REPORT. A comparative study of the mechanical and biomechanical behaviour of natural turf and hybrid turf for the practise of sports

REPORT A comparative study of the mechanical and biomechanical behaviour of natural turf and hybrid turf for the practise of sports Addressed to: PSF - PALAU TURF Date: May 2015

Table of Contents SHEET OF SIGNATURES AND CONDITIONS 1. INTRODUCTION AND OBJECTIVES 2. MATERIAL AND METHODS 3. RESULTS AND DISCUSSION 4. CONCLUSIONS 5/21

1. INTRODUCTION AND OBJECTIVES The conditions that sport surfaces offer for the practise of sports are becoming increasingly important in the market. Players are increasingly appreciating the influence of the sport surfaces on sports performance, comfort and the risk of injury. These conditions are complex, as they derive from the interaction of a variety of aspects, such as the surface's capacity to adapt to the foot movements that are enabled by the footwear, the shock absorption, and the friction or grip on the ground. The key determining factors of a sport surfaces comfort, functionality and safety include: Shock absorption: this is the surface's capacity to reduce the forces of the impact that the player receives when running or jumping during sports. It relates to aspects of both safety and performance. Deformation: the capacity of the surface to deform on impact. Excessive deformation may cause instabilities when the player's foot hits the ground, which can increase the risk of injury. Traction: this is the characteristic of the surface related to a player's surface grip capacity. High traction values mean that the surface offers increased grip and therefore there is less risk of falls due to slipping. However, excessively high traction values can increase the risk of injury, making some movements more difficult to perform by obstructing the foot's movement, which will transmit higher tensions to the joints. These properties may vary depending on the type of sport surface, due to the characteristics of the materials from which they are made. The aim of this study is to make a comparison of two sport surfaces intended for football: a hybrid turf pitch and a high-quality natural turf pitch. To make this comparison, both the mechanical and biomechanical characteristics of the turfs will be analysed. 7/21

2. MATERIAL AND METHODS Tests were conducted on two football pitches. In this section we present the characteristics of the two fields and the measurement procedures used to assess them. 2.1. TEST SAMPLES Table 1 shows the location and turf type of the assessed pitches, as well as the date and the environmental conditions under which the tests were conducted. Location Avià (Barcelona) Location Salou (Tarragona) Turf type Hybrid Turf type Natural Date of tests 15 April 2015 Date of tests 15 April 2015 Environmental conditions 20-23 C and 54-60% RH Environmental conditions 17-20 C and 46-53% RH Table 1. Assessed football pitches 2.2. TESTS CARRIED OUT The tests carried out on the football pitches may be classified into two distinct groups. - Mechanical behaviour assessment tests. Refers to tests with machines, of which the following were carried out: 8/21 Mechanical shock absorption test. Mechanical deformation test. Mechanical rotational torque test. - Biomechanical behaviour assessment tests. Refers to tests with players, of which the following were carried out: Biomechanical shock absorption test.

Each of the aforementioned tests are described below in detail. 2.2.1. Mechanical behaviour assessment tests The aspects evaluated in the assessment tests with machines are: Shock absorption (%). Vertical deformation (mm). Rotational torque (N m). Figure 1 shows the points where the assessment tests were performed with machines: - Points designated with LETTERS (19 points): Shock absorption and vertical deformation test. - Points designated with NUMBERS (6 points): Rotational resistance test. Figure 1. Points of assessment tests with machines The assessment tests with machines were performed according to the current high-level football method 1. The equipment used in the assessment tests with machines was as follows: Advanced Artificial Athlete (AAA) (Figure 2, left): simulates the impact of a player's heel while running and measures the shock absorption capacity of the running surface and the deformation resulting from this impact. Rotational resistance machine (Figure 2, right): simulates a player's rotation on the supporting foot. The machine has a base with studs that are dropped from a certain 1 FIFA Quality Concept January 2012 (http://www.fifa.com/mm/document/afdeveloping/pitchequip/fqc_football_turf_folder_342.pdf) 9/21

