Golf Coaches’ Perceptions Of Key Technical Swing Parameters (End)
When discussing posture the coaches also referred to the key technical parameter ‘Body Rotation’ [Q13]. Several terms were used to communicate the idea of body rotation, including ‘Core’, ‘Upper Torso,’ ‘Trunk’, ‘Shoulders’, ‘Hips’ and ‘Pelvis’. Nevertheless, the most common terms used were Shoulder and Hip rotations as these were deemed the most appropriate words to communicate clearly with the golfers during coaching sessions. Due to the various terms used to describe Body Rotation, the terms trunk rotation and pelvis rotation will be used to aid clarity in this paper.
The rotation of the trunk and pelvis was referred to throughout the swing. The coaches believed that the rotation of the trunk and pelvis during the backswing was an opportunity to generate a powerful, repeatable and simple swing by producing torque or energy, which could then be transferred to the ball at impact.
…if there was minimal rotation…you’re not going to be able to create as big torque in the backswing, create as much pressure in your right leg, therefore, you’re not going to be able to shift that back across through into your left side and transfer that energy back through your arms and your club. [Q16]
I want the club to come down…I don’t particularly want that to be fine movement with hands and arms…that can vary enormously…whereas hip turn can’t vary as much. [Q17]
Only a selection of coaches offered preferences for the degree of rotation they would like to see during the swing, whereas other coaches commented that the degree of rotation was golfer specific, depending on elements such as a golfer’s degree of flexibility.
[At] impact we’re looking for the hips to be more turned open than the shoulders, within about 10 degrees…40 degrees with the hips and 35 to 30 degrees with shoulders is fine, as long as we’ve got the right tilts and right shifts into the left side. [Q18]
It was also recognised that body rotations would be influenced by movements within other planes and should not be disregarded:
Pelvic rotation… [is] rotation around its mid axis…but it doesn’t just rotate…it shifts, it turns, it tilts as well so it’s not simple rotation. [Q19]
The coaches believed that the separation between the trunk and pelvis was more important than the independent rotations of the segments. Many of the coaches spoke about the ‘Disassociation’, ‘Resistance’, ‘Storing power’ or ‘Separation’ between the trunk and pelvis segments. Others used the coined term ‘X-factor’ to describe the relationship between the trunk and pelvis rotations.
hips beginning the downswing…you can see as the left foot, pulls the rest of the body through so the hips pull through the abs, the abs pull the chest and it all comes through and the big disassociation you can get between the hips and the shoulders, the more power. [Q20]
You get a good golfer who is stable…there will be a big difference between the hips and shoulders at the top of the backswing…that is one of the key factors of powerful golf swings, but it’s not the key factor, the ability is to be able to separate the hips on the way down from the upper torso and then … close that gap down as quick as we possibly can. [Q21]
In the golf biomechanical literature, body rotation, typically quantified by axial rotation of the central body segments, has been widely investigated and linked to performance outcomes, such as clubhead velocity. Many studies have reported pelvis and trunk axial rotational angles at various stages of the swing including at TA, TB, IMP, middownswing, last 40ms prior to impact as well as the peak magnitudes. Hume et al. reported trunk axial rotation of 78 – 102º and pelvis axial rotation of 47-55º at TB depending on golfer ability and the club being used and also suggested that trunk flexion, lateral bend and knee angles should be observed as they could influence axial rotation.
Several authors have suggested that the separation between pelvis and trunk axial rotation (i.e., X-factor) was more important for power generation. Chu et al. reported that X-factor at TB explained approximately 25% of ball velocity with a driver and the authors suggested that golfers should focus on increasing separation between trunk and pelvis rotation in order to increase ball velocity. Maximum X-factor during the downswing was shown to strongly correlate with clubhead linear velocity at IMP (~ 74%) and a moderate correlation was found between ball velocity using a driver and X-factor at TB (~ 30%). The authors concluded that X-factor at TB and downswing maximum contributed to the rotation velocities of the upper torso which, in turn contributed to increased club and ball velocity.
