- Email: [email protected]
Validity and reliability of two standard test devices in assessing mechanical properties of different sport surfaces
Polymer Testing 62 (2017) 61e67
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Validity and reliability of two standard test devices in assessing mechanical properties of different sport surfaces nchez-Sa nchez b, Jorge García-Unanue b, Enrique Colino a, *, Javier Sa lien Le Blan c, Leonor Gallardo a Esther Ubago-Guisado a, Pascal Haxaire c, Aure a b c
IGOID Research Group, University of Castilla-La Mancha, Avda. Carlos III, s/n, 45071, Toledo, Spain n, Madrid, Spain School of Sport Sciences, European University of Madrid, Calle Tajo, s/n, 28670, Villaviciosa de Odo Labosport International, Technoparc Du Circuit des 24 Heures, Chemin Aux Bœufs, 72100, Le Mans, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Accepted 14 June 2017 Available online 17 June 2017
The Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA) devices are the two key test methods for the assessment of shock absorption (SA) and vertical deformation (VD) of sports surfaces. The aim of this study was to investigate the relationship between them. Laboratory tests were carried out in accordance with international regulations and standards on 50 athletics tracks and 44 artificial turf systems using both test methods. No significant differences between methods were observed. Measurements with the AA and the AAA were compared using intraclass correlation coefficients (ICCs), showing acceptable to excellent inter-method reliability (ICC ranged from 0.47 to 0.91). The BlandAltman test revealed an overall SA overestimation and VD underestimation with the AAA. Linear regression analysis was performed, obtaining excellent agreement between test methods for the assessment of SA (R2 ¼ 0.994) and VD (R2 ¼ 0.985). It is concluded that, by applying Equations (1) and (2) in this study, the AA and the AAA could be used interchangeably when assessing SA and VD on athletics tracks and artificial turf surfaces. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Sports surfaces Shock absorption Vertical deformation Mechanical test methods Artificial athlete Advanced artificial athlete
1. Introduction A sports surface should provide appropriate properties such that user to surface interaction can be conducted safely, while the user can perform sporting skills to an appropriate level [1]. Therefore, surface properties, understood as those parameters describing the dynamic behaviour and performance of the ‘as constructed’ whole surface system, condition sports practice since they affect athletesurface interaction. Athletes and sports players adjust their leg stiffness when running, hopping or landing on surfaces of differing mechanical properties [2e6], resulting in subtle changes in different aspects such as lower limb kinematic patterns, ground reaction force dynamics and peak impact acceleration [7e12]. In addition, surface properties have been reported to cause
* Corresponding author. E-mail addresses: [email protected] (E. Colino), javier.sanchez2@ universidadeuropea.es (J. S anchez-S anchez), [email protected] (J. García-Unanue), [email protected] (E. Ubago-Guisado), pascal.haxaire@ labosport.com (P. Haxaire), [email protected] (A. Le Blan), leonor. [email protected] (L. Gallardo). http://dx.doi.org/10.1016/j.polymertesting.2017.06.011 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
statistically significant differences in athletes' physical and physiological responses [13e16]. Therefore, properties of sports surfaces represent a determinant factor of sports practice, as they are significant in relation to the performance and safety of the athlete. Mechanical devices are generally used when assessing the surface properties involved in the athlete-surface interaction. These devices enable properties to be measured directly on the manufactured surface by performing tests that in some way reproduce athletes' contact with the ground. They can, in general, be categorized into two different groups: tests assessing surface properties conditioning impact actions and tests assessing surface properties conditioning friction/traction actions [1]. Although some studies have suggested that these devices are excessively simplistic and lack representativeness when mimicking human movement [11,17e19], they are still recognized by most of the international standards and sports federations as being appropriate for the assessment and regulation of sports surfaces [20e23]. They represent a practical and reproducible tool for predicting surface behaviour during athletic movements, allowing surfaces to be compared and providing evidence that they meet specification standards. The present paper aims to gain insight into the
62
E. Colino et al. / Polymer Testing 62 (2017) 61e67
mechanical devices that are available for assessing the surface properties involved in impact actions. Impacts, interpreted as the actions when an athlete collides with the ground during walking, running or jumping, are a widely studied topic in sports research since they are the most likely reason for athletes' physical injuries [24]. The repetitive application of relatively small loads over many repetitive cycles is the main cause of chronic injuries in sports [25]. Additionally, excessive repetitive impact forces during sports practice are also known to be major contributors to the appearance of overload or overuse injuries [9,26e28]. Since surface properties themselves have a significant role in peak impact accelerations experienced by athletes [29], they are also relevant in terms of injury risk [11,30,31]. Increasing the impact-absorbing properties of the surface has been demonstrated to lower impact forces during standardized athletic movements [18], although it is also associated with an increased fatigue rate compared with surfaces with less damping capacity [14]. Conversely, decreases in the ability of floor surfaces to reduce impact forces (increased stiffness) have been reported to increase peak ground reaction forces [32,33]. Moreover, increased surface stiffness during different running tasks has been related to the risk of bone injuries such as knee osteoarthritis and stress fractures [34,35]. Thus, it is assumed that, to some extent, softer surfaces with more resilient materials and better shock attenuation will reduce injury risk during sports practice [36]. When testing impact characteristics during athlete-surface interactions, there are two main variables traditionally considered: shock absorption (SA) and vertical deformation (VD). SA, also known as force reduction (FR), is the most significant parameter in sports surfaces and has been used as an injury prevention criterion [24], since it reflects the capability of the surface to reduce impact forces when athletes or objects make contact with the ground [37]. SA on artificial polymeric surfaces such as athletics tracks or artificial turf systems depends on different factors, such as the structure of the manufactured surface (its thickness mainly), the inherent mechanical properties of its constituent materials and even the substrate over which they are placed [38]. It is assumed that the ability to absorb impact forces is influenced by the maximum possible displacement of the surface [39], since greater surface displacement will typically extend the time of contact, allowing force to be distributed over a larger time frame [18]. Therefore VD, which refers to the ability of the surface to deform under load, is the second major component of the foot-surface interaction. If surface deformation is too low, the deceleration forces experienced by athletes when making contact with the ground will be high, and injuries might result due to impact forces [7,31,34]. On the other hand, if deformation under foot load is too high, the risk of injury will increase due to instability of the foot and a waste of kinetic energy will occur that affects athletes' performance [15,40,41]. Therefore, the ability to accurately determine surface behaviour during athletes' landings in sports actions is important. In order to ensure safe and controlled conditions for sports practice, the International Association of Athletics Federade ration Internationale de Football Associtions (IAAF) and the Fe ation (FIFA) include tests to assess SA and VD in their certification procedures [20,21], along with other international sports federations such as Rugby World and the International Hockey Federation. Thus, every certified facility must meet predetermined criteria regarding these properties. There are two portable mechanical devices currently in use for the assessment of SA and VD in sports surfaces, the Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA), both consisting of drop tests that attempt to reproduce the impact of an athlete's heel on the surface while running [42]. The AA apparatus has traditionally been considered the ‘gold standard’ test and is the only
method included in European standards. It is given two different names depending on whether it is configured to assess SA (AA Berlin) or VD (AA Stuttgart). Since its appearance in the mid 1970s, many international federations have arranged tests performed with this apparatus for the assessment and certification of sports surfaces. However, in 2004 an alternative method for the evaluation of these parameters was developed [1]. This apparatus, simpler than the AA, was called the Advanced Artificial Athlete (AAA). Following its definitive design in 2012, the AAA was adopted by FIFA and World Rugby (formerly the International Rugby Board (IRB)) as the only test method approved for assessing SA and VD on artificial turf pitches used for competitions dependent on these federations [23,43]. However, the AA continues to be the only test described in European standards and is recognized by most other international sports federations, with cases even existing where both apparatus coexist [22]. To the best of the authors' knowledge, no comparison between these two key test methods has been performed before. The only reference found in the scientific literature corresponds to a modest correlation study mentioned by Kolitzus [44] where both apparatus were compared in a sample of two artificial turf pitches. The results of that work revealed that the value of SA measured with the AAA (SAAAA) is slightly lower than that measured with the AA Berlin (SAAA), whereas the VD obtained with the AAA (VDAAA) is larger than the one obtained with the AA Stuttgart (VDAA). However, the small size of the sample and the lack of concordance analysis make it impossible to make consistent inferences from that study. Therefore, the objective of the present study was to determine the relationship between the AA and the AAA apparatus when assessing SA and VD on sports flooring. Athletics tracks and artificial turf pitches have been included in this study since they are the most representative surfaces to be measured with each test method. This study should provide relevant information for the evaluation of impacts during athlete-surface interactions. 2. Methods 2.1. Sample A total of 50 athletics tracks and 44 artificial turf systems were evaluated using both the AA and the AAA apparatus. SA and VD were measured according to the standards and protocols currently set by the respective international federation [20,21], with the exception that two devices instead of only the one stipulated were used (AA and AAA). 2.2. Equipment 2.2.1. Artificial Athlete Tests in this study were carried out with an AA produced by Labosport. For the assessment of SA, this device consists of a 20 kg dropping mass that falls from a standard height of 55 mm, before hitting a spring with 2000 N/mm stiffness that is positioned over a test foot with a spherical base placed on the ground. The test foot is a cylinder with a diameter of 70 mm that is fitted with a force transducer and is capable of recording impact forces applied during the test process by using a load cell with a 10 kN capacity and 0.1% accuracy. The peak force recorded is compared with a reference value from a rigid concrete floor, and thus the SA of the synthetic surface is obtained [45]. For the assessment of SA in athletics track surfaces, the IAAF states that the low-pass filter must have a 9th order Butterworth characteristic [21]. For the assessment of VD, the AA must be slightly modified, although the main structure of the apparatus remains the same. The test is carried out in a similar way to that explained before,
