Twenty-four right-handed male golfers (mean mass 75.3 SD 9.1 Kg) volunteered for the study. Eight golfers had a low handicap (0-7), eight had a medium (8-14), and eight had a high handicap (15+). All played three times or more a month, with the highest handicap being 26 and the lowest 0. Following an explanation of the proposed research each subject provided written informed consent to participation, in accordance with the conditions of the ethical approval of the research investigation.

Golfers wore three golf shoe designs with different outer sole configurations and leather uppers. The metal spike shoe (Figure 1) had an ethylene vinyl acetate (EVA) mid-sole, thermoplastic urethane (TPU) Adidas Stripe Tournament outsole and was fitted with 7 Fast Twist™ 8mm metal spikes. The alternative spike shoe (Figure 2) had an EVA mid-sole, TPU outsole (Adidas Z-Traction Tour) fitted with 7 Fast Twist™ alternative Adidas spikes. The flat-soled golf shoe (Figure 3) had an EVA mid-sole, Stilo adapted flat sole and was not fitted with any spikes to provide additional traction. All shoes were new for the research and a range of sizes was available.

Data collection
Golfers adopted their natural stance to perform a full swing golf shot with each foot on a force platform in an outdoor flat field in good sunny weather. The platforms were embedded in the ground and covered in a natural grass turf surface, similar to that found on a teeing off area on a golf course. The turf (30mm) was attached using clay to smooth plates, which were screwed onto the top of each force platform (Janaway and Dyson, 2000). The Kistler 9851 force platforms' signals were passed to two Kistler 9865 amplifiers, which were connected to an Amplicon 12-bit analogue to digital converter. Kistler Bio-Ware 3.1 software controlled 1000 Hz data sampling and recording. A 200Hz Peak Performance Technologies camera was placed in front of the golfer to capture whole body and club movement and this was recorded on a high speed Panasonic AG-MD830 video recorder. A small instantaneous impact signal delivered to the surface of the force platform prior to the start of each golf swing enabled the 200Hz video recording and force platform systems to be synchronised. The time of ball impact was determined by calculating the number of frames from a force plate synchronization signal to ball impact from the video footage to the nearest 0.005 second. Posterior lower leg and foot movements were also recorded using a 50Hz JVC Compact VHS GR-FX 12EK video camcorder to aid in subsequent analysis.

Once the golfer had become accustomed to the test environment he performed 5 shots using his own driver, 3 iron and 7 iron towards a directional indicator located at 350 m. Golfers were asked to play straight shots (±8° approximately) as they would normally with each club without drawing or fading to simulate the shot landing on the fairway. The outcome of each shot was recorded. Club and shoe order were randomly assigned for each participant. Grass turf moisture level was maintained at level 2 indicated by a Rapitest moisture probe for all testing and the grass covered plates were replaced as soon as any wear became apparent.

Data analysis
The maximal vertical force (Fz max) occurring at the back and front foot during each golf swing was identified in each swing and the mean and SD calculated for all swings. In addition the greatest amount of force generated at the shoe sole to ground interface during each golf swing was determined by identification of the minimum forces occurring during the backswing and maximal forces occurring during the downswing at each foot, which is a similar methodology to that used by Williams and Sih, 1998 to compare golf shoe performance. The amount of force generated in each orthogonal plane was normalised to the body weight of each participant. In notational terms foot action anterior forces were positive and posterior foot forces were negative for both feet. In the case of mediolateral action forces for the left (front) foot were medial-negative and lateral-positive and for the right (back) foot medial-positive and lateral-negative. The coefficient of friction was determined from the ratio of vertical to shear forces using the equation (|Fx|+ |Fy|)/ Fz.

Straight shots were achieved in 89%, 71% and 46% of cases by the low, medium and high handicap players respectively reflecting the low handicap experienced golfers ability to hit straight shots. Mean and standard deviation force values were calculated for each of the five shots played by each golfer for each club, shoe and handicap condition. Data integrity checks for sphericity using Mauchley's test, homogeneity using a Levene's test and normality using a Kolmogorov-Smirnov test were performed. Data was then analysed using three way ANOVA with repeated measures at a 5% significance level. Significant differences were detected by Post Hoc Tukey HSD test (p < 0.05). No handicap group was identified as producing consistent differences between club and shoe conditions. Consequently there was support for the three handicap groups to be amalgamated. Following the same data integrity checks two way ANOVA with repeated measures was then applied to the revised 3 x 3 design (shoe by club). Differences were identified using a Post Hoc Tukey HSD test (p < 0.05).

Figure 4 and post hoc analysis revealed the predominant feature that greater maximal vertical forces occurred when using the 3 iron and 7 iron compared to the driver at both the back and front foot (p < 0. 05). Shoe outer sole design features and golfer's handicap level did not influence maximal forces (Fz max). The coefficient of friction determined for each shoe design was very similar at a value of 0.62 ± 0.03 for both the back and front foot, and this was not influenced by the type of shoe outer sole or club adopted.

The golf swing force action trace (Figure 5) shows the right-handed golfer's weight distributed approximately equally between the front and back foot in the address stance position. The greatest forces are in the vertical (Fz) plane with the back foot vertical force reaching a maximal at the end of the backswing and then a rapid weight transfer shown to the front foot with the vertical force rising to a maximal in the downswing. After ball impact a rapid decrease in force occurs as the follow-through of the club creates upwards force on the golfer. The front foot Fz increases as the club reaches the top of the follow-through. The anterior-posterior (Fy) plane forces show similar patterns with a directional change within the forces of both feet as the backswing-downswing transition occurs and then the associated generation of a couple effect giving hip and shoulder rotation. The front foot medial-lateral (Fx) force shows a medial to lateral transfer with backswing to downswing progression. The back foot medial-lateral trace indicates medial movement as body weight transfers to the front foot at the beginning of the follow- through.

Further analysis indicated that back and front foot vertical force (Fz) generated was also predominantly related to club type with similar trends evident whichever shoe was worn (Table 1). When wearing the traditional metal spike shoe during the performance of the golf swing the front foot Fz generated was significantly greater when the 3 iron or shorter 7 iron were used (0.96 W and 0.92 BW respectively) than when the driver was used at 0.82 BW. Similarly the back foot Fz generated was 0. 67 BW for the 3 and 7 iron but less at 0.33 BW when the driver was used. The back and front foot Fy with the driver was significantly less than when the irons were used reflecting a more upright maintained stance during driving. A significant difference was also identified at the front foot in that for each shoe the anterior-posterior Fy range was greater with the 3 iron (0.33-0.34 BW) than the 0.31BW recorded for the short range 7 iron. Consideration of Table 1 indicated that when using the driver and wearing the metal spike and alternative spike shoes the mean Fx forces generated were consistently greater (indicating more sideways force across each foot) than when the 3 iron and the 7 iron were used.