Study Six: The effect of the impact point on golf ball roll kinematics.

7.2.1 Abstract

Background: Study Five identified a significant association between the impact variables and the variables horizontal launch angle and whether the ball was pushed or pulled when using a putting robot, this needed to be investigated with human participants. Aim: To investigate the effects of the impact point on the golf ball and the impact point on a putter on the resulting ball roll kinematics. Method: After the subjects habituated themselves to the task they completed six successful 3.2 m putt trials with two different putters (one groove faced and the other a traditional face). The impact point was identified by firstly identifying the centroid location (centre of dimple pattern). From this the following impact variables were measured; the distance and angle from the centroid location and surface area of the impact zone. Variability of the impact location on the putter face and golf ball between groups was assessed using the X and Y coordinates from the centroid location. Multiple regression analysis was used to identify if any significant associations existed between the impact variables and the following kinematic ball roll variables; velocity, side spin, initial ball roll, forward roll, vertical and horizontal launch angles and whether the golf ball was pushed (ball ended right of the target line) or pulled (ball ended left of the target line). An independent samples t-test (or non parametric alternative) was used to assess the variability of the impact point on the golf ball and putter face. Results: A significant association was identified between the impact variables and the ball velocity. No other significant associations were observed for both putters for any of the other variables. The only significant difference between the variation of the impact point between both groups was the Y coordinate on the putter face. Conclusion: No significant associations between the impact variables and horizontal launch angle and whether the ball was pushed or pulled may have been due to the fact the variance in the putter face angle and putter path rendered the effects of dimple error negligible. No differences for intra-subject variability of the X coordinate on the club head and ball may not have been significant due to the club head velocity being not significantly different between the two groups.

7.2.2 Introduction

Very little research to date has investigated the effect of the impact point on the golf ball and ball roll kinematics (velocity, side spin, initial ball roll, forward roll, vertical launch angle, horizontal launch angle and whether the ball was pushed or pulled). Pelz (2000) identified that dimples do affect the direction variability during the golf putt via the mechanism shown in Figure 6.14. However, the data presented is very limited and needs expanding on using human participants. Study Five of this thesis identified a significant association between the impact variables (length and angle from the centroid location and surface area of the impact zone) and the kinematic ball roll variables horizontal launch angle and whether the ball was

pushed or pulled for three of the four putter-ball combinations while using a mechanical putting robot. During this protocol the face angle at impact, putter path and horizontal impact point on the putter remained constant. Karlsen et al. (2008) and Pelz (2000) have previously identified these variables to affect direction consistency and did not consider the influence of the impact point on the golf ball.

Therefore, the aim of this study was to investigate the effects of the impact point on the golf ball and the impact point on a putter on the resulting ball roll kinematics using golfers with a range of ability. Based on the results observed in Study Five it was hypothesised that significant associations would exist between the impact variables and the kinematic ball roll variables horizontal launch angle and whether the ball was pushed or pulled. It was additionally hypothesised that increased variability of the impact point on the golf ball and putter face would be observed in the < 80% success rate group.

7.2.3 Methods

Participants

Following institutional ethical approval, a total of 22 right handed golfers participated in the study (age 42 ± 12.38 years; handicap 13.6 ± 7.4 (handicap range 0-28); height 1.76 ± 0.21 metres; mass 88.6 ± 23.8 kg). All golfers played a minimum of once a week and wore their own personal golfing attire and golf shoes. Signed informed consent was gained before testing.

Experimental set - up

All testing was completed in the University of Hertfordshire human performance laboratory. A Huxley Golf artificial putting green (3.66 x 4.27 metres) was used that registered 11 on the stimpmeter. A level straight 3.2 metre putt was setup with a regulation 108 mm hole. Two putters were used for the experiment a GEL® Vicis putter (grooved faced, 69º lie and 2.5º loft) and Odyssey White Hot #3 (non grooved face, 69º lie and 2.5º loft). Putters were standardised for the protocol as differences were identified in kinematic ball roll variables between putters in Study Three and Five. A thin layer of pigmented emollient was applied to the putter face and smoothed out before each trial for subsequent impact point analysis. The golf ball used for the protocol was the Srixon Z-STAR, and each trial completed used the same ball. Each trial was completed with subjects standing with both feet on an RS Scan FootScan pressure plate with a total 4096 sensors, sampling at 120 Hz was used to record CPE movements during the putting stroke.