E. Colino et al. / Polymer Testing 62 (2017) 61e67
although in this case the 20 kg weight is dropped from a height of 120 mm instead of 55 mm, the spring has a 40 N/mm stiffness instead of 2000 N/mm, the spherical base test foot is swapped for a flat one, and two movement transducers are incorporated. Deformation of the surface is recorded through the two movement transducers, which are mounted on either side of the foot and held by a simple structure placed independently from the main structure. The total distance between both sensors should not be greater than 125 mm, keeping the weight drop axis in the middle. Overall, VD simulates the deformation of the surface under a dynamic load of 1500 N [46]. 2.2.2. Advanced Artificial Athlete Tests with the AAA were carried out with a Labosport manufactured device. The AAA, which is a modification and amalgamation of the two test methods described before, enables the assessment of SA and VD at the same time and with only one signal. The principle of the apparatus consists of a drop test similar to that of the AA, although in this case the spring and the test foot through which the impact load is applied to the ground are attached to the bottom of the falling mass instead of resting over the surface. The spring rate is 2000 N/mm, the drop mass is 20 kg in weight and it is also dropped from a height of 55 mm. A test foot with the same diameter and similar characteristics to those of the AA is attached to the lower side of the spring. Instead of the force transducer, the AAA incorporates a piezoresistive accelerometer with a frequency range of up to 1000 Hz and a 2% linearity over the operating range. The G sensor is firmly attached to the falling mass on its vertical line of gravity and as much as possible on the lower side of it. During the impact, the signal of the accelerometer is conditioned and recorded, and a curve representing falling mass acceleration versus time is obtained. The AAA is thoroughly described in FIFA Test Method 04a [20], including test apparatus, test procedure and specifications for laboratory and field tests.
63
displacement of the falling weight during the course of impact are calculated by integration and double integration of the acceleration signal, respectively. For the calculation of SAAAA, peak force during the impact test (Fmax) must be obtained first:
Fmax ¼ m g Gmax þ m g where: Fmax is the peak force during the test, expressed in N. Gmax is the peak acceleration during the impact, expressed in g (1 g ¼ 9.81 m/s2) m is the falling mass including the spring, test foot and accelerometer, expressed in kg. g is the acceleration by gravity (9.81 m/s2) Then, SAAAA is calculated with the same formula as with the AA Berlin, with the exception that the reference force, Fr, is not previously obtained but fixed at 6760 N, which is the theoretical value for a concrete floor:
SAAAA ¼ ½ð1 ðFmax =FrÞ 100 For the calculation of VDAAA, a time interval is considered from the time when the test foot makes initial contact with the surface (T1) until the time when the maximum absolute velocity of the mass is obtained after the impact on the test specimen (T2). Then, the displacement of the falling mass (Dmax) and displacement of the spring (Dspring) during that interval are calculated through relatively complex formulas [20]. Finally, the vertical deformation of the test specimen is defined as:
VDAAA ¼ Dmax Dspring
2.3. Data collection 2.4. Protocol 2.3.1. SA with the Artificial Athlete (SAAA) SAAA is calculated according to the equation below from the maximum impact force recorded during the test (Ft) normalized with respect to a reference one (Fr), which is obtained by previously performing the same test (with more repetitions) on a rigid concrete surface. Both the Ft and the Fr are expressed in newtons (N). The SAAA value obtained from this test is considered representative of the shock absorption capability of a determined surface, and is expressed as a percentage:
SAAA ¼ ½ð1 ðFt=FrÞ 100
2.3.2. VD with the Artificial Athlete (VDAA) Vertical displacement of the surface during the impact is recorded through the movement transducers. Then, VDAA is calculated from the maximum deformation of the surface on the weight drop axis (fmax) and the maximum impact force recorded during the test (Fmax) according to the following expression:
VDAA ¼ ð1500N=Fmax Þ fmax
2.3.3. SA and VD with the Advanced Artificial Athlete (SAAAA, VDAAA) When measuring surface properties with the AAA, the acceleration-time curve during the impact of the falling mass is obtained from the accelerometry signal. Then, the speed and the
2.4.1. Athletics track surfaces Control samples using the same materials and techniques as those used during the construction of the tracks were used. The SA and VD were assessed using apparatus described in the EN 14808 [45] and EN 14809 [46] standards, respectively, through suitable laboratory tests according to the procedures specified by the IAAF in its IAAF Track and Runway Synthetic Surface Testing Specifications [21]. The absolute thickness of the samples was the same as that listed on the IAAF Product Certificate for each synthetic material [47]. The size of the samples was not less than 600 mm 600 mm. All tests were undertaken on samples at a temperature of 23 C.
2.4.2. Artificial turf systems Laboratory tests were carried out on test specimens constructed and conditioned following the FIFA requirements with a size of at least 1.0 m 1.0 m. The SA and VD were evaluated in all test specimens according to the procedures specified in the FIFA Handbook of Test Methods for Football Turf [20]. All the tests were performed over a concrete floor with a minimum thickness of 10 cm and a minimum hardness of 40 MPa, verified according to EN 12504e2 [48]. For each artificial turf system sample, the test was repeated three times on the same spot of the test specimen, with no brushing or adjusting of the surface between impacts. The test was repeated in three different positions, each at least 100 mm apart and at least 100 mm from the sides of the test specimen. The temperature during the tests was 23 ± 2 C.
64
E. Colino et al. / Polymer Testing 62 (2017) 61e67
2.5. Statistical analysis For data analysis, sports surfaces were evaluated separately, with the sample being divided into two different groups: athletics tracks (N ¼ 50) and artificial turf systems (N ¼ 44). Results are presented for each independent surface. Descriptive data are presented as mean and standard deviation (sd). Normality was tested through the KolmogoroveSmirnov test and graphic methods (normal probability plot). SA and VD values obtained with the AA and AAA test methods were compared using an independent sample t-test and the Bland-Altman method. Bland-Altman agreement was assessed by calculating the mean bias between the test methods and the means between methods, and these values were plotted on a scatterplot. The lower limit of agreement (LLA) and upper limit of agreement (ULA) were calculated as ± 1.96 times the standard deviation of the mean bias (95%LOA). Absolute agreement intraclass correlation coefficients (ICCs) were calculated to determine the inter-method reliability between the AA and the AAA test methods. ICC values represented low (ICC < 0.4), moderate (ICC between 0.4 and 0.75) and excellent (ICC > 0.75) reliability according to Shrout and Fleiss [49]. Finally, a linear regression analysis was conducted to assess the relationship between measurements obtained with both apparatus. The statistical analysis was carried out using IBM SPSS Statistics software (v. 21.0 SPSS Inc, Chicago, IL, USA), and Bland-Altman plots were also obtained using the same software. The significance level was set at p < 0.05.
3. Results Table 1 shows descriptive results of the study. The t-test for independent samples revealed no statistical differences between the AA and AAA test methods for any sports surface, either for SA or for VD variables (p > 0.05). Table 2 represents inter-method agreement and reliability. Results of the Bland-Altman test show modest deviations between the AA and the AAA for both the SA and the VD assessment, and ICC values show moderate reliability (0.4e0.75) between the two test methods for both the SA and VD variables and for both surfaces. Plots obtained through the Bland-Altman analysis are presented in Fig. 1. The results indicate a slight underestimation of SA values when obtained with the AAA. This underestimation is more relevant for the hard surfaces (athletics tracks) than for the soft surfaces such as artificial turf systems (5.82± 0.92% vs. 1.21± 1.18%). Conversely, the VD is slightly overestimated when assessed with the AAA apparatus, with that overestimation being higher when the hardness of the surface decreases. The results of the linear regression analysis show a close relationship between overall measures obtained with the AA and the AAA. Regression equations for each mechanical property have been obtained considering both surfaces together, so hard surfaces with low values of SA and VD (athletics tracks) and soft surfaces with
Type of surface
Variables
Test method
Mean
sd
p*
Artificial turf (N ¼ 44)
SA (%)
AAA AA Berlin AAA AA Stuttgart AAA AA Berlin AAA AA Stuttgart
59.74 60.95 8.30 7.09 29.36 35.18 2.37 1.62
3.28 2.62 0.83 0.96 4.44 4.47 0.57 0.50
0.16
Athletic track (N ¼ 50)
SA (%) VD (mm)
*Level of significance: p < 0.05.
