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Footwear Science Vol. 2, No. 1, March 2010, 37 49 Traction on artificial turf: development of a soccer shoe outsole Thorsten Sterzing*, Clemens Mu ller and Thomas L. Milani Human Locomotion, Chemnitz University of Technology, Thu ringer Weg 11, Chemnitz 09126, Germany (Received 19 September 2009; final version received 8 February 2010) Purpose: Official game play on high quality third-generation artificial soccer turf was approved by the FIFA already in 2004. However, it is still unknown how the new surface affects traction requirements and thus potentially calls for specific footwear, especially with respect to the shoe outsole. This research project aimed to develop an artificial soccer turf outsole that provides very good traction performance to players. Methods: The whole project consisted of three phases that were carried out over three years: (I) status quo evaluation, (II) modified prototype testing, and (III) market comparison. In each phase an identical, comprehensive testing design, incorporating performance, perception and biomechanical testing procedures, was applied on FIFA 2-star artificial soccer turf. Four different shoe models were comparatively examined in each research phase and respective findings guided the selection of shoes for the following phase. Results: Soccer shoes that were traditionally designed for playing on natural grass were not (soft ground) or only limitedly (firm ground) suited for playing on artificial turf. Better traction performance on artificial soccer turf was achieved by usage of multiple and rather low studs being evenly distributed across the rearfoot and the forefoot areas. The final prototype shoe outperformed three commercially available artificial turf soccer shoes on the market at time of testing. Conclusion: This research provides an improved understanding of the mechanisms of artificial soccer shoe traction. Solid recommendations for the requirements of artificial soccer turf outsoles are stated, which generally confirm players intuitive choice of soccer footwear. Keywords: football; soccer shoe; artificial turf; traction; performance; perception; biomechanics 1. Introduction Artificial turf as sports playing surface came up in the 1960s and was originally directed towards American Football. One of the first installations was the Astrodome in Houston, TX, USA (Levy et al. 1990). The first generation of artificial soccer turf consisted of a concrete bottom layer and an unfilled top layer of rather short and dense artificial grass fibres. In the 1980s, the second-generation featured an elastic bottom layer and longer grass fibres with an additional sand infill. However, its implementation during soccer match play was obstructed as the new surface changed the nature of the game by alteration of ball bounce and ball roll (Lees and Lake 2003). This was due to higher ball rebound resilience and lower ball rolling resilience on artificial turf (Lees 1996). Also, players experienced difficulties to keep their footing and carpet burns were sustained during sliding tacklings which prevented players to perform those. A third generation of artificial soccer turf was developed, also containing an elastic bottom layer and an artificial grass fibre layer filled with sand. Additionally, there is a top layer infill of rubber particles, specifically meant to provide traction by functional interaction with the shoe outsoles of the players. Official game play on high quality, FIFA 2-star, artificial soccer turf was then approved by the FIFA in 2004 and included in the official rules of the game (FIFA 2009g). Recent comparisons of game characteristics on elite level did not show general differences between match play on natural grass and artificial turf (FIFA 2009a,b,c,d). FIFA tournaments, like the U17 World Championships in Peru (2005) or the U20 World Championships in Canada (2007), were held completely or in part on artificial turf already. Currently, there are 144 2-star certified artificial turf installations around the globe. These were manufactured by 17 different companies, according to the FIFA (2009h). An argument for resisting the use of artificial turf in top level soccer was the reported high incidence of injuries. On previous, first- and second-generation, types of artificial turf, surveys in fact revealed a considerably higher injury risk on artificial turf compared to natural grass. Injury incidence on *Corresponding author. Email: thorsten.sterzing@hsw.tu-chemnitz.de ISSN 1942 4280 print/issn 1942 4299 online ß 2010 Taylor & Francis DOI: 10.1080/19424281003685678 http://www.informaworld.com