To record the ball roll kinematics, a Quintic high speed camera sampling at 220 Hz was positioned perpendicular to the putting line. The Quintic Ball Roll v2.4 launch monitor software was used to analyse the recorded videos of the ball roll, allowing for analysis of the kinematic ball roll variables. Kinematic variables measured were initial velocity (m·s-1, calculated across the first 6 recorded frames); side spin (the amount of side spin (rpm) placed on the ball during impact); launch angle measured in degrees (vertical (whether the ball was launched in the air) and horizontal (the degree to which the ball deviates from the original putting line), initial ball roll (whether the golf ball has positive rotation (topspin) or negative rotation (backspin) at the point of impact (rpm)), forward roll (the distance at which the ball starts positive rotation (cm)) and whether the putt was pushed (right) or pulled (left) (a calculation of the final resting point of the golf ball based on the kinematic variables recorded (cm)). A Canon EOS 1000d camera was situated on a stationary Velbon CX-440 tripod was placed away from the putting line where it did not disturb the view of the participant during the trial.

Procedure

Participants were allowed as much time as they needed to habituate themselves to the first putter that had been randomly selected. This habituation period was repeated when the putters were swapped mid protocol. Once the participant was comfortable and ready to proceed with the trial the investigator lined up the putt with the Superline 2D line lasers, before the trial was completed the laser lines were turned off so not to distract the participant. The FootScan pressure plate was zeroed before the participant stepped on before each trial. Following the participant stepping onto the FootScan pressure plate the Quintic Ball Roll v2.4 software was activated. The participant was then verbally instructed the trial was due to start once the RS Scan FootScan pressure plate was activated via the external trigger this was due to the 8- second recording time limit while recording at 120 Hz.

Once the trial had been completed and the result clear, the participant was asked to step back off the pressure plate, the trial was then saved on the Quintic Ball Roll software, RS Scan FootScan software. After each trial two pictures were taken with the Canon EOS 1000d camera with the pigmented emollient imprint on the ball and of the imprint of the dimple pattern left on the putter face. If the putt was missed, the putt result was manually measured as the radial distance from the centre of the hole, and identified as long, short, left and right or a combination of two of the directions in accordance with McLaughlin and Best (2013) and Wilson et al. (2007). This process was completed until six successful putts had been completed with each putter; however, missed putts were included within analysis.

Data Processing

The data identifying the centroid location, length and angle from the centroid location was processed in the manner described in Study Four (pp. 112 – 116). The X, Y coordinates for the impact point on the putter face where processed using the polygon technique as described in Study Four (Figure 6.5, pp. 114). Additional information about this process is described in Study Five (Figure 6.15, pp. 134).

Data Analysis

Data was exported to statistical software packages Microsoft Excel 2011 and SPSS v19 for analysis. To compare the variance between scales could be, variability of the X, Y coordinates of the impact point on the golf ball and putter face were assessed using the coefficient of variation (𝐶𝑉 = 𝜎/𝜇).

Firstly, the data was tested for normality by assessing the data set skewness and kurtosis values, histogram and box-plot graphs. Any data sets that were found to be highly skewed or displaying high levels of kurtosis was log transformed to increase uniformity to a normal distribution curve (Atkinson & Nevill, 1998; Hopkins et al., 2009). Further information on boundaries set for skewness and kurtosis values is found in Study Five. Normality statistics are presented in full in Appendix F.

For the multiple regression analysis the independent variables used were the length of the impact point from the centroid location (mm), angle of the impact point (°) from the centroid location and surface area of the impact zone (mm2). These were used as the predictors for the multiple regression analysis. The dependent variables were the kinematic ball roll variables (velocity, side spin, launch angle (vertical and horizontal), initial ball roll, forward roll, and whether the putt was pushed (right) or pulled (left)), these were analysed individually from one another. Level of significance was set at p < 0.05.

Statistical analysis completed assessing the variation of the X, Y coordinates on the golf ball and putter face was completed by grouping the participants by putting success rate (> 80% success rate and < 80% success rate). An independent samples t-test (or the non parametric alternative (Mann-Whitney U test) if the data was found to be not normally distributed) statistically compared the groups. All trials were tested together as not all participants missed a putt with either putters, however, all trials (successful putts and missed putts) are presented. The multiple regression analysis was completed with all participants’ data in an attempt to identify the relationship between the impact and kinematic ball roll variables.

7.2.4 Results