SAAA ¼ 0:852 SAAAA þ 10:117
(1)
With regard to VD, the following equation predicts 98.5% of the variance in the SAAA parameter from data obtained with the AAA:
VDAA ¼ 0:925 VDAAA 0:583
(2)
4. Discussion The aim of the present study was to discover the extent to which the AA and AAA test methods provide similar results when assessing SA and VD variables on sports surfaces. Although no statistical differences were observed between the methods, the analysis of the level of agreement revealed relevant deviations that could be corrected by using Equations (1) and (2). Thus, the main finding of the present study is that, although AA and AAA apparatus should not be used interchangeably for the assessment of SA and VD, values obtained with the AA can be predicted with a high level of accuracy when using the AAA (99.4% and 98.5%, respectively). Surface behaviour during impact actions is of special relevance due to its association with athletes' performance and the risk of injury, with SA and VD being the main surface properties conditioning these actions. Currently, the AA and the AAA are the key test methods in use for assessing these properties in sports surfaces, although a lack of consensus exists about which method should be used. The heterogeneity of apparatus and regulations concerning test methods makes the analysis of sports surface properties confusing and could lead to conflicting information. At the present time, no scientific evidence exists on whether the AA and the AAA provide equivalent results. This paper aimed to contribute to the clarification of sports surface testing by comparing both apparatus and stating the relationship between them. Athletics tracks and artificial turf systems were selected in this study since they are the most representative sports surfaces to be evaluated with each test method. In athletics tracks, a modest SA underestimation of 5.82± 0.92% was described for the AAA when compared with the AA, as well as an overestimation of 0.75 ± 0.23 mm for VD. Given that the IAAF requirements for SA in athletics track surfaces range from 35% to 50%, and the VD requirements go from 0.6 mm to 2.5 mm [50], the discrepancy in AAA measures is greater than onethird of the allowed range for both parameters. Differences of that magnitude could easily lead to a wrong diagnosis when assessing performance parameters of athletics track surfaces using the AAA apparatus. Thus, given an accuracy of 95%, the results for those variables assessed with the AAA device could be simplified as:
SAAAA zSAAA 5:82%
Table 1 Descriptive characteristics of surfaces.
VD (mm)
higher SA and VD (artificial turf systems) are represented under the same regression (Fig. 2). For the assessment of SA, the equation below predicts 99.4% of the variance in the SAAA parameter from data obtained with the AAA.
0.24 0.88 0.66
VDAAA zVDAA þ 0:75mm To the best of the authors' knowledge, no previous comparison between AA and AAA devices has been carried out on athletics tracks, with this study being the first approach towards the standardization and unification of test methods in these surfaces. Assessing the mechanical properties of athletics tracks with the AAA could provide some benefits, such as easier calibration of the apparatus and significant time savings, since SA and VD are measured with one instead of two different tests. Even if SA and VD
E. Colino et al. / Polymer Testing 62 (2017) 61e67
65
Table 2 Agreement and reliability between test methods for the assessment of SA and VD. Surface
Artificial turf (N ¼ 44) Athletic track (N ¼ 50) a b c
Variables
SA (%) VD (mm) SA (%) VD (mm)
Differences between methods
AAA AAA AAA AAA
e e e e
AA AA AA AA
Bland-Altman analysis
Confidence intervals
biasa ± sd
95% LOAb
ICCc
95% CI
1.21 ± 1.18 1.22 ± 0.40 5.82 ± 0.92 0.75 ± 0.23
3.52 to 1.10 0.44 to 2.00 7.62 to 4.02 0.30 to 1.20
0.91 0.60 0.69 0.47
0.44 to 0.97 0.12 to 0.88 0.02 to 0.90 0.20 to 0.98
Bias: average difference between methods. LOA: Limits of agreement. LOA: Media ± (1.96*SD). ICC: Infraclass Correlation Coefficient.
Fig. 1. Bland-Altman plots identifying differences when comparing measurements with the Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA) apparatus. Central line represents the inter-method difference (bias). Upper and lower lines represent the 95% limits of agreement (bias ± 1.96 sd of the differences). A e Shock Absorption in athletics track; B e Vertical Deformation in athletics track; C e Shock Absorption in artificial turf; D e Vertical Deformation in artificial turf.
values obtained with the AA were needed, the present paper would enable them to be calculated with a high degree of accuracy (99.4% and 98.5%, respectively) by using the equations from the linear regression analysis. As regards artificial turf systems, a modest study exists comparing the AA (FIFA test method) and the AAA (European standards test method) in two artificial turf surfaces [44]. Compared with that work, a higher underestimation in the SA test
was found in the present study (1.21% vs. 0.6%), while a slightly lower overestimation was obtained in the VD test (þ1.22 mm vs. þ 1.6 mm). Given that requirements for SA in artificial turf pitches range from 55% to 70% (both FIFA and European standards), and the VD requirements go from 4.0 to 11.0 mm (FIFA) or 4.0e9.0 mm (European standards), these deviations represent smaller proportions than those found for the athletics track surfaces, thereby providing greater representativeness for both test
66
E. Colino et al. / Polymer Testing 62 (2017) 61e67
Fig. 2. Linear regression analysis for Shock Absorption (SA) and Vertical Deformation (VD) measurements obtained with the Artificial Athlete and the Advanced Artificial Athlete apparatus. A e Shock Absorption (R2 ¼ 0.994); B e Vertical Deformation (R2 ¼ 0.985).
methods. Thus, if AA values for SA and VD parameters were required in order to meet European standards, they could also be obtained by assessing the surface with the AAA and then applying Equations (1) and (2). The t-test for independent samples indicates that the AA and the AAA could be used interchangeably on both athletics tracks and artificial turf surfaces and no statistical error would occur. However, the results of the Bland-Altman test show moderate deviations between test methods in both surfaces, with hard surfaces (athletics tracks) having a greater bias for SA and slightly lower bias for VD. Notwithstanding the foregoing, the overall results of the linear regression analysis indicate an excellent replication of AA outcomes when using the AAA apparatus. With this in mind, it seems that both the AA and the AAA could be used interchangeably when assessing SA and VD on both athletics tracks and artificial turf surfaces by applying Equations (1) and (2) as stated in point 3. This study could represent a substantial contribution to guaranteeing that athletics and artificial-turf football events at any level and at any venue take place on surfaces with comfort and safety for athletes and football players, since it promotes and facilitates linkages between test laboratories and facility managers in order to carry out quality control tests on the surfaces and assess their degree of compliance with the requirements established by international bodies. 5. Conclusions No statistical differences were observed between the AA and AAA test methods when assessing SA and VD on athletics tracks and artificial turf pitches. Slight deviations occur between both apparatus that seem to be dependent on the surface hardness, with SA differing more in hard surfaces than in soft surfaces and VD working the opposite way. Equations (1) and (2) allow those deviations to be minimized and results predicted with both apparatus by using either of them with almost perfect reliability. The present paper opens new possibilities for the assessment of surface properties and lays the foundation for the unification of both test methods. This work should enable research institutions and accredited laboratories to use the AA and AAA interchangeably when evaluating SA and VD on athletics tracks and artificial turf pitches.