38 T. Sterzing et al. artificial turf was found to be 1.5 times higher as on natural grass in Norway (Engebretsen and Kase 1987). In Icelandic soccer players injuries occurred 2.5 times more often when summarizing match and training incidents (Arnason et al. 1996). However, third-generation artificial turf injury statistics show improvement compared to earlier generations. Prospective studies did not show differences in injury occurrence between natural and artificial surfaces, so far. Thereby, the overall risk of injury did not differ for elite professional male soccer players across Europe, although injury sites are controversially discussed (Ekstrand et al. 2006). Similar injury incidence was also shown for match and training injuries in an investigation of American college and university soccer players (Fuller et al. 2007a,b). The authors reported no major differences in severity, nature or cause of injuries between the two surfaces. Also, no higher incidence of overall injury occurrence on artificial turf was observed for teenage female soccer players (Steffen et al. 2007). Soccer shoes and their outsoles have been discussed with respect to injury occurrence and playing performance in soccer (Lees 1996, Lake 2000, Sterzing et al. 2007). Rodano et al. (1988) stated the soccer shoe to be the most important piece of equipment for the player referring specifically to the shoe ground interface. Naturally, soccer shoes so far were mainly developed for playing on natural grass. Those shoes feature outsoles that were designed for hard ground, firm ground, and soft ground surface conditions in response to temporary change of pitch and weather conditions across the season. At starting time of the present research project, specific artificial soccer turf shoes hardly existed. Those that did were based only on unsystematic research and development processes. Currently, players use either hard ground or firm ground natural grass designs also when playing on artificial turf (Kunz 2009). The soft ground design is avoided by almost all players when playing on artificial turf solely based on individual preference. Generally, traction was rated second among the most important soccer shoe features after comfort and prior to features like stability, ball sensing, weight, kicking velocity, and kicking accuracy (Sterzing et al. 2007). As traction influences all movements of the players during the match, various movement patterns need to be considered during traction evaluation. Better traction was found to translate into faster running and corresponding perception of running speed (Krahenbuhl 1974, Mu ller et al. 2009b, Sterzing et al. 2009). In contrast, increased shoe weight of 70 g was shown not to be a confounding factor for running speed in slalom courses at all (Sterzing et al. 2009). Functional traction is also needed to achieve higher ball velocity during full instep kicking (Sterzing and Hennig 2008). Biomechanically, shear forces and force ratios of horizontal divided by vertical forces were shown to be crucial descriptors of traction properties (Valiant 1988; Morag and Johnson 2001). However, discrepancies were reported between the functionality of mechanical and biomechanical traction properties. Higher mechanical traction properties did not automatically lead to higher biomechanical shear forces and shear force ratios during cutting movements. This shows that biological human structures have to be strongly considered during the evaluation of traction properties (Sterzing et al. 2008). For systematic improvement processes of athletic footwear, one approach, suggested by a group of researchers, combines various interdependent research areas like performance, perception, biomechanics and mechanics (Hennig and Milani 1996, Lafortune 2001, Sterzing et al. 2007). Thereby, performance and perception variables may be classified as direct descriptors for the quality of certain shoe properties. For instance, running time or ball speed variables do not need further interpretation when assessing the suitability of shoe properties. The same is true for subjects perception which directly tells us the quality level of shoe properties. In contrast, indirect biomechanical and mechanical variables need to be carefully interpreted when transferring research data into shoe design recommendations. Currently, when playing on artificial turf, players predominantly use soccer shoes with a hard ground or firm ground stud configuration, originally designed for playing on natural grass (Kunz 2009). Therefore, the ultimate goal of this research project was to develop a soccer shoe outsole specifically designed for playing on artificial turf. This outsole should provide very good traction performance properties to players on a fairly new but rapidly spreading artificial soccer surface all around the globe. A second goal was to better understand the fundamental interface characteristics between soccer shoes and third-generation artificial turf. By this, solid general recommendations for artificial turf outsole configurations may be provided. 2. Methods The 3-year research project consisted of three subsequent phases: (I) status quo evaluation, (II) modified prototype testing, and (III) market comparison. For status quo evaluation, three shoe models, originally designed for natural grass, and a first artificial turf prototype were examined regarding