height so that they penetrate the surface; once riveted, a torque wrench is used to assess the force required to rotate them. Figure 2. Test machines: left, AAA; right, rotational resistance machine N.B.: Although the tests were performed on natural turf and hybrid turf, the test results were compared with the current regulations for artificial turf football pitches, since no international rules currently exist that regulate the properties of natural turf sport surfaces. However, please note that although it is specified that the regulations apply to artificial turf sports surfaces, the requirements that a football-playing surface should meet must be the same, regardless of the surface type used, as must the test method used. 2.2.2. Biomechanical behaviour assessment tests During sports, impacts occur in the area where the foot makes contact with the playing field. The magnitude of these impacts largely depends on the characteristics of the surface, of the footwear and of the individual technique. These impacts are transmitted to the users' entire musculoskeletal system in the form of vibrations. To assess the degree of shock absorption provided by the two types of turf (hybrid and natural turf), five football players participated in the tests. The size, weight, age and length of each player's tibia were recorded before the test was performed. Each subject used their regular training clothes and footwear (football boots). 2.2.2.1. Description of the analysed movement The biomechanical behaviour assessment was carried out using a validated test to measure the player's jumping power, called Drop Jump (DJ), which consists of dropping from a certain height (drop phase) and jumping again after contact with the ground, trying to reach the maximum height (flight phase). Table 2. Analysed movement sequence shows the sequence of the analysed movement. The height of the drop was set at 40 cm. Each player made a minimum of five valid movements, on each of the two football pitches assessed. 10/21

Initial position Start of drop Reception 1 Maximum flexion Start of jump Flight phase Reception 2 Final position Table 2. Analysed movement sequence A movement was deemed valid if the following conditions were met: While performing it, the subject keeps their hands on their waist. The movement begins with the right foot. During the drop phase, the knees and torso remain fully extended. Upon contact with the football pitch surface, the subject generates a sudden and maximum effort, propelling themselves vertically upwards. 2.2.2.2. Instruments used to assess the vibrations transmitted to the human body upon impact with the surface may be described using special accelerometer equipment (see Table 3). Measurement channels 4 Accelerometers Uniaxial of 10 G (x2) Uniaxial of 40 G (x2) Max. acquisition frequency 2500 Hz Resolution 0.01 G Error 0.1 G Table 3. Description of the accelerometer equipment used 11/21

Each player is fitted with two accelerometers, firmly fastened against their skin. The first accelerometer is fastened onto the inner surface of the distal half of the tibia on the right leg and the second accelerometer to the front of the head, centred on the frontal bone. The signal issuing from the accelerometers is transmitted to a computer via a telemetry system. Marker 1 Proximal area of the line that joins the trochanter with the lateral condyle Marker 2 Lateral condyle Marker 3 Lateral malleolus Figure 3. Location of markers for video analysis Each movement was video-recorded on the player's sagittal plane, to study the reproducibility of the jump's execution. By analysing the movement captured in the recording, it can be determined whether the movements made are similar. To facilitate the video analysis, each player was fitted with three flat markers located on anatomical points of the right leg, (see Figure 3. Location of markers for video analysis). 2.2.2.2. Description of the analysed parameters Figure 2 shows a signal registered by the tibia and head accelerometers. For the subsequent data analysis, the following parameters were defined: - Maximum acceleration of the tibia (MT1) after the drop phase. - Maximum acceleration of the tibia (MT2) after the flight phase. - Maximum acceleration of the head (MC) after the flight phase. 12/21

Drop Fase de phase caída Reception, Recepción, flexion flexión and e inicio start del of jump salto Flight Fase de Phase vuelo 20 15 MT1 MT2 Accel. Acel. Head Cabeza Accel. Acel. Tibia Acceleration Aceleración (G's) (G) 10 5 0 MC -5 1.25 1.5 1.75 2 2.25 2.5 2.75 Time Tiempo (s) (s) Figure 2. Example of accelerometer signals obtained in the tests with subjects The acceleration in the musculoskeletal system experienced by the players is linked to both the shock absorption capacity of the surface and the player's jump pattern. In the case of acceleration in the tibia, the measurement obtained after the drop phase is more closely related to the player's jump pattern as they have mechanisms (such as flexion of the knee) to absorb the impact. However, in the case of the acceleration measured after the flight phase, given that the drop following the jump is with the knee fully extended, the shock absorption capacity of the surface plays a key role in reducing the accelerations measured on the player's tibia. Video analysis was used to determine the maximum knee flexion angle during the jump (see Figure ). Figure 5. Obtaining the maximum knee flexion angle (centre) through video analysis. Moments before (left), and after (right) 13/21

The knee flexion angle was used as a control variable of the jump technique, in order to discard movements that were not performed correctly or that varied from the other movements repeated by the same player. 14/21