Declaration of conflicting interests The authors declare that there is no conflict of interest. Funding Enrique Colino has been funded by the Spanish Ministry of Education, Culture and Sport (Grant number: FPU15/04700) within the ‘National Programme for Training University Lecturers’ (FPU Grant) e Ref. 998758. References [1] P. Fleming, C. Young, M. Carre, Mechanical testing and characterisation of sports surfaces, in: S.J. Dixon, P. Fleming, I. James, M. Carre (Eds.), The Science and Engineering of Sport Surfaces, 2015, pp. 26e69. [2] C.T. Farley, H.H. Houdijk, C. Van Strien, M. Louie, Mechanism of leg stiffness adjustment for hopping on surfaces of different stiffnesses, J. Appl. Physiol. 85 (1998) 1044e1055. [3] D.P. Ferris, K. Liang, C.T. Farley, Runners adjust leg stiffness for their first step on a new running surface, J. Biomech. 32 (1999) 787e794. [4] D.P. Ferris, M. Louie, C.T. Farley, Running in the real world: adjusting leg stiffness for different surfaces, Proc. Biol. Sci. 265 (1998) 989e994. [5] J. Hackney, S. Brummel, K. Jungblut, C. Edge, The effect of sprung (suspended) floors on leg stiffness during grand jete landings in ballet, J. Dance Med. Sci. 15 (2011) 128e133. [6] C.T. Moritz, S.M. Greene, C.T. Farley, Neuromuscular changes for hopping on a range of damped surfaces, J. Appl. Physiol. 96 (2004) 1996e2004. [7] E.M. Bigelow, N.G. Elvin, A.A. Elvin, S.P. Arnoczky, Peak impact accelerations during track and treadmill running, J. Appl. Biomech. 29 (2013) 639e644. [8] M.F. Bobbert, M.R. Yeadon, B.M. Nigg, Mechanical analysis of the landing phase in heel-toe running, J. Biomech. 25 (1992) 223e234. [9] S.J. Dixon, A.C. Collop, M.E. Batt, Surface effects on ground reaction forces and lower extremity kinematics in running, Med. Sci. Sports Exerc. 32 (2000) 1919e1926. [10] B.M. Nigg, R.W. De Boer, V. Fisher, A kinematic comparison of overground and treadmill running, Med. Sci. Sports Exerc. 27 (1995) 98e105. [11] B.M. Nigg, M.R. Yeadon, Biomechanical aspects of playing surfaces, J. Sports Sci. 5 (1987) 117e145. [12] V. Wank, U. Frick, D. Schmidtbleicher, Kinematics and electromyography of lower limb muscles in overground and treadmill running, Int. J. Sports Med. 19 (1998) 455e461. [13] R. Di Michele, A.M. Di Renzo, S. Ammazzalorso, F. Merni, Comparison of physiological responses to an incremental running test on treadmill, natural grass, and synthetic turf in young soccer players, J. Strength Cond. Res. 23 (2009) 939e945. nchez-Sa nchez, J. García-Unanue, J.L. Felipe, P. Jime nez-Reyes, D. Viejo[14] J. Sa mez-Lo pez, E. Hernando, P. Burillo, L. Gallardo, Physical and Romero, M. Go physiological responses of amateur football players on third-generation artificial turf systems during simulated game situations, J. Strength Cond. Res. 30
E. Colino et al. / Polymer Testing 62 (2017) 61e67 (2016) 3165e3177. [15] J. Sanchez-Sanchez, J. Garcia-Unanue, P. Jimenez-Reyes, A. Gallardo, P. Burillo, J.L. Felipe, L. Gallardo, Influence of the mechanical properties of thirdgeneration artificial turf systems on soccer players' physiological and physical performance and their perceptions, PLoS One 9 (2014) e111368. [16] A. Sassi, A. Stefanescu, P. Menaspa, A. Bosio, M. Riggio, E. Rampinini, The cost of running on natural grass and artificial turf surfaces, J. Strength Cond. Res. 25 (2011) 606e611. [17] S.J. Dixon, V. Stiles, Impact absorption of tennis shoe-surface combinations, Sports Eng. 6 (2003) 1e9. [18] D. McGhie, G. Ettema, Biomechanical analysis of surface-athlete impacts on third-generation artificial turf, Am. J. Sports. Med. 41 (2013) 177e185. [19] H. Yukawa, R. Aduma, S. Kawamura, K. Kobayashi, Shock attenuation properties of sports surfaces with two dimensional impact test, Procedia Eng. 34 (2012) 855e860. [20] FIFA, FIFA Quality Programme for Artificial Turf - Handbook of Test Methods, 2015. [21] IAAF, in: IAAF (Ed.), IAAF Track and Runway Synthetic Surface Testing Specifications, 2016. [22] FIH, Handbook of Performance, Durability and Construction Requirements for Synthetic Turf Hockey Pitches, 2013. [23] IRB, IRB Artificial Rugby Turf Performance Specification, 2012. [24] B. Farhang, F.R. Araghi, A. Bahmani, F. Moztarzadeh, M. Shafieian, Landing impact analysis of sport surfaces using three-dimensional finite element model, J. Sports Eng. Tech. 230 (2016) 180e185. [25] T.F. Novacheck, The biomechanics of running, Gait Posture 7 (1998) 77e95. asson, L. Peterson, Effects of shoe and surface characteristics on lower [26] G. Andre limb injuries in sports, Int. J. Sport Biomech. 2 (1986) 202e209. [27] G.K. Cole, B.M. Nigg, A.J. van Den Bogert, K.G. Gerritsen, Lower extremity joint loading during impact in running, Clin. Biomech. 11 (1996) 181e193. pez-Fern [28] E. Ubago-Guisado, J. García-Unanue, J. Lo andez, J. S anchez-S anchez, L. Gallardo, Association of different types of playing surfaces with bone mass in growing girls, J. Sports Sci. (2016) 1e9. [29] E.M. Zanetti, C. Bignardi, G. Franceschini, A.L. Audenino, Amateur football pitches: mechanical properties of the natural ground and of different artificial turf infills and their biomechanical implications, J. Sports Sci. 31 (2013) 767e778. [30] K.R. Ford, N.A. Manson, B.J. Evans, G.D. Myer, R.C. Gwin, R.S. Heidt Jr., T.E. Hewett, Comparison of in-shoe foot loading patterns on natural grass and synthetic turf, J. Sci. Med. Sport 9 (2006) 433e440. [31] A. Hreljac, Impact and overuse injuries in runners, Med. Sci. Sports Exerc. 36 (2004) 845e849.
67
[32] J.A. Mercer, J. Vance, A. Hreljac, J. Hamill, Relationship between shock attenuation and stride length during running at different velocities, Eur. J. Appl. Physiol. 87 (2002) 403e408. [33] H.K. Reeve, L.S. Hopper, B.C. Elliott, T.R. Ackland, Lower limb kinematic variability in dancers performing drop landings onto floor surfaces with varied mechanical properties, Hum. Mov. Sci. 32 (2013) 866e874. [34] R.J. Butler, H.P. Crowell 3rd, I.M. Davis, Lower extremity stiffness: implications for performance and injury, Clin. Biomech. 18 (2003) 511e517. [35] S.K. Grimston, J.R. Engsberg, R. Kloiber, D.A. Hanley, Bone mass, external loads, and stress fracture in female runners, Int. J. Sport Biomech. 7 (1991) 293e302. [36] L. Andena, F. Briatico-Vangosa, A. Ciancio, A. Pavan, A finite element model for the prediction of force reduction of athletics tracks, Procedia Eng. 72 (2014) 847e852. [37] R.M. Silva, J.L. Rodrigues, V.V. Pinto, M.J. Ferreira, R. Russo, C.M. Pereira, Evaluation of shock absorption properties of rubber materials regarding footwear applications, Polym. Test. 28 (2009) 642e647. [38] M. Benanti, L. Andena, F. Briatico-Vangosa, A. Pavan, Viscoelastic behavior of athletics track surfaces in relation to their force reduction, Polym. Test. 32 (2013) 52e59. [39] M. Shorten, J. Himmelsbach, Shock attenuation of sports surfaces, Eng. Sport 4 (2002) 152e159. [40] A.E. Kerdok, A.A. Biewener, T.A. McMahon, P.G. Weyand, H.M. Herr, Energetics and mechanics of human running on surfaces of different stiffnesses, J. Appl. Physiol. 92 (2002) 469e478. [41] N.M. Schrier, J.W. Wannop, R.T. Lewinson, J. Worobets, D. Stefanyshyn, Shoe traction and surface compliance affect performance of soccer-related movements, Footwear Sci. 6 (2014) 69e80. [42] L. Andena, F. Briatico-Vangosa, E. Cazzoni, A. Ciancio, S. Mariani, A. Pavan, Modeling of shock absorption in athletics track surfaces, Sports Eng. 18 (2015) 1e10. [43] FIFA, FIFA Quality Concept - Handbook of Test Methods for Football Turf, 2012. [44] H.J. Kolitzus, Scrutiny of advanced artificial athlete, ISSS Technical Conference, Personal communication. Shanghai, 2012. [45] EN 14808:2005 Surface for sports areas - determination of shock absorption. [46] EN 14809:2005 Surface for sports areas - determination of vertical deformation. [47] IAAF, in: IAAF (Ed.), IAAF Certified Track Surfacing Products, 2016. [48] EN 12504e2:2012 Testing concrete in structures - Part 2: non-destructive testing - determination of rebound number. [49] P.E. Shrout, J.L. Fleiss, Intraclass correlations: uses in assessing rater reliability, Psychol. Bull. 86 (1979) 420e428. [50] IAAF, in: IAAF (Ed.), IAAF Track and Field Facilities Manual, Editions EGC, 2008.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Validity and reliability of two standard test devices in assessing mechanical properties of different sport surfaces nchez-Sa nchez b, Jorge García-Unanue b, Enrique Colino a, *, Javier Sa lien Le Blan c, Leonor Gallardo a Esther Ubago-Guisado a, Pascal Haxaire c, Aure a b c
IGOID Research Group, University of Castilla-La Mancha, Avda. Carlos III, s/n, 45071, Toledo, Spain n, Madrid, Spain School of Sport Sciences, European University of Madrid, Calle Tajo, s/n, 28670, Villaviciosa de Odo Labosport International, Technoparc Du Circuit des 24 Heures, Chemin Aux Bœufs, 72100, Le Mans, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Accepted 14 June 2017 Available online 17 June 2017
The Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA) devices are the two key test methods for the assessment of shock absorption (SA) and vertical deformation (VD) of sports surfaces. The aim of this study was to investigate the relationship between them. Laboratory tests were carried out in accordance with international regulations and standards on 50 athletics tracks and 44 artificial turf systems using both test methods. No significant differences between methods were observed. Measurements with the AA and the AAA were compared using intraclass correlation coefficients (ICCs), showing acceptable to excellent inter-method reliability (ICC ranged from 0.47 to 0.91). The BlandAltman test revealed an overall SA overestimation and VD underestimation with the AAA. Linear regression analysis was performed, obtaining excellent agreement between test methods for the assessment of SA (R2 ¼ 0.994) and VD (R2 ¼ 0.985). It is concluded that, by applying Equations (1) and (2) in this study, the AA and the AAA could be used interchangeably when assessing SA and VD on athletics tracks and artificial turf surfaces. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Sports surfaces Shock absorption Vertical deformation Mechanical test methods Artificial athlete Advanced artificial athlete