Footwear Science 39 their suitability on artificial soccer turf. Based on phase I findings, three new prototypes were constructed in phase II and compared to the first prototype. Eventually, the final prototype was produced in phase III and examined in comparison to three, commercially available, artificial soccer turf shoes. In the following this article is specifically structured: First, the general methods of the research project are introduced as they apply consistently to all three research phases throughout. Then, phase specific methods are presented which primarily describe the different shoe conditions and respective reasons for their selection. The phase specific methods are directly accompanied by corresponding results and discussion. Finally, a comprehensive discussion, covering all three phases of the research project, is presented. 2.1. General methods In all phases of the research project the same methods were applied. The comprehensive research design consisted of performance, perception, and biomechanical testing. In each research phase, four shoe models were examined. Shoe order was randomized between subjects for all testing procedures. The surface for field and laboratory testing was FIFA 2-star artificial soccer turf (Polytan Liga Turf 240 22/4 RPU brown; Polytan, Burgheim/Germany). Surface construction consisted of a 35-mm resilient, elastic base layer (DIN V 18035/7) which was covered by the artificial turf fibre carpet (40 mm) being filled with sand (18 kg/m 2 ) and rubber (0.5 1.7 granulation diameter, 5 kg/m 2 ). The subject pool used over the 3-year project consisted of 47 experienced soccer players (23.0 3.2 years, 177.3 4.4 cm, and 71.4 5.9 kg). All participants had shoe size UK eight as required by the shoe samples available. All participants signed informed consent and procedures adhered to the requirements of Chemnitz University of Technology for subject testing. Performance testing was carried out by use of two soccer-specific Functional Traction Courses (FTC), slalom and acceleration (Figure 1). Running time (RT) and respective running time perception (RTP) of the subjects running in different shoe conditions were examined according to the protocol by Sterzing et al. (2009). In each shoe condition subjects performed three runs for each course. Subjects switched shoes after each single run in order to account for potential fatigue effects. There was a 2-minute mandatory rest between runs. RT was measured by double light barriers (TAG Heuer HL 2-31, Marin-Epagnier, Switzerland) as recommended by Yeadon et al. (1999). Better performance was indicated by shorter RT. Additionally, players ranked their perception of running time in each shoe condition (1, best RTP to 4, worst RTP). Perception testing also required players to give a rating of their traction suitability perception (TSP) TSP was rated by use of a nine-point perception scale (1, very good to 9, very bad, Figure 2). Similar scales were already used earlier in athletic footwear testing (Coyles et al. 1998, NSRL 2003, Sterzing and Hennig 2005, Brauner et al. 2009). Prior to providing their rating, players performed several, non-standardized, cutting, turning, acceleration, and deceleration movements in the respective shoe condition. During biomechanical laboratory testing subjects performed three standardized soccer specific movements: Straight acceleration (0 ), cutting 45, and turning 180 (Figure 3). Subjects performed all movements with a two-step (left, right) approach placing their right foot on the force plate. Movements were executed as dynamic as possible. Approach speed and orientation of foot placement were not standardized in order to avoid any artefacts of movement execution by the subjects. In each shoe condition five repetitive trials were collected for all three movements. The force plate (Kistler 9287 BA, 60 90 cm, 1 khz) was covered with a wooden box containing the in-filled artificial turf. The surrounding floor was elevated to match the box height. It was covered with unfilled artificial soccer turf in order to allow a smooth approach to and return from the force plate in all shoe conditions. Evaluated variables and their directional components were oriented towards Valiant (1988) and Morag and Johnson (2001), directions are defined in Figure 3. For acceleration, peak vertical force (PVF) and peak anterior-posterior shear force (PSF a-p ) were measured. For cutting and turning, additionally, peak medio-lateral shear force (PSF m-l ) and peak resultant shear force (PSF res ) were obtained. Also, respective force ratios for acceleration (PSF a-p / PVF), cutting (PSF res /PVF), and turning (PSF res /PVF) were looked at. 2.2. Statistics Means and standard deviations of all variables were calculated across all subjects. For shoe comparison repeated measures ANOVA (p 5 0.05) and Bonferroni post-hoc tests (p 5 0.0083) were carried out. Running times of the FTC are presented as percentages (Sterzing et al. 2009). Therefore, each subject s individual performance was normalized to the mean running time of the slowest shoe condition across all subjects. This was done for each study separately.

