During a game of golf, every shot from the tee or from the fairway comprises both a backswing phase and a downswing phase of the golf swing (Joyce, Burnett and Ball, 2010). The main aim of the golfer is to displace the ball the optimum distance and direction to achieve success (Chu, Sell and Lephart, 2010; Healy et al., 2011). For tee shots, a critical performance factor for the golfer is the ability to produce long drives with consistency and control (Higdon, Finch, Leib and Dugan, 2012).
The backswing phase of the swing positions the club and the body into the most advantageous postural position in which to accelerate the club during the downswing phase of the swing (Hume, Keogh and Reid, 2005; Hellstrom, 2009). One of the most important postural advantages is a rotation of the shoulders further from the target than the hips (Myers et al., 2008). The result of this is a separation of the hip-shoulder alignment at the start of the downswing phase, known as the ‘X-Factor’ (Mclean, 1992).
Throughout the downswing phase, many body parts are coordinated in order to maximise the transfer of elastic energy from the club to the ball via increasing rotational velocities in distal segments (Hume et al., 2005; Cheetham, Martin and Mottram, 1994) therefore maximising ball velocity at impact (Bechler et al., 1995). These movements of the body should also follow a sequence from the ground up (Chu et al., 2010). For example, the ground reaction force (GRF) the golfer applies will transmit from the back foot onto the front foot in combination with the uncocking of the wrists and the maximisation of the X-Factor. This is caused by sequential eccentric stretches of the musculature of the trunk, upper limbs and hip to increase the velocity of the clubhead and consequently ball displacement (Adlington, 1996; Hume et al., 2005).
The evidence that suggests the greater the X-Factor at the top of the backswing, the greater displacement of the ball is, however, equivocal. It is postulated that a maximisation of the hip-shoulder separation angle is capable of increasing the utilisation of the stretch shortening cycle with connotations for extending hitting distance (Cheetham et al.,1994; Myers et al., 2008). This is caused by the hips counter rotating before the shoulders (Cheetham et al.,1994). Studies examining the benefits of the X-Factor discovered that both a significant increase in clubhead velocity (McTeigue, Lamb, Mottram and Pirozzolo, 1994; Cole and Grimshaw, 2009) and increased driving distance occurred (McLean, 1992). These findings also displayed increased separation in hip-shoulder orientation in professionals (Cheetham et al., 1994) and golfers with a low handicap (Watanabe, Kuroki, Hokari, & Nishizawa, 1998) compared with amateurs. In addition, it has been suggested that the difference in the magnitude and the timing of the rotation increases distance (Myers et al.,2008), due to faster sequential trunk rotation in professionals compared with amateurs (McTeigue et al.,1994). This research therefore suggests that an increased, stronger and faster X-Factor is different for skilled and non-skilled golfers. This difference provides evidence to suggest that an increase in the X-Factor is beneficial for golfing performance.
On the other hand, research has also reported no significant difference in ball displacement with an increased X-Factor (Hellstrom, 2009). Furthermore, contradictory to previous literature, research has reported no significant difference in the separation angle between professionals and amateurs (Cheetham, Martin, Mottram and St. Laurent, 2001). The ambiguity in the results regarding the X-Factor may be due to different testing methods (Hellstrom, 2009). For example, studies placing medially located retroreflective markers close to the spine found no significant association between pelvic-upper torso rotation at the top of the backswing phase and the displacement of the ball or the skill of the player (Cheetham et al.,2001). Conversely, markers placed laterally to the acromion found significant results (Myers et al.,2008). Also, many studies have examined the shoulder-hip separation angle in only two dimensions with the golf swing executed in three dimensions combining tilting, bending and axial rotation of the spine (Hellstrom, 2009). Additionally, these studies also failed to conduct a comparison of multiple segments (e.g., shoulders to pelvis, etc.) therefore not incorporating analysis in all planes of motion and producing data that is non-reproducible (Hellstrom, 2009; Joyce et al., 2010).
In addition to sporting performance is the potential risk of injury with an increase in the X-Factor in the golfers’ swing, elevating the possibility of lower back pain (Lindsay, Horton and Paley, 2002; Joyce et al., 2010). It is reported that the aggravation and even development of lower back pain is produced by exaggerated strain on the spine including damage to the inter-vertebral discs and facet joints (Lindsay et al., 2002; Gluck, Bendo and Spivak, 2007). The literature is also equivocal in terms of the onset of injury due to a high X-Factor separation with other research reporting no difference in the likelihood of injury between skilled and non-skilled players (McTeigue et al.,1994; Cole et al., 2009). The inconsistency in these results is also caused by unreliable measurement techniques (Evans et al., 2012; Hellstrom, 2009).
Due to the inconsistency and equivalence of the evidence displayed in the literature it is evident that the X-Factor mechanism is a key component of a golf swing, in terms of the contribution of force produced throughout the rotational stretch between the upper torso and pelvis, and therefore should be investigated further (Hellstrom, 2009). It would be of great benefit if future studies implemented a universal method for measuring rotational angles of the golf swing. Findings could then be compared in confidence, in terms of sporting performance and, especially, potential injury (Hellstrom, 2009). Finally, in terms of aiding the enhancement of sporting performance, by measuring the kinematics of the thorax and the pelvis, the evolution of a three-segment model to analyse the golf swing in all planes of motion is essential (Evans et al.,2012; Joyce et al.,2010).
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