1. Introduction A sports surface should provide appropriate properties such that user to surface interaction can be conducted safely, while the user can perform sporting skills to an appropriate level [1]. Therefore, surface properties, understood as those parameters describing the dynamic behaviour and performance of the ‘as constructed’ whole surface system, condition sports practice since they affect athletesurface interaction. Athletes and sports players adjust their leg stiffness when running, hopping or landing on surfaces of differing mechanical properties [2e6], resulting in subtle changes in different aspects such as lower limb kinematic patterns, ground reaction force dynamics and peak impact acceleration [7e12]. In addition, surface properties have been reported to cause
* Corresponding author. E-mail addresses: [email protected] (E. Colino), javier.sanchez2@ universidadeuropea.es (J. S anchez-S anchez), [email protected] (J. García-Unanue), [email protected] (E. Ubago-Guisado), pascal.haxaire@ labosport.com (P. Haxaire), [email protected] (A. Le Blan), leonor. [email protected] (L. Gallardo). http://dx.doi.org/10.1016/j.polymertesting.2017.06.011 0142-9418/© 2017 Elsevier Ltd. All rights reserved.
statistically significant differences in athletes' physical and physiological responses [13e16]. Therefore, properties of sports surfaces represent a determinant factor of sports practice, as they are significant in relation to the performance and safety of the athlete. Mechanical devices are generally used when assessing the surface properties involved in the athlete-surface interaction. These devices enable properties to be measured directly on the manufactured surface by performing tests that in some way reproduce athletes' contact with the ground. They can, in general, be categorized into two different groups: tests assessing surface properties conditioning impact actions and tests assessing surface properties conditioning friction/traction actions [1]. Although some studies have suggested that these devices are excessively simplistic and lack representativeness when mimicking human movement [11,17e19], they are still recognized by most of the international standards and sports federations as being appropriate for the assessment and regulation of sports surfaces [20e23]. They represent a practical and reproducible tool for predicting surface behaviour during athletic movements, allowing surfaces to be compared and providing evidence that they meet specification standards. The present paper aims to gain insight into the
62
E. Colino et al. / Polymer Testing 62 (2017) 61e67
mechanical devices that are available for assessing the surface properties involved in impact actions. Impacts, interpreted as the actions when an athlete collides with the ground during walking, running or jumping, are a widely studied topic in sports research since they are the most likely reason for athletes' physical injuries [24]. The repetitive application of relatively small loads over many repetitive cycles is the main cause of chronic injuries in sports [25]. Additionally, excessive repetitive impact forces during sports practice are also known to be major contributors to the appearance of overload or overuse injuries [9,26e28]. Since surface properties themselves have a significant role in peak impact accelerations experienced by athletes [29], they are also relevant in terms of injury risk [11,30,31]. Increasing the impact-absorbing properties of the surface has been demonstrated to lower impact forces during standardized athletic movements [18], although it is also associated with an increased fatigue rate compared with surfaces with less damping capacity [14]. Conversely, decreases in the ability of floor surfaces to reduce impact forces (increased stiffness) have been reported to increase peak ground reaction forces [32,33]. Moreover, increased surface stiffness during different running tasks has been related to the risk of bone injuries such as knee osteoarthritis and stress fractures [34,35]. Thus, it is assumed that, to some extent, softer surfaces with more resilient materials and better shock attenuation will reduce injury risk during sports practice [36]. When testing impact characteristics during athlete-surface interactions, there are two main variables traditionally considered: shock absorption (SA) and vertical deformation (VD). SA, also known as force reduction (FR), is the most significant parameter in sports surfaces and has been used as an injury prevention criterion [24], since it reflects the capability of the surface to reduce impact forces when athletes or objects make contact with the ground [37]. SA on artificial polymeric surfaces such as athletics tracks or artificial turf systems depends on different factors, such as the structure of the manufactured surface (its thickness mainly), the inherent mechanical properties of its constituent materials and even the substrate over which they are placed [38]. It is assumed that the ability to absorb impact forces is influenced by the maximum possible displacement of the surface [39], since greater surface displacement will typically extend the time of contact, allowing force to be distributed over a larger time frame [18]. Therefore VD, which refers to the ability of the surface to deform under load, is the second major component of the foot-surface interaction. If surface deformation is too low, the deceleration forces experienced by athletes when making contact with the ground will be high, and injuries might result due to impact forces [7,31,34]. On the other hand, if deformation under foot load is too high, the risk of injury will increase due to instability of the foot and a waste of kinetic energy will occur that affects athletes' performance [15,40,41]. Therefore, the ability to accurately determine surface behaviour during athletes' landings in sports actions is important. In order to ensure safe and controlled conditions for sports practice, the International Association of Athletics Federade ration Internationale de Football Associtions (IAAF) and the Fe ation (FIFA) include tests to assess SA and VD in their certification procedures [20,21], along with other international sports federations such as Rugby World and the International Hockey Federation. Thus, every certified facility must meet predetermined criteria regarding these properties. There are two portable mechanical devices currently in use for the assessment of SA and VD in sports surfaces, the Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA), both consisting of drop tests that attempt to reproduce the impact of an athlete's heel on the surface while running [42]. The AA apparatus has traditionally been considered the ‘gold standard’ test and is the only
method included in European standards. It is given two different names depending on whether it is configured to assess SA (AA Berlin) or VD (AA Stuttgart). Since its appearance in the mid 1970s, many international federations have arranged tests performed with this apparatus for the assessment and certification of sports surfaces. However, in 2004 an alternative method for the evaluation of these parameters was developed [1]. This apparatus, simpler than the AA, was called the Advanced Artificial Athlete (AAA). Following its definitive design in 2012, the AAA was adopted by FIFA and World Rugby (formerly the International Rugby Board (IRB)) as the only test method approved for assessing SA and VD on artificial turf pitches used for competitions dependent on these federations [23,43]. However, the AA continues to be the only test described in European standards and is recognized by most other international sports federations, with cases even existing where both apparatus coexist [22]. To the best of the authors' knowledge, no comparison between these two key test methods has been performed before. The only reference found in the scientific literature corresponds to a modest correlation study mentioned by Kolitzus [44] where both apparatus were compared in a sample of two artificial turf pitches. The results of that work revealed that the value of SA measured with the AAA (SAAAA) is slightly lower than that measured with the AA Berlin (SAAA), whereas the VD obtained with the AAA (VDAAA) is larger than the one obtained with the AA Stuttgart (VDAA). However, the small size of the sample and the lack of concordance analysis make it impossible to make consistent inferences from that study. Therefore, the objective of the present study was to determine the relationship between the AA and the AAA apparatus when assessing SA and VD on sports flooring. Athletics tracks and artificial turf pitches have been included in this study since they are the most representative surfaces to be measured with each test method. This study should provide relevant information for the evaluation of impacts during athlete-surface interactions. 2. Methods 2.1. Sample A total of 50 athletics tracks and 44 artificial turf systems were evaluated using both the AA and the AAA apparatus. SA and VD were measured according to the standards and protocols currently set by the respective international federation [20,21], with the exception that two devices instead of only the one stipulated were used (AA and AAA). 2.2. Equipment 2.2.1. Artificial Athlete Tests in this study were carried out with an AA produced by Labosport. For the assessment of SA, this device consists of a 20 kg dropping mass that falls from a standard height of 55 mm, before hitting a spring with 2000 N/mm stiffness that is positioned over a test foot with a spherical base placed on the ground. The test foot is a cylinder with a diameter of 70 mm that is fitted with a force transducer and is capable of recording impact forces applied during the test process by using a load cell with a 10 kN capacity and 0.1% accuracy. The peak force recorded is compared with a reference value from a rigid concrete floor, and thus the SA of the synthetic surface is obtained [45]. For the assessment of SA in athletics track surfaces, the IAAF states that the low-pass filter must have a 9th order Butterworth characteristic [21]. For the assessment of VD, the AA must be slightly modified, although the main structure of the apparatus remains the same. The test is carried out in a similar way to that explained before,