40 T. Sterzing et al. Figure 1. Soccer-specific Functional Traction Courses. Figure 2. Perception testing scale for TSP (1, very good to 9, very bad). Hence, 100% running time marks the shoe mean across all subjects of the slowest shoe condition. Thus, lower percentages mark faster running performance in the respective shoe condition. This procedure allows the comparison of running time differences between the slalom and the acceleration FTC. It also allows comparing results of the different phases of this research project as this type of data representation is independent of the subject pool used. It is acknowledged that findings of this research are only valid for the specific artificial soccer turf used in this study. Perception testing was carried out with the different design features and brands (phase III) visible to the subjects. This may have caused some bias, although explicit instruction of subjects included precaution of this. 3. Specific methods, results and discussion 3.1. Phase I: status quo evaluation 3.1.1. Shoe conditions In phase I the three basic types of soccer shoes, originally developed for playing on natural grass, were evaluated: Hard ground (HG, 244 g), firm ground (FG, 209 g), and soft ground (SG, 222 g). Additionally, an innovative outsole design (ID, 252 g) was examined which marked the first artificial turf prototype of

Footwear Science 41 Figure 3. Biomechanical testing movement pathways. Figure 4. Shoe conditions Phase I. this project. This prototype featured a regular firm ground outsole at the rearfoot. At the forefoot there were multiple double cylindrical TPU stud elements, each being able to be telescoped into each other (DuoCell Technology). The shoe upper of all four shoe conditions was similar, so that it was justified to relate potential differences between shoes to their different outsoles. Shoe conditions of phase I are displayed in Figure 4. 3.1.2. Results and discussion Performance and perception variables showed that SG was the least suited shoe condition. It provoked the slowest RT in the slalom and acceleration course which was also matched by subjects perception (Figure 5, Table 1). Post-hoc tests underlined these findings for both courses for RTP but for RT only for the slalom course. TSP was also perceived worst for SG (Table 1). The best suited stud configurations during slalom running (RT and RTP) were ID and HG, both characterized by multiple, and fairly short, less protruding studs. FG was positioned between SG and HG/ID for slalom RT and RTP. However, no significant differences were found between ID, HG, and FG for RT and RTP in either slalom or acceleration course. The TSP rating displayed that players perceived ID significantly better suited

42 T. Sterzing et al. Figure 5. Slalom RT, RTP, and TSP Phase I (means and SD). Table 1. Performance and perception variables Phase I (means and SD). Variable Protocol Subjects [n] Unit HG FG SG ID p value ANOVA RT Slalom 20 [%] 97.32 5.90 97.98 6.10 100.00 6.11 97.36 6.34 p 5 0.01 RTP Slalom 20 [1 4] 1.80 0.95 2.50 0.76 3.85 0.37 1.85 0.88 p 5 0.01 RT Acceleration 19 [%] 98.81 4.95 98.73 5.57 100.00 5.62 98.53 4.82 p 5 0.05 RTP Acceleration 16 [1 4] 2.56 1.03 2.13 0.81 3.63 0.89 1.69 0.79 p 5 0.01 TSP Non-standardized 20 [1 9] 3.20 1.32 3.85 2.05 6.75 2.75 2.25 1.62 p 5 0.01 compared to FG. Findings indicated that less protruding stud designs nevertheless seem to provide sufficient functional traction to players on artificial turf. Biomechanically, these differences in performance and perception variables between shoe conditions can be explained by the magnitude of the ground reaction force variables (Figure 6, Table 2). During cutting, force ratios for HG, FG, and ID were significantly higher compared to SG. This was due to higher a-p and m-l shear forces, leading to higher resultant shear forces, while PVF remained unchanged. Previous studies have shown that SG, due to its protruding six-studded outsole configuration, exhibited