factor Stretch is something different and here’s an explanation. If you measure the X-Factor at the time the hips transition from backswing to downswing and then again at its maximum, which should be a little later in the downswing, when the shoulder turning speed has reached the same speed as the turning speed of the hips, then find the difference between the two, that is the X-Factor Stretch.
X-Factor Example:
If the shoulders are turned to 90 degrees at the top of backswing and the hips to 45 degrees then the X-Factor is 90 – 45 = 45 degrees.
X-Factor Stretch Example:
If the X-Factor is 45 degrees at the transition of the hips and if the hips are leading the shoulders in the downswing the X-Factor will continue to increase until the hips no longer outpace the shoulders. If the X-Factor increases to say 50 degrees, then the X-Factor Stretch is 50 – 45 = 5 degrees, that is, the X-Factor has stretched an additional 5 degrees. This extra stretch provides for extra power in the downswing.
The Paper
I presented this paper at the Pre-Olympic Congress in Brisbane, Australia in 2000. I also submitted this research to Golf Magazine and it won second place in their first annual Science in Golf Prize. It was also published in 2001 in a book edited by Patrick Thomas called Optimizing Performance in Golf, unfortunately this book had only a limited publication and is no longer available, Dan Parks republished it with permission in his first edition of the Journal of Golf Research (2011) online, and I am also making it available with a link to the PDF file at the end of this article. Here is the abstract:
Abstract
The “X-Factor” is a popular term for the relative rotation of shoulders with respect to hips during the golf swing. A relatively large X-Factor at top of backswing is thought to facilitate high club head speed at impact. Little consideration, however, has been given to how the X-Factor changes early in the downswing. We tested the hypotheses that highly skilled golfers have a higher X-Factor at the top of the backswing and a greater increase in the X-Factor early in the downswing (“X-Factor Stretch”) than less skilled golfers. Multiple swings of 10 highly skilled (handicap 0 or better plus one long drive champion) and 9 less-skilled (handicap ≥ 15) golfers were captured with a SkillTec 3D-Golf™ swing analysis system (Skill Technologies Inc., Phoenix AZ). The X-Factor was measured at the top of backswing and at its maximum in the downswing. A contrast of the X-Factor means at the top of backswing showed no significant difference between the highly skilled and less skilled golfers, (t=1.017, p=0.326). Further, the effects of skill level (highly skilled and less skilled) and swing position (top and maximum) on the X-Factor were assessed using a two-factor ANOVA. The X-Factor averaged for both the top and maximum was 11% higher in highly skilled golfers than the less-skilled players, but this difference was not statistically significant (group main effect F1,17=1.93, p=.18). The X-Factor Stretch occurred during the early stages of the downswing for both highly skilled and less skilled golfers (swing position main effect F1,17=131.57, p<.001), but was significantly greater for the highly skilled players (19%) than the less skilled golfers (13%; group x swing position interaction F1,17=6.90, p=.02). This suggests X-Factor Stretch early in the downswing is more important to an effective swing than simply the X-Factor at the top of backswing.
For the full paper please Stretching-the-X-Factor-Paper.