E. Colino et al. / Polymer Testing 62 (2017) 61e67
although in this case the 20 kg weight is dropped from a height of 120 mm instead of 55 mm, the spring has a 40 N/mm stiffness instead of 2000 N/mm, the spherical base test foot is swapped for a flat one, and two movement transducers are incorporated. Deformation of the surface is recorded through the two movement transducers, which are mounted on either side of the foot and held by a simple structure placed independently from the main structure. The total distance between both sensors should not be greater than 125 mm, keeping the weight drop axis in the middle. Overall, VD simulates the deformation of the surface under a dynamic load of 1500 N [46]. 2.2.2. Advanced Artificial Athlete Tests with the AAA were carried out with a Labosport manufactured device. The AAA, which is a modification and amalgamation of the two test methods described before, enables the assessment of SA and VD at the same time and with only one signal. The principle of the apparatus consists of a drop test similar to that of the AA, although in this case the spring and the test foot through which the impact load is applied to the ground are attached to the bottom of the falling mass instead of resting over the surface. The spring rate is 2000 N/mm, the drop mass is 20 kg in weight and it is also dropped from a height of 55 mm. A test foot with the same diameter and similar characteristics to those of the AA is attached to the lower side of the spring. Instead of the force transducer, the AAA incorporates a piezoresistive accelerometer with a frequency range of up to 1000 Hz and a 2% linearity over the operating range. The G sensor is firmly attached to the falling mass on its vertical line of gravity and as much as possible on the lower side of it. During the impact, the signal of the accelerometer is conditioned and recorded, and a curve representing falling mass acceleration versus time is obtained. The AAA is thoroughly described in FIFA Test Method 04a [20], including test apparatus, test procedure and specifications for laboratory and field tests.
63
displacement of the falling weight during the course of impact are calculated by integration and double integration of the acceleration signal, respectively. For the calculation of SAAAA, peak force during the impact test (Fmax) must be obtained first:
Fmax ¼ m g Gmax þ m g where: Fmax is the peak force during the test, expressed in N. Gmax is the peak acceleration during the impact, expressed in g (1 g ¼ 9.81 m/s2) m is the falling mass including the spring, test foot and accelerometer, expressed in kg. g is the acceleration by gravity (9.81 m/s2) Then, SAAAA is calculated with the same formula as with the AA Berlin, with the exception that the reference force, Fr, is not previously obtained but fixed at 6760 N, which is the theoretical value for a concrete floor:
SAAAA ¼ ½ð1 ðFmax =FrÞ 100 For the calculation of VDAAA, a time interval is considered from the time when the test foot makes initial contact with the surface (T1) until the time when the maximum absolute velocity of the mass is obtained after the impact on the test specimen (T2). Then, the displacement of the falling mass (Dmax) and displacement of the spring (Dspring) during that interval are calculated through relatively complex formulas [20]. Finally, the vertical deformation of the test specimen is defined as:
VDAAA ¼ Dmax Dspring
2.3. Data collection 2.4. Protocol 2.3.1. SA with the Artificial Athlete (SAAA) SAAA is calculated according to the equation below from the maximum impact force recorded during the test (Ft) normalized with respect to a reference one (Fr), which is obtained by previously performing the same test (with more repetitions) on a rigid concrete surface. Both the Ft and the Fr are expressed in newtons (N). The SAAA value obtained from this test is considered representative of the shock absorption capability of a determined surface, and is expressed as a percentage:
SAAA ¼ ½ð1 ðFt=FrÞ 100
2.3.2. VD with the Artificial Athlete (VDAA) Vertical displacement of the surface during the impact is recorded through the movement transducers. Then, VDAA is calculated from the maximum deformation of the surface on the weight drop axis (fmax) and the maximum impact force recorded during the test (Fmax) according to the following expression:
VDAA ¼ ð1500N=Fmax Þ fmax
2.3.3. SA and VD with the Advanced Artificial Athlete (SAAAA, VDAAA) When measuring surface properties with the AAA, the acceleration-time curve during the impact of the falling mass is obtained from the accelerometry signal. Then, the speed and the
2.4.1. Athletics track surfaces Control samples using the same materials and techniques as those used during the construction of the tracks were used. The SA and VD were assessed using apparatus described in the EN 14808 [45] and EN 14809 [46] standards, respectively, through suitable laboratory tests according to the procedures specified by the IAAF in its IAAF Track and Runway Synthetic Surface Testing Specifications [21]. The absolute thickness of the samples was the same as that listed on the IAAF Product Certificate for each synthetic material [47]. The size of the samples was not less than 600 mm 600 mm. All tests were undertaken on samples at a temperature of 23 C.
2.4.2. Artificial turf systems Laboratory tests were carried out on test specimens constructed and conditioned following the FIFA requirements with a size of at least 1.0 m 1.0 m. The SA and VD were evaluated in all test specimens according to the procedures specified in the FIFA Handbook of Test Methods for Football Turf [20]. All the tests were performed over a concrete floor with a minimum thickness of 10 cm and a minimum hardness of 40 MPa, verified according to EN 12504e2 [48]. For each artificial turf system sample, the test was repeated three times on the same spot of the test specimen, with no brushing or adjusting of the surface between impacts. The test was repeated in three different positions, each at least 100 mm apart and at least 100 mm from the sides of the test specimen. The temperature during the tests was 23 ± 2 C.
64
E. Colino et al. / Polymer Testing 62 (2017) 61e67
2.5. Statistical analysis For data analysis, sports surfaces were evaluated separately, with the sample being divided into two different groups: athletics tracks (N ¼ 50) and artificial turf systems (N ¼ 44). Results are presented for each independent surface. Descriptive data are presented as mean and standard deviation (sd). Normality was tested through the KolmogoroveSmirnov test and graphic methods (normal probability plot). SA and VD values obtained with the AA and AAA test methods were compared using an independent sample t-test and the Bland-Altman method. Bland-Altman agreement was assessed by calculating the mean bias between the test methods and the means between methods, and these values were plotted on a scatterplot. The lower limit of agreement (LLA) and upper limit of agreement (ULA) were calculated as ± 1.96 times the standard deviation of the mean bias (95%LOA). Absolute agreement intraclass correlation coefficients (ICCs) were calculated to determine the inter-method reliability between the AA and the AAA test methods. ICC values represented low (ICC < 0.4), moderate (ICC between 0.4 and 0.75) and excellent (ICC > 0.75) reliability according to Shrout and Fleiss [49]. Finally, a linear regression analysis was conducted to assess the relationship between measurements obtained with both apparatus. The statistical analysis was carried out using IBM SPSS Statistics software (v. 21.0 SPSS Inc, Chicago, IL, USA), and Bland-Altman plots were also obtained using the same software. The significance level was set at p < 0.05.
3. Results Table 1 shows descriptive results of the study. The t-test for independent samples revealed no statistical differences between the AA and AAA test methods for any sports surface, either for SA or for VD variables (p > 0.05). Table 2 represents inter-method agreement and reliability. Results of the Bland-Altman test show modest deviations between the AA and the AAA for both the SA and the VD assessment, and ICC values show moderate reliability (0.4e0.75) between the two test methods for both the SA and VD variables and for both surfaces. Plots obtained through the Bland-Altman analysis are presented in Fig. 1. The results indicate a slight underestimation of SA values when obtained with the AAA. This underestimation is more relevant for the hard surfaces (athletics tracks) than for the soft surfaces such as artificial turf systems (5.82± 0.92% vs. 1.21± 1.18%). Conversely, the VD is slightly overestimated when assessed with the AAA apparatus, with that overestimation being higher when the hardness of the surface decreases. The results of the linear regression analysis show a close relationship between overall measures obtained with the AA and the AAA. Regression equations for each mechanical property have been obtained considering both surfaces together, so hard surfaces with low values of SA and VD (athletics tracks) and soft surfaces with
Type of surface
Variables
Test method
Mean
sd
p*
Artificial turf (N ¼ 44)
SA (%)
AAA AA Berlin AAA AA Stuttgart AAA AA Berlin AAA AA Stuttgart
59.74 60.95 8.30 7.09 29.36 35.18 2.37 1.62
3.28 2.62 0.83 0.96 4.44 4.47 0.57 0.50
0.16
Athletic track (N ¼ 50)
SA (%) VD (mm)
*Level of significance: p < 0.05.