The difference in X-factor between TB and downswing maximum value (termed X-factor stretch), has been suggested as more important than the maximum X-factor alone. The greater X-factor stretch (mean 13.4º) in highly skilled golfers (handicap < 0) compared to a lower skilled golfer (handicap > 15) (mean 0.5º) was considered to contribute to the greater shot distance for the higher skilled golfers. It is important to note that the differences in how TB is defined could affect the value of X-factor at this part in the swing and subsequent X-factor stretch calculations. The rate of stretch and recoil describes the speed with which the trunk and pelvis separate and align providing a measure of rotational power. Golfers with greater driving distance are suggested to display greater maximum rates of recoil in the downswing. Nevertheless, there are limited studies that have investigated this idea further. The proposed mechanism for increased separation between trunk and pelvis and the timings of rotations was due to a stretch-shortening cycle within the spinal rotator muscles, leading to increased trunk acceleration and in turn increased club acceleration. In contrast, studies on female golfers have not provided support for the stretchshortening mechanism of trunk muscles during the downswing for increasing clubhead linear velocity.
The repeatability of rotational parameters have also been investigated. Peak trunk axial rotation has been shown to have lower variability as shot intensity increased, while peak pelvis rotation repeatability was greater than trunk rotation repeatability across all shot intensities. The rotational parameters at IMP displayed larger coefficient of variation (COV) than the peak values, which may be a consequence of consistently identifying the IMP position or how COV is defined. Horan et al. examined movement variability of rotational parameters using standard deviations (SD) at key stages of the swing (TB, mid-downswing, IMP) and using spanning sets across continuous phases of the swing in male and female golfers. Female golfers were reported to have greater axial rotation variability for the pelvis at mid-downswing and IMP and trunk at IMP than males. However, the authors could not explain these differences in variability.
The majority of previous studies calculated trunk and pelvis axial rotation as 2D projection angles. These methods include simply using marker positions (e.g., acromion and anterior superior illiac spine (ASIS) markers, to define trunk and pelvis segment vectors. Two-dimensional axial rotation angles are then calculated by projecting the vectors onto the global co-ordinate system horizontal plane. The X-factor calculated by the 2D projection method would be the angle between the projected pelvis and trunk vectors; however, limitations have been identified with this method. In reality, the golfer rotates about a flexed trunk (2D trunk ~ 30º) and projecting the trunk vector onto the global co-ordinate system horizontal plane could lead to errors in axial rotation angle measurements. Recognising that the 2D projection angles do not account for the six degrees of freedom of golf swing motion more recent studies have used 3D measurements to calculate trunk and pelvis axial rotation. However, there has not been a direct comparison of Xfactor magnitude between 2D projection methods and 3D measurement methods until recently. Both studies reported statistically significant differences in X-factor values between the different computation methods. In particular, Kwon et al. reported larger maximum X-factor values when using the 2D projection method compared to 3D angles (59.1 ± 10.6º vs. 55.7 ± 10.0º).
In summary, the separation between trunk and pelvis was viewed as more important than rotations of individual segments by golf coaches [Q20-21], which is in agreement with most of the biomechanical literature. Some coaches alluded to other important aspects of X-factor, such as rate of recoil [Q21]; there have been few studies to investigate this premise. Although coaches were largely concerned with body rotation they did not discount the effect of movement in other directions such as shifts or translation [Q18-19], which is sometimes discounted when biomechanical studies report 2D methods. This will require determination of the most appropriate methodologies to account for both rotations and translations. Coaches would also link body rotation to powerful, repeatable and simple swings [Q17]. Although, body rotation varied at discrete stages, the variability throughout the swing and across golfers needs further investigation.
The purpose of this study was to identify the key technical parameters that high-level golf coaches associate with a successful golf swing and to compare them to current golf biomechanical literature to identify areas for future golf biomechanical research. Five key technical parameters emerged from the inductive analysis which were, ‘Club Motion’, ‘Posture’, ‘Body Rotation’, ‘Sequential Movement of Body Segments’ and ‘Arm and Wrist Action’. Coaches were keen to analyse the golf swing as a whole or during different phases of the movement (e.g., initial downswing), which the current golf biomechanical analysis methods do not readily report. Posture and body rotation were the most common themes discussed by golf coaches and were often discussed together. Posture was identified as a key biomechanical parameter in the literature; however, there were limitations with the methodologies used to measure trunk flexion and lateral bend and the relationship between postural kinematics and postural balance was not examined. Body rotation was also related to posture in the biomechanical literature, but again methodological limitations make it difficult to examine the coupled movement. The separation between pelvis and trunk axial rotation was viewed as key to producing powerful swings, but the mechanisms are not fully understood. The results of this study have led to the formation of two prevailing research questions: Are existing biomechanical data collection and analysis methods appropriate for measuring posture and body rotation of the golf swing?; and How can we biomechanically analyse posture and body rotation for individual golfers and throughout the swing to further understand their relationship with performance?