3. RESULTS AND DISCUSSION 3.1. TESTS RESULTS OF MECHANICAL BEHAVIOUR ASSESSMENT Table 4 shows the values obtained for each of the mechanical parameters tested: Pitch Average Standard deviation Maximum Minimum SHOCK ABSORPTION (%) Avià: Hybrid turf 61.5 2.0 65.5 60.0 Salou: Natural turf 50.1 4.6 58.5 42.5 DEFORMATION (mm) Avià: Hybrid turf 7.7 0.7 8.5 6.5 Salou: Natural turf 5.1 0.8 4.0 7.0 ROTATIONAL TORQUE (Nm) Avià: Hybrid turf 46.2 6.3 54.0 36.0 Salou: Natural turf 50.8 2.8 54.0 47.0 Table 4. Results of the test with machines The average values obtained in the Avià pitch (hybrid turf) fall within the range of high-level criteria for playing pitches, which is separated into basic level and advanced level. The values for shock absorption and deformation are in the advanced level range (see Table 5). Test Basic Level Advanced Level Minimum Maximum Minimum Maximum Shock absorption (%) 55 70 60 70 Vertical deformation (mm) 4 11 4 10 Rotational resistance (Nm) 25 50 30 45 Table 5. High-level criteria for football pitches The shock-absorption and deformation properties of the Avià pitch (hybrid turf) are much more homogeneous than those of the Salou pitch (natural turf). This fact is reflected in the standard deviations obtained in each of the analysed parameters (Table 4. Results of the test with machines) and can be observed graphically in Figure 3. Values obtained in the assessment test with machines 15/21

Avià Hybrid Turf Salou Natural Turf Vertical deformation (mm) Test point Shock absorption (%) Test point Rotational resistance (Nm) 16/21 Test point Figure 3. Values obtained in the assessment test with machines The green-shaded area of Figure 3 indicates the range of values accepted according to the high-level criteria for advanced level football pitches. It can be seen that most of the points in the Avià pitch (turf hybrid) fall within said range. However, some points are in the lower limit. Tables 6, 7 and 8 show the comparison between the tested pitches and the fulfilment of the high-level criteria for football pitches (basic level and advanced level).

Shock absorption (%) Vertical deformation (mm) Point Avià (hybrid) Salou (natural) Avià (hybrid) Salou (natural) Values Criterion Values Criterion Values Criterion Values Criterion A 59 56 6.5 6 B 65.5 48.5 8.5 5 C 62 46 8.5 4 D 62.5 53 8.5 5.5 E 60 53.5 6.5 6 F 62 53.5 8 6 G 64.5 58.5 8.5 7 H 59.5 43.5 6.5 4 I 60 49.5 7.5 5 J 59.5 42.5 7.5 4 K 63 45 8 4 L 63.5 43.5 8.5 4 M 62 52 7 5.5 N 63.5 49.5 8 5 O 60 53.5 7.5 5 P 61.5 52.5 7.5 5 Q 57.5 46.5 7 4.5 R 62.5 52.5 8.5 5 S 61 53 8 5.5 Table 6. Values of shock absorption and vertical deformation per point, compared with high-level criteria (basic level) 17/21

Shock absorption (%) Vertical deformation (mm) Point Avià (hybrid) Salou (natural) Avià (hybrid) Salou (natural) Values Criterion Values Criterion Values Criterion Values Criterion A 59 56 6.5 6 B 65.5 48.5 8.5 5 C 62 46 8.5 4 D 62.5 53 8.5 5.5 E 60 53.5 6.5 6 F 62 53.5 8 6 G 64.5 58.5 8.5 7 H 59.5 43.5 6.5 4 I 60 49.5 7.5 5 J 59.5 42.5 7.5 4 K 63 45 8 4 L 63.5 43.5 8.5 4 M 62 52 7 5.5 N 63.5 49.5 8 5 O 60 53.5 7.5 5 P 61.5 52.5 7.5 5 Q 57.5 46.5 7 4.5 R 62.5 52.5 8.5 5 S 61 53 8 5.5 Table 7. Values of shock absorption and vertical deformation per point, compared with high-level criteria (advanced level) Point Basic level criteria Rotational resistance (N m) Advanced level criteria Avià (hybrid) Salou (natural) Avià (hybrid) Salou (natural) Values Criterion Values Criterion Values Criterion Values Criterion 1 36 51 36 51 2 50 47 50 47 3 48 50 48 50 4 47 49 47 49 5 42 54 42 54 6 54 54 54 54 Table 8. Values of rotational resistance per point, compared with high-level criteria 18/21