higher mechanical traction properties on the given surface, compared to HG, FG, and ID (Sterzing et al. 2008). By combination of the mechanical and biomechanical information, it is concluded that in the SG shoe condition subjects did not utilize the full amount of mechanical traction available. These considerations follow the concept presented by Fong et al. (2009) who distinguishes between availability and utilization of traction properties. Movement patterns of subjects thus were interpreted as more cautious. This is assumed to have led to slower RT in the running performance measurements respectively. During acceleration ID revealed lower PSF a-p compared to HG in post-hoc comparison. However, this did not affect RT, RTP, or TSP. Most likely this finding is due to the relatively soft forefoot DuoCell elements of ID compared to HG. For turning significant differences between PVF and all PSF displayed a general influence of the shoe conditions on the movement strategy. In general, FG and SG exhibited lower values than HG and ID. This supports the cutting observations that less protruding outsole configurations (HG, ID) may allow players to exhibit higher PSF than more protruding outsole configurations (FG, HG). However, no differences were found for PSF res /PVF which indicates that potential traction effects among these shoe conditions might have been overshadowed by the assumed general alteration of the movement. The most important finding that SG was unsuited for performance on artificial turf confirmed players preference shown in their everyday use. Based on the findings of research phase I, ID was carried over to phase II. 3.2. Phase II: modified prototype testing 3.2.1. Shoe conditions In phase II three new prototypes compared to the carried over ID were evaluated. As the ID shoe

Footwear Science 43 Figure 6. Force ratios of acceleration, cut, and turn Phase I (means and SD). Table 2. Ground reaction force variables Phase I (means and SD). Variable Protocol Subjects [n] Unit HG FG SG ID p value ANOVA PVF Acceleration 18 [N] 1637 219 1628 206 1632 185 1618 211 p ¼ 0.91 PSF a-p Acceleration 18 [N] 531 103 509 123 507 112 491 110 p 5 0.05 PSF a-p /PVF Acceleration 18 [ ] 0.33 0.09 0.32 0.09 0.32 0.08 0.31 0.09 p ¼ 0.21 PVF Cut 18 [N] 1836 533 1772 522 1739 464 1773 648 p ¼ 0.55 PSF a-p Cut 18 [N] 696 400 652 382 546 300 736 403 p 5 0.05 PSF m-l Cut 18 [N] 762 342 719 313 619 246 748 383 p 5 0.01 PSF res Cut 18 [N] 1067 447 1002 423 850 327 1071 511 p 5 0.01 PSF res /PVF Cut 18 [ ] 0.57 013 0.56 0.16 0.48 0.12 0.60 0.15 p 5 0.01 PVF Turn 18 [N] 1769 360 1633 227 1618 287 1740 323 p 5 0.01 PSF a-p Turn 18 [N] 1291 362 1189 267 1186 289 1286 311 p 5 0.05 PSF m-l Turn 18 [N] 292 184 254 142 237 132 297 169 p 5 0.01 PSF res Turn 18 [N] 1339 345 1226 253 1221 253 1334 293 p 5 0.05 PSF res /PVF Turn 18 [ ] 0.76 0.11 0.75 0.11 0.76 0.11 0.77 0.10 p ¼ 0.72 condition worked well in phase I, three new prototypes were systematically oriented towards this outsole design. One aspect of phase II was to investigate the effect of placing the DuoCell elements not only at the forefoot but also at the rearfoot. Furthermore, the effect of slightly different DuoCell hardness was examined. Shoe conditions were ID (252 g), one modified shoe model also with DuoCell Technology at forefoot and firm ground studs at rearfoot (DC FG, 230 g), and two models with DuoCell Technology at forefoot and rearfoot. The latter two differed only in TPU stud hardness, 85 and 90 Asker C (DC 85, 237 g; DC 90, 240 g). Shoe uppers were identical for all shoe conditions in this research phase. All shoes of phase II are displayed in Figure 7. 