Néstor Arroyo Gómez; Enric Medina i Ripoll; Rafael Mengual Ortolá; Mario Aguado Vírseda; Enrique Alcántara Alcover; Juan Vicente Durá Gil; Begoña Mateo Martínez; Marta Valero Martínez; Mercedes Sanchis Almenara; Laura Magraner Llavador

Instituto de Biomecánica (IBV)
Universitat Politècnica de València
Camino de Vera s/n. Edificio 9C
46022 Valencia, España

Increasing importance is being given to the influence that playing surfaces have on aspects such as performance, comfort or the prevention of injuries. Soccer is no exception, and new types of grass are being developed that endeavor to obtain the best possible performance. One such example is hybrid grass which comprises a layer of porous artificial turf and another layer of elastic. The grass roots are able to pass through the two layers, thereby allowing the natural grass to grow properly. To characterize this type of grass, the Instituto de Biomecánica (IBV) has conducted a comparative study of a soccer field made from hybrid grass with another field of high quality natural grass."

INTRODUCTION

The main factorsthat determine the comfort, functionality and safety of a playing field are:

♦ Shock absorption:The ability to reduce the impact forces that the user receives during a game.

♦ Deformation:The ability to deform when confronted with a vertical force such as an impact. Excessive deformation may result in players experiencing instability, which can increase the risk of injury.

♦ Traction: A characteristic related to the sportsperson’s ability to grip the surface. High values of traction mean that the surface provides greater grip and therefore less risk of slipping and falling. However, excessively high traction values make it harder for players to move as they can cause the foot to jam which places high levels of stress on the joints. Rotational traction refers to grip during a turn. Excessive grip can cause serious knee injuries.

These factors vary - for a particular type of surface -- depending on the materials, features, proportions and the quality of the elements of which it is made.

The objective of this study is to compare two different types of surfaces used for the practice of soccer: a field of natural grass and a field of hybrid grass. 

MATERIAL AND METHODS

Mechanical and biomechanical tests were conducted on the two soccer fields in order to compare their characteristics: a soccer field of high-quality natural grass and another of hybrid grass. Both fields are used intensively both for training and for actual games.

Given the diversity of types of grass playing fields, both natural and artificial, the results of this study are applicable only to the fields analyzed.

Mechanical behavior assessment tests

Measurements of a mechanical nature were carried out in accordance with the existing methodology for soccer fields used in top-level competitions. We used the following testing machines:

♦ Advanced Artificial Athlete (AAA):This machine simulates the impact of a player’s heel as he or she runs and it measures the surface’s ability to cushion impacts and its deformation due to these impacts (Figure 1 – left-).

♦ Rotational Traction Machine: This machine simulates the rotation of a player on the supporting foot. The unit is equipped with a base with soccer boot studs that is dropped from a specific height in order to ensure they penetrate the surface. Once the studs are in place, a torque wrench is used to assess the force required to make them rotate (Figure 1 – right-). 

Figure 1. Advanced artificial athlete (left) and rotational traction machine (right).

Biomechanical tests

During these trials, we compared the ability of both surfaces to absorb impacts that occur during the practice of sports at the area of contact between the foot and the playing field. These impacts are transmitted to the player’s musculoskeletal system in the form of vibrations. Therefore, in order to assess the vibrations caused by each type of grass, we called on five soccer players to assist us. Each player was fitted with two accelerometers, fixed firmly to their skin:

♦ On the internal face of the distal half of the tibia of the right leg (Figure 2 -A- and Figure 2 -B-).

♦ On the front of the head, in the middle of the frontal bone (Figure 2 -C-). 

Figure 2. Attaching instruments to one of the soccer players: (A) Measuring the length of the tibia, (B) Attaching the accelerometer to the tibia, (c) Attaching the accelerometer to the frontal bone and (D) Adjusting the telemetry equipment.

The signal from the accelerometers was transmitted to a computer using a telemetry system, so that the soccer players’ mobility was not restricted by the cables (Figure 2 -D-).

On the other hand, the flexion of the knee is a natural mechanism of the human body designed to reduce impact forces. Therefore, in order to measure the degree of bending, we used a video camera to record the test in the player’s sagittal plane. To facilitate the analysis of the video, we made marks on the following anatomic points of the right leg (Figure 3):

♦ Proximal zone of the line that joins the trochanter with the lateral condyle.

♦ Lateral condyle.

♦ Lateral malleolus.

Figure 3. Location of the markers to facilitate the analysis of the video.

In order to measure the power of their jump, each player made at least five correct executions of a validated test known as Drop Jump (DJ). This test consists of letting oneself fall from a certain height and then, after having made contact with the surface, jumping as high as possible again.

Figure 4 shows an example of the accelerometer results for of the tibia and the head during the execution of the movement. The signal indicates the studied parameters corresponding to the peaks of acceleration of the tibia (red) and the head (blue) for the two contacts made with the ground during the jump.

Figure 4. Records from the accelerometers during the executed movement.

RESULTS AND DISCUSSION

Results of the mechanical behavior assessment tests

The values obtained for each of the mechanical parameters that were measured at each test point are represented graphically in figure 5:

Figure 5. Results of the mechanical tests by point.

The area shaded in green indicates the admitted range of values according to the criterion of top-level competition for advanced level soccer pitches. Note that most of the points of the hybrid grass playing field are within the range mentioned although some points are however located at the lower limit. On the contrary, most of the results from the natural grass playing field are outside these ranges.

In addition, the shock absorption and deformation properties of the hybrid grass field present greater homogeneity than the natural grass field.

Finally, on both playing fields, the resistance to rotation presented values generally superior to those set by the aforementioned criterion.

Results of the biomechanical tests

Table 1shows the mean acceleration values obtained on each field during the assessment tests in which people took part:

The hybrid grass field returned mean acceleration values that were lower than the natural grass field. As can be seen in table 2, the statistical analysis of the data (ANOVA) showed that the acceleration that was measured at the second reception (MT2) was statistically lower in the hybrid grass field:

Acceleration by the musculoskeletal system during the execution of the movement is related both to the surface’s cushioning capacity and to the players’ jumping pattern. In the case of the acceleration in the tibia, the measurement obtained in the first reception (MT1) is more influenced by the player’s jumping pattern, given that he or she possesses such mechanisms as knee flexion to cushion the impact. However, in the case of the acceleration measured at the second reception (MT2), the surface’s cushioning capacity has a greater influence on the acceleration in the players’ tibia, given that the fall after the jump is performed with the knee fully extended. In this way, the surface’s superior cushioning capacity translates into a lower acceleration in the players’ tibias at that moment in the execution of the movement.

On the other hand, scientific evidence exists that relates a surface’s lesser cushioning capacity with a higher risk of injury [[1],[2],[3]] which would make the hybrid grass field safer than the natural grass field from the point of view of shock absorption.

Although the differences between the other parameters that we analyzed have not been statistically significant, both the average value of acceleration in the tibia measured after the fall phase and the acceleration in the head are less on the hybrid grass field

As far as knee angles are concerned, there are no noticeable differences between both fields (Table 3).