SAAA ¼ 0:852 SAAAA þ 10:117
(1)
With regard to VD, the following equation predicts 98.5% of the variance in the SAAA parameter from data obtained with the AAA:
VDAA ¼ 0:925 VDAAA 0:583
(2)
4. Discussion The aim of the present study was to discover the extent to which the AA and AAA test methods provide similar results when assessing SA and VD variables on sports surfaces. Although no statistical differences were observed between the methods, the analysis of the level of agreement revealed relevant deviations that could be corrected by using Equations (1) and (2). Thus, the main finding of the present study is that, although AA and AAA apparatus should not be used interchangeably for the assessment of SA and VD, values obtained with the AA can be predicted with a high level of accuracy when using the AAA (99.4% and 98.5%, respectively). Surface behaviour during impact actions is of special relevance due to its association with athletes' performance and the risk of injury, with SA and VD being the main surface properties conditioning these actions. Currently, the AA and the AAA are the key test methods in use for assessing these properties in sports surfaces, although a lack of consensus exists about which method should be used. The heterogeneity of apparatus and regulations concerning test methods makes the analysis of sports surface properties confusing and could lead to conflicting information. At the present time, no scientific evidence exists on whether the AA and the AAA provide equivalent results. This paper aimed to contribute to the clarification of sports surface testing by comparing both apparatus and stating the relationship between them. Athletics tracks and artificial turf systems were selected in this study since they are the most representative sports surfaces to be evaluated with each test method. In athletics tracks, a modest SA underestimation of 5.82± 0.92% was described for the AAA when compared with the AA, as well as an overestimation of 0.75 ± 0.23 mm for VD. Given that the IAAF requirements for SA in athletics track surfaces range from 35% to 50%, and the VD requirements go from 0.6 mm to 2.5 mm [50], the discrepancy in AAA measures is greater than onethird of the allowed range for both parameters. Differences of that magnitude could easily lead to a wrong diagnosis when assessing performance parameters of athletics track surfaces using the AAA apparatus. Thus, given an accuracy of 95%, the results for those variables assessed with the AAA device could be simplified as:
SAAAA zSAAA 5:82%
Table 1 Descriptive characteristics of surfaces.
VD (mm)
higher SA and VD (artificial turf systems) are represented under the same regression (Fig. 2). For the assessment of SA, the equation below predicts 99.4% of the variance in the SAAA parameter from data obtained with the AAA.
0.24 0.88 0.66
VDAAA zVDAA þ 0:75mm To the best of the authors' knowledge, no previous comparison between AA and AAA devices has been carried out on athletics tracks, with this study being the first approach towards the standardization and unification of test methods in these surfaces. Assessing the mechanical properties of athletics tracks with the AAA could provide some benefits, such as easier calibration of the apparatus and significant time savings, since SA and VD are measured with one instead of two different tests. Even if SA and VD
E. Colino et al. / Polymer Testing 62 (2017) 61e67
65
Table 2 Agreement and reliability between test methods for the assessment of SA and VD. Surface
Artificial turf (N ¼ 44) Athletic track (N ¼ 50) a b c
Variables
SA (%) VD (mm) SA (%) VD (mm)
Differences between methods
AAA AAA AAA AAA
e e e e
AA AA AA AA
Bland-Altman analysis
Confidence intervals
biasa ± sd
95% LOAb
ICCc
95% CI
1.21 ± 1.18 1.22 ± 0.40 5.82 ± 0.92 0.75 ± 0.23
3.52 to 1.10 0.44 to 2.00 7.62 to 4.02 0.30 to 1.20
0.91 0.60 0.69 0.47
0.44 to 0.97 0.12 to 0.88 0.02 to 0.90 0.20 to 0.98
Bias: average difference between methods. LOA: Limits of agreement. LOA: Media ± (1.96*SD). ICC: Infraclass Correlation Coefficient.
Fig. 1. Bland-Altman plots identifying differences when comparing measurements with the Artificial Athlete (AA) and the Advanced Artificial Athlete (AAA) apparatus. Central line represents the inter-method difference (bias). Upper and lower lines represent the 95% limits of agreement (bias ± 1.96 sd of the differences). A e Shock Absorption in athletics track; B e Vertical Deformation in athletics track; C e Shock Absorption in artificial turf; D e Vertical Deformation in artificial turf.
values obtained with the AA were needed, the present paper would enable them to be calculated with a high degree of accuracy (99.4% and 98.5%, respectively) by using the equations from the linear regression analysis. As regards artificial turf systems, a modest study exists comparing the AA (FIFA test method) and the AAA (European standards test method) in two artificial turf surfaces [44]. Compared with that work, a higher underestimation in the SA test
was found in the present study (1.21% vs. 0.6%), while a slightly lower overestimation was obtained in the VD test (þ1.22 mm vs. þ 1.6 mm). Given that requirements for SA in artificial turf pitches range from 55% to 70% (both FIFA and European standards), and the VD requirements go from 4.0 to 11.0 mm (FIFA) or 4.0e9.0 mm (European standards), these deviations represent smaller proportions than those found for the athletics track surfaces, thereby providing greater representativeness for both test
66
E. Colino et al. / Polymer Testing 62 (2017) 61e67
Fig. 2. Linear regression analysis for Shock Absorption (SA) and Vertical Deformation (VD) measurements obtained with the Artificial Athlete and the Advanced Artificial Athlete apparatus. A e Shock Absorption (R2 ¼ 0.994); B e Vertical Deformation (R2 ¼ 0.985).
methods. Thus, if AA values for SA and VD parameters were required in order to meet European standards, they could also be obtained by assessing the surface with the AAA and then applying Equations (1) and (2). The t-test for independent samples indicates that the AA and the AAA could be used interchangeably on both athletics tracks and artificial turf surfaces and no statistical error would occur. However, the results of the Bland-Altman test show moderate deviations between test methods in both surfaces, with hard surfaces (athletics tracks) having a greater bias for SA and slightly lower bias for VD. Notwithstanding the foregoing, the overall results of the linear regression analysis indicate an excellent replication of AA outcomes when using the AAA apparatus. With this in mind, it seems that both the AA and the AAA could be used interchangeably when assessing SA and VD on both athletics tracks and artificial turf surfaces by applying Equations (1) and (2) as stated in point 3. This study could represent a substantial contribution to guaranteeing that athletics and artificial-turf football events at any level and at any venue take place on surfaces with comfort and safety for athletes and football players, since it promotes and facilitates linkages between test laboratories and facility managers in order to carry out quality control tests on the surfaces and assess their degree of compliance with the requirements established by international bodies. 5. Conclusions No statistical differences were observed between the AA and AAA test methods when assessing SA and VD on athletics tracks and artificial turf pitches. Slight deviations occur between both apparatus that seem to be dependent on the surface hardness, with SA differing more in hard surfaces than in soft surfaces and VD working the opposite way. Equations (1) and (2) allow those deviations to be minimized and results predicted with both apparatus by using either of them with almost perfect reliability. The present paper opens new possibilities for the assessment of surface properties and lays the foundation for the unification of both test methods. This work should enable research institutions and accredited laboratories to use the AA and AAA interchangeably when evaluating SA and VD on athletics tracks and artificial turf pitches.