3.2.2. Results and discussion Performance and perception variables showed hardly any statistically significant differences among the shoes (Figure 8, Table 3). Only the slalom course revealed statistical differences by the ANOVA for RT. However, no respective post-hoc comparison was significant at Bonferroni level. In addition, subjects did not reveal any RTP differences for both running courses. Also, no TSP differences between the shoe conditions were observed. It was concluded that slightly different DuoCell hardness of DC 85 and DC 90 did not meaningful affect running performance. Furthermore, the DuoCell Technology at the rearfoot was not inferior compared to the traditional firm ground rearfoot design (DC FG). ID, although not statistically secured, was placed last among the four shoe conditions. This was regarded as an effect due to positive modifications in the overall construction of the three new prototypes in this research phase. Also biomechanically, no major differences were observed between the four shoe conditions for the three movement tasks (Figure 9, Table 4). None of the acceleration and cutting variables displayed any statistically significant differences

44 T. Sterzing et al. Figure 7. Shoe conditions Phase II. Figure 8. Slalom RT, RTP, and TSP Phase II (means and SD). Table 3. Performance and perception variables Phase II (means and SD). Variable Protocol Subjects [n] Unit ID DC FG DC 85 DC 90 p value ANOVA RT Slalom 19 [%] 100.00 6.34 99.90 6.45 99.62 6.12 98.88 4.80 p 5 0.05 RTP Slalom 19 [1 4] 2.68 1.20 2.47 1.07 2.84 1.07 2.00 1.05 p ¼ 0.21 RT Acceleration 18 [%] 99.35 4.05 100.00 4.05 99.46 4.65 99.09 3.62 p ¼ 0.37 RTP Acceleration 18 [1 4] 2.39 1.29 2.67 1.14 2.72 1.13 2.22 0.94 p ¼ 0.63 TSP Non-standardized 19 [1 9] 4.11 1.85 3.21 1.44 3.21 1.13 2.89 1.24 p ¼ 0.10 Figure 9. Force ratios of acceleration, cut, and turn Phase II (means and SD).

Footwear Science 45 Table 4. Ground reaction force variables Phase II (means and SD). Variable Protocol Subjects [n] Unit ID DC FG DC 85 DC 90 p value ANOVA PVF Acceleration 20 [N] 1563 237 1598 230 1626 234 1571 197 p ¼ 0.12 PSF a-p Acceleration 20 [N] 515 115 508 134 525 143 517 126 p ¼ 0.43 PSF a-p /PVF Acceleration 20 [ ] 0.34 0.10 0.33 0.11 0.33 0.11 0.34 0.10 p ¼ 0.67 PVF Cut 20 [N] 1735 372 1830 418 1827 446 1833 508 p ¼ 0.07 PSF a-p Cut 20 [N] 706 239 730 264 729 245 725 270 p ¼ 0.60 PSF m-l Cut 20 [N] 694 196 743 218 749 221 729 223 p ¼ 0.12 PSF res Cut 20 [N] 1001 269 1054 300 1055 294 1039 314 p ¼ 0.17 PSF res /PVF Cut 20 [ ] 0.57 0.08 0.57 0.08 0.58 0.08 0.57 0.08 p ¼ 0.77 PVF Turn 18 [N] 1583 248 1654 260 1677 309 1710 313 p 5 0.01 PSF a-p Turn 18 [N] 1140 234 1242 299 1291 320 1299 342 p 5 0.01 PSF m-l Turn 18 [N] 211 104 222 147 237 133 232 141 p ¼ 0.27 PSF res Turn 18 [N] 1162 243 1267 311 1316 331 1324 353 p 5 0.01 PSF res /PVF Turn 18 [ ] 0.74 0.11 0.76 0.12 0.78 0.12 0.77 0.13 p 5 0.01 between the shoe conditions. For turning, ID exhibited the lowest PSF res /PVF, indicating a relatively weaker resultant shear force component. However, significantly decreased PVF pointed towards a modified turning movement strategy in general. Due to the lack of solid and coherent statistical differences between shoe conditions the decision which prototype was the carry over to phase III was based on descriptive statistics. These favoured DC 90 according to performance and perception testing. 3.3. Phase III: market comparison 3.3.1. Shoe conditions In phase III traction characteristics of the final prototype (DC 90f, 235 g) were compared to three shoe models recommended for use on artificial turf (AP, 316 g; NT, 324 g; PK, 306 g). Those were from three different brands and available on the market at time of testing. Based on phase II findings a final prototype (DC 90f) was constructed with minor overall improvements with respect to manufacturing compared to its predecessor. Naturally, the fit of all these shoes may have been considerably different. Thus, fit preference of subjects was additionally examined by a fit rating on a nine-point perception scale (1, very good to 9, very bad). Shoe conditions of phase III are displayed in Figure 10. 