3.2. TESTS RESULTS OF BIOMECHANICAL BEHAVIOUR ASSESSMENT Table 9 shows the average acceleration values obtained on each pitch in the assessment with subjects. Pitch IMPACT of TIBIA 1 (G) IMPACT of TIBIA 2 (G) IMPACT of HEAD (G) Average Standard Dev. Average Standard Dev. Average Standard Dev. Avià (hybrid turf) 16.7 5.1 18.5 8.0 5.9 2.2 Salou (natural turf) 17.5 5.5 25.2 10.5 6.6 2.0 Table 9. Average acceleration values obtained in the biomechanical tests In all of the analysed parameters, the hybrid turf pitch has an average value lower than the natural turf pitch. To determine whether this difference is statistically significant, an ANOVA was performed for each parameter, using the assessed pitch as a factor (Table 10). To prevent the intrinsic variability of the subjects concealing the differences between the assessed pitches, the data were normalised between zero and one. Sum of squares gl Root Mean Square F Sigma Tibia 1 (MT1) Tibia 2 (MT2) Head (MC) Inter-groups 0.242 1 0.242 3.438 0.068 Intra-groups 4.776 68 0.070 - - Total 5.018 69 - - - Inter-groups 0.746 1 0.746 9.225 0.003 Intra-groups 5.500 68 0.081 - - Total 6.246 69 - - - Inter-groups 0.241 1 0.241 2.015 0.160 Intra-groups 8.125 68 0.119 - - Total 8.365 69 - - - Table 10. ANOVA of biomechanical parameters The maximum acceleration of the tibia after the flight phase (MT2) is statistically lower in the hybrid turf pitch (Avià). As previously discussed, the acceleration measured after the flight phase is related to the shock absorption capacity of the playing surface, so that a higher shock absorption capacity in the tibia translates into a lower acceleration being measured in the players' tibia following this phase. 19/21

Furthermore, there is scientific evidence that links a lower shock absorption capacity of the surface with a higher risk of injury 2,3,4 which would make the hybrid turf surface of the Avià pitch safer than the Salou natural turf pitch from the point of view of shock absorption. Although the other analysed parameters have not turned out to be statistically significant, both the average value of acceleration in the tibia measured after the drop phase and the acceleration in the head are lower in the hybrid turf pitch (Avià). 2 HOEBERINGS, J. H (1992). Factors related to the incidence of running injuries: a review. Sports Med. 13:408 422 3 JONES, B. H (1993). Overuse injuries of the lower extremities associated with marching jogging and running: a review. Mil. Med. 148:783 787 4 MACERA, C. A (1992). Lower extremity injuries in runners: advances in prediction. Sports Med. 13:50 57 20/21

4. CONCLUSIONS In the light of the results obtained, it may be concluded that: 1. The hybrid turf pitch (Avià) presents a more homogeneous mechanical behaviour than the natural turf pitch. It also has a higher number of points whose shock absorption, deformation and rotational resistance values meet the requirements of the high-level criteria for advanced-level football pitches. 2. The rotational resistance measured in the hybrid turf pitch (Avià) presents lower values than those measured in the natural turf pitch (Salou), meaning that the former presents a lower risk of the player's supporting foot being obstructed during a turn, which reduces the likelihood of injury. 3. The results of the mechanical assessment demonstrate that the shock absorption capacity of the hybrid turf pitch (Avià) is higher, which is corroborated by the results obtained from the biomechanical assessment, yielding lower acceleration values in the tibia following the flight phase in this pitch compared to the natural turf pitch (Salou). 4. Taking into account current legislation and scientific studies that demonstrate that a lower shock absorption and a higher rotational resistance increase the likelihood of injury 5,6, Avià's hybrid turf pitch would present a lower risk of injury than the natural turf pitch of Salou. Limitations of the study: The above conclusions only apply to the pitches assessed under the surface conditions and specific environmental conditions that concurred on the date of the tests. 5 Torg, J. S., Quedenfeld, T. C., & Landau, S. (1974). The shoe-surface interface and its relationship to football knee injuries. The American Journal of Sports Medicine, 2(5), 261-269. 6 Livesay GA, Reda DR, Nauman EA (2006). Peak Torque and Rotational Stiffness Developed at the Shoe-Surface Interface. The Effect of Shoe Type and Playing Surface. Am J Sports Med 34 (3): 415-422 21/21