X-factor seeks to account for the dissociation between the shoulder and the pelvis girdles. It has been believed to be a key parameter for golf swing performance. However, several computational methods have been used and there is currently no consensus. Even if some comparison studies were performed, they remain incomplete. Methodologies used for its computation are based on pelvic markers on the one hand, and either on acromions markers (Kwon et al. 2013) or thorax ones (Brown et al. 2013) on the other hand. These markers permit to define an upper and a lower lines. These lines are then projected either in the horizontal plane or in the swing plane. Then, this factor could be defined either as the angle at the beginning of the downswing phase (or top of the backswing), or as the maximal value of this separation angle. In the latter case, it is named the X-factor stretch (Cheetham et al. 2001). The objective of this study was to evaluate if the methodological choice, i.e., based on acromion versus thorax markers, and depending on the plane of projection, have a significant influence on both the X-factor and X-factor stretch estimations.
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