3.3.2. Results and discussion Neither fit of the four shoe models was preferred over any other shoe model by the subjects (p ¼ 0.35, Table 5). Thus, differences in the investigated variables were related to outsole configuration. The DC 90f stud configuration enabled players to run faster compared to AP and NT in the slalom but not in the acceleration course. RTP of DC 90f was found to be superior for both running courses. Also, DC 90f was placed best with respect to TSP (Figure 11, Table 5). Biomechanically, the following results were observed (Figure 12, Table 6). No differences in biomechanical variables were found for acceleration. However, significantly higher PSF res /PVF were observed for DC 90f compared to AP and NT during cutting and compared to AP, NT, and PK during turning. These higher force ratios are due to statistically higher PSF res and statistically similar PVF. Therefore, it is assumed that these findings point directly towards increased functional traction performance. These values are also in line with better RT, RTP, and TSP values for DC 90f during performance and perception testing. It is concluded that traction of DC 90f outperformed the three other shoe models with respect to performance, perception, and biomechanical testing. Thus, its design features are recommended to serve as a guideline for the development of traction outsoles specifically for performing on artificial soccer turf. 4. Results and discussion The following general discussion provides a summary and an overview of this research including all three phases. The performance test with its two running courses was suited to detect small but significant RT differences between soccer shoes as already shown in previous studies (Sterzing et al. 2009). In the present

46 T. Sterzing et al. Figure 10. Shoe conditions Phase III. Table 5. Performance and perception variables Phase III (means and SD). Variable Protocol Subjects [n] Unit AP NT PK DC 90f p value ANOVA RT Slalom 16 [%] 100.00 6.16 99.93 5.29 98.99 6.23 97.92 5.76 p 5 0.01 RTP Slalom 16 [1 4] 3.06 1.12 2.88 1.03 2.44 0.97 1.63 0.89 p 5 0.01 RT Acceleration 16 [%] 100.00 3.99 99.68 4.66 99.95 3.15 99.72 4.39 p ¼ 0.91 RTP Acceleration 16 [1 4] 2.81 1.11 3.19 0.83 2.50 1.10 1.50 0.73 p 5 0.01 TSP Non-standardized 16 [1 9] 4.25 1.61 4.69 1.45 3.69 1.40 2.69 1.49 p 5 0.01 FIT Non-standardized 16 [1 9] 3.88 1.45 4.06 1.29 3.81 1.97 3.19 1.68 p ¼ 0.35 Figure 11. Slalom RT, RTP, and TSP Phase III (means and SD). research strong RT differences were found for phase I and phase III shoes, whereas only weak differences were found for phase II shoes. In case shoes conditions are very similar RT differences either do not exist or are overshadowed by movement variability of subjects, which makes it impossible to provide scientific differences. Generally, the slalom showed better potential to distinguish between shoes than the acceleration. This is due to being longer and due to the multiple changes of direction that are incorporated. RTP of subjects was found to be generally well matched to RT. Thus running performance and its perception served as a good first indicator for the functionality of the different shoe models in this research project. Also, TSP was able to distinguish between shoe conditions in phase I and III. It appears that direct variables would

Footwear Science 47 Figure 12. Force ratios of acceleration, cut, and turn Phase III (means and SD). Table 6. Ground reaction force variables Phase III (means and SD). Variable Protocol Subjects [n] Unit AP NT PK DC 90f p value ANOVA PVF Acceleration 20 [N] 1416 135 1434 160 1420 144 1420 143 p ¼ 0.62 PSF a-p Acceleration 20 [N] 544 77 533 64 536 61 544 58 p ¼ 0.32 PSF a-p /PVF Acceleration 20 [ ] 0.39 0.06 0.38 0.06 0.38 0.06 0.39 0.05 p ¼ 0.27 PVF Cut 20 [N] 1652 413 1648 414 1600 366 1649 397 p 5 0.47 PSF a-p Cut 20 [N] 547 181 544 193 571 197 572 188 p 5 0.23 PSF m-l Cut 20 [N] 736 210 727 212 725 220 784 230 p 5 0.05 PSF res Cut 20 [N] 925 246 918 251 931 267 979 269 p 5 0.05 PSF res /PVF Cut 20 [ ] 0.56 0.08 0.56 0.07 0.58 0.80 0.59 0.09 p 5 0.01 PVF Turn 20 [N] 1475 310 1514 335 