Declaration of conflicting interests The authors declare that there is no conflict of interest. Funding Enrique Colino has been funded by the Spanish Ministry of Education, Culture and Sport (Grant number: FPU15/04700) within the ‘National Programme for Training University Lecturers’ (FPU Grant) e Ref. 998758. References [1] P. Fleming, C. Young, M. Carre, Mechanical testing and characterisation of sports surfaces, in: S.J. Dixon, P. Fleming, I. James, M. Carre (Eds.), The Science and Engineering of Sport Surfaces, 2015, pp. 26e69. [2] C.T. Farley, H.H. Houdijk, C. Van Strien, M. Louie, Mechanism of leg stiffness adjustment for hopping on surfaces of different stiffnesses, J. Appl. Physiol. 85 (1998) 1044e1055. [3] D.P. Ferris, K. Liang, C.T. Farley, Runners adjust leg stiffness for their first step on a new running surface, J. Biomech. 32 (1999) 787e794. [4] D.P. Ferris, M. Louie, C.T. Farley, Running in the real world: adjusting leg stiffness for different surfaces, Proc. Biol. Sci. 265 (1998) 989e994. [5] J. Hackney, S. Brummel, K. Jungblut, C. Edge, The effect of sprung (suspended) floors on leg stiffness during grand jete landings in ballet, J. Dance Med. Sci. 15 (2011) 128e133. [6] C.T. Moritz, S.M. Greene, C.T. Farley, Neuromuscular changes for hopping on a range of damped surfaces, J. Appl. Physiol. 96 (2004) 1996e2004. [7] E.M. Bigelow, N.G. Elvin, A.A. Elvin, S.P. Arnoczky, Peak impact accelerations during track and treadmill running, J. Appl. Biomech. 29 (2013) 639e644. [8] M.F. Bobbert, M.R. Yeadon, B.M. Nigg, Mechanical analysis of the landing phase in heel-toe running, J. Biomech. 25 (1992) 223e234. [9] S.J. Dixon, A.C. Collop, M.E. Batt, Surface effects on ground reaction forces and lower extremity kinematics in running, Med. Sci. Sports Exerc. 32 (2000) 1919e1926. [10] B.M. Nigg, R.W. De Boer, V. Fisher, A kinematic comparison of overground and treadmill running, Med. Sci. Sports Exerc. 27 (1995) 98e105. [11] B.M. Nigg, M.R. Yeadon, Biomechanical aspects of playing surfaces, J. Sports Sci. 5 (1987) 117e145. [12] V. Wank, U. Frick, D. Schmidtbleicher, Kinematics and electromyography of lower limb muscles in overground and treadmill running, Int. J. Sports Med. 19 (1998) 455e461. [13] R. Di Michele, A.M. Di Renzo, S. Ammazzalorso, F. Merni, Comparison of physiological responses to an incremental running test on treadmill, natural grass, and synthetic turf in young soccer players, J. Strength Cond. Res. 23 (2009) 939e945. nchez-Sa nchez, J. García-Unanue, J.L. Felipe, P. Jime nez-Reyes, D. Viejo[14] J. Sa mez-Lo pez, E. Hernando, P. Burillo, L. Gallardo, Physical and Romero, M. Go physiological responses of amateur football players on third-generation artificial turf systems during simulated game situations, J. Strength Cond. Res. 30
E. Colino et al. / Polymer Testing 62 (2017) 61e67 (2016) 3165e3177. [15] J. Sanchez-Sanchez, J. Garcia-Unanue, P. Jimenez-Reyes, A. Gallardo, P. Burillo, J.L. Felipe, L. Gallardo, Influence of the mechanical properties of thirdgeneration artificial turf systems on soccer players' physiological and physical performance and their perceptions, PLoS One 9 (2014) e111368. [16] A. Sassi, A. Stefanescu, P. Menaspa, A. Bosio, M. Riggio, E. Rampinini, The cost of running on natural grass and artificial turf surfaces, J. Strength Cond. Res. 25 (2011) 606e611. [17] S.J. Dixon, V. Stiles, Impact absorption of tennis shoe-surface combinations, Sports Eng. 6 (2003) 1e9. [18] D. McGhie, G. Ettema, Biomechanical analysis of surface-athlete impacts on third-generation artificial turf, Am. J. Sports. Med. 41 (2013) 177e185. [19] H. Yukawa, R. Aduma, S. Kawamura, K. Kobayashi, Shock attenuation properties of sports surfaces with two dimensional impact test, Procedia Eng. 34 (2012) 855e860. [20] FIFA, FIFA Quality Programme for Artificial Turf - Handbook of Test Methods, 2015. [21] IAAF, in: IAAF (Ed.), IAAF Track and Runway Synthetic Surface Testing Specifications, 2016. [22] FIH, Handbook of Performance, Durability and Construction Requirements for Synthetic Turf Hockey Pitches, 2013. [23] IRB, IRB Artificial Rugby Turf Performance Specification, 2012. [24] B. Farhang, F.R. Araghi, A. Bahmani, F. Moztarzadeh, M. Shafieian, Landing impact analysis of sport surfaces using three-dimensional finite element model, J. Sports Eng. Tech. 230 (2016) 180e185. [25] T.F. Novacheck, The biomechanics of running, Gait Posture 7 (1998) 77e95. asson, L. Peterson, Effects of shoe and surface characteristics on lower [26] G. Andre limb injuries in sports, Int. J. Sport Biomech. 2 (1986) 202e209. [27] G.K. Cole, B.M. Nigg, A.J. van Den Bogert, K.G. Gerritsen, Lower extremity joint loading during impact in running, Clin. Biomech. 11 (1996) 181e193. pez-Fern [28] E. Ubago-Guisado, J. García-Unanue, J. Lo andez, J. S anchez-S anchez, L. Gallardo, Association of different types of playing surfaces with bone mass in growing girls, J. Sports Sci. (2016) 1e9. [29] E.M. Zanetti, C. Bignardi, G. Franceschini, A.L. Audenino, Amateur football pitches: mechanical properties of the natural ground and of different artificial turf infills and their biomechanical implications, J. Sports Sci. 31 (2013) 767e778. [30] K.R. Ford, N.A. Manson, B.J. Evans, G.D. Myer, R.C. Gwin, R.S. Heidt Jr., T.E. Hewett, Comparison of in-shoe foot loading patterns on natural grass and synthetic turf, J. Sci. Med. Sport 9 (2006) 433e440. [31] A. Hreljac, Impact and overuse injuries in runners, Med. Sci. Sports Exerc. 36 (2004) 845e849.
67
[32] J.A. Mercer, J. Vance, A. Hreljac, J. Hamill, Relationship between shock attenuation and stride length during running at different velocities, Eur. J. Appl. Physiol. 87 (2002) 403e408. [33] H.K. Reeve, L.S. Hopper, B.C. Elliott, T.R. Ackland, Lower limb kinematic variability in dancers performing drop landings onto floor surfaces with varied mechanical properties, Hum. Mov. Sci. 32 (2013) 866e874. [34] R.J. Butler, H.P. Crowell 3rd, I.M. Davis, Lower extremity stiffness: implications for performance and injury, Clin. Biomech. 18 (2003) 511e517. [35] S.K. Grimston, J.R. Engsberg, R. Kloiber, D.A. Hanley, Bone mass, external loads, and stress fracture in female runners, Int. J. Sport Biomech. 7 (1991) 293e302. [36] L. Andena, F. Briatico-Vangosa, A. Ciancio, A. Pavan, A finite element model for the prediction of force reduction of athletics tracks, Procedia Eng. 72 (2014) 847e852. [37] R.M. Silva, J.L. Rodrigues, V.V. Pinto, M.J. Ferreira, R. Russo, C.M. Pereira, Evaluation of shock absorption properties of rubber materials regarding footwear applications, Polym. Test. 28 (2009) 642e647. [38] M. Benanti, L. Andena, F. Briatico-Vangosa, A. Pavan, Viscoelastic behavior of athletics track surfaces in relation to their force reduction, Polym. Test. 32 (2013) 52e59. [39] M. Shorten, J. Himmelsbach, Shock attenuation of sports surfaces, Eng. Sport 4 (2002) 152e159. [40] A.E. Kerdok, A.A. Biewener, T.A. McMahon, P.G. Weyand, H.M. Herr, Energetics and mechanics of human running on surfaces of different stiffnesses, J. Appl. Physiol. 92 (2002) 469e478. [41] N.M. Schrier, J.W. Wannop, R.T. Lewinson, J. Worobets, D. Stefanyshyn, Shoe traction and surface compliance affect performance of soccer-related movements, Footwear Sci. 6 (2014) 69e80. [42] L. Andena, F. Briatico-Vangosa, E. Cazzoni, A. Ciancio, S. Mariani, A. Pavan, Modeling of shock absorption in athletics track surfaces, Sports Eng. 18 (2015) 1e10. [43] FIFA, FIFA Quality Concept - Handbook of Test Methods for Football Turf, 2012. [44] H.J. Kolitzus, Scrutiny of advanced artificial athlete, ISSS Technical Conference, Personal communication. Shanghai, 2012. [45] EN 14808:2005 Surface for sports areas - determination of shock absorption. [46] EN 14809:2005 Surface for sports areas - determination of vertical deformation. [47] IAAF, in: IAAF (Ed.), IAAF Certified Track Surfacing Products, 2016. [48] EN 12504e2:2012 Testing concrete in structures - Part 2: non-destructive testing - determination of rebound number. [49] P.E. Shrout, J.L. Fleiss, Intraclass correlations: uses in assessing rater reliability, Psychol. Bull. 86 (1979) 420e428. [50] IAAF, in: IAAF (Ed.), IAAF Track and Field Facilities Manual, Editions EGC, 2008.