1536 255 1495 275 p ¼ 0.49 PSF a-p Turn 20 [N] 1000 227 1001 242 1071 190 1109 234 p 5 0.01 PSF m-l Turn 20 [N] 172 86 185 103 217 92 166 83 p 5 0.01 PSF res Turn 20 [N] 1019 225 1023 243 1097 187 1126 230 p 5 0.01 PSF res /PVF Turn 20 [ ] 0.69 0.06 0.68 0.08 0.72 0.06 0.75 0.06 p 5 0.01 have allowed picking the favourite shoe condition of each phase already. Ground reaction forces then helped to understand the related biomechanical mechanisms during foot ground contact in different soccer shoe conditions. Subjects were asked to go for maximum performance also during biomechanical testing. Therefore, a more dynamic foot strike, shown by higher shear forces and force ratios, was interpreted as indicator for better performance (Valiant 1988). The underlying assumption was that higher force variables indicate more dynamic braking and/or propulsion during the different movements. Biomechanically, it was noteworthy that ground reaction forces were less suited to distinguish between shoe conditions during acceleration and better suited during cutting and turning. Ground reaction forces of the acceleration movement were in fact shown to be a poor indicator between shoe conditions. This also explains the weaker distinguishing potential of the acceleration running course in contrast to the slalom course. The discussed findings allow to judge functional traction properties of soccer shoes to be most important during cutting and turning instead. All three testing procedures showed interrelated results. It was possible to interpret the different findings comprehensively in line to each other. The outcome of performance and perception testing was supported by the characteristics of the biomechanical ground reaction forces. 5. Conclusions Based on scientific methods, this research project helped to develop a specific artificial soccer turf outsole configuration. Its functionality was shown to be superior compared to three already available soccer shoes designed for use on artificial turf. General conceptual aspects for outsole configurations on third-generation artificial soccer turf were derived.

48 T. Sterzing et al. It appears that artificial soccer turf does not favour outsole configurations consisting of rather few and relatively long studs. These findings reflect players intuitive choice of outsole configuration, avoiding soft ground designs. It also questions the use of firm ground designs which actually is observed in training and match situations by about 50% of the players (Kunz 2009). Shoes that were shown to provide better functional traction to players featured multiple, relatively low stud elements. With respect to the general strategy for the development of innovative (soccer) footwear the used systematic research approach was shown to be successful. Thus the application of multiple testing procedures is strongly recommended in order to take footwear on a higher level and to better understand the underlying motor performance and biomechanical mechanisms of athletes. 6. Perspective This research project only measured running times, perception of running time, and perceived traction suitability as well as ground reaction force variables. Thus, shoe differences were elaborated with respect to performance only. No statement may be derived for the corresponding ankle and knee loading characteristics and related lower extremity kinematics that act on the human body due to the different outsole configurations. This was part of a follow-up research that examined lower extremity kinetics and kinematics during a soccer turning movement (Mu ller et al. 2009a). This research elaborated on the findings of Park et al. (2005) who investigated the influence of soccer shoe traction properties on lower extremity kinetics and kinematics. All these information should lead to a better understanding of the mechanisms of player-surface interaction and potential adaptation mechanisms in general. 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