Influence of the artificial turf certification on physical performance and muscle damage in football players (QUALTURF PROJECT)

Introduction

Football is experienced by actions of great intensity such as jumps, changes of direction, accelerations, decelerations and sprints along with moments of recovery^1,2. Therefore, physical³ and physiological⁴ parameters influence the performance of football players. On the other hand, an influential external variable in the game is the sports surface. It has been proven that the state of the playing field affects the performance^5,6 and the injury risk⁷ of the football player. The first comparative studies between surfaces were aimed at comparing the rate of injuries with respect to natural grass compared to artificial grass [(male match rate ratio 1.0 (95% CI 0.9–1.2); and female match rate ratio 1.2 (95% CI 0.8–1.8)]⁸.

Most of the research has focused on comparing the properties of artificial turf compared to natural grass, especially in terms of safety, with the risk of injury being the most analysed component^7–9. Meanwhile, Andersson, Ekblom and Krustrup¹⁰ analysed the impact of these two surfaces on movement patterns in total distance (10.19 km, s = 0.19 vs. 10.33 km, s = 0.23), high-intensity running (1.86 km, s = 0.10 vs. 1.87 km, s = 0.14), number of sprints (21, s = 1 vs. 22, s = 2), both p < 0.05 on artificial turf than natural grass. Another line of research has been related to the lifecycle of the artificial turf, in order to control the variables that ensure the maximum durability of the surface. In this regard, the effect of the specification and maintenance¹¹ related to the deterioration of the mechanical properties of the surface as a result of their exploitation has been more addressed¹². The results of these studies have generated a great development in the last generation of artificial turf, allowing the existence of a large of combinations of structural components, resulting in different types of artificial turf fields. Although it may seem an unimportant fact, some authors believe that the differences between the different types may be greater than those between natural grass and artificial turf¹³. Previous investigations¹⁴ noted the significant differences between the different types of surface in their mechanical properties, focusing mainly on the support structure (sub-base and elastic layer). In this sense, it is very important to consider that the development and evolution of safe and quality artificial turf systems guarantee substantial savings in water consumption. Thereby, this situation consolidating the goal 6 in the 2030 Water and Agenda 'Ensure access to water and sanitation for all ', especially in underdeveloped and developing countries.

However, one of the areas that needs more study on the use of artificial turf in elite football is the influence on sport performance, which has been avoided by the scientific investigations so far. Some previous studies showed a clear influence of the playing surface (sand, asphalt and artificial turf) on physical parameters, physiological and psychological parameters. In this context, one physiological factor to consider is muscle damage. Different studies have evaluated muscle damage between very disparate surfaces, such as sand and grass or wood, showing that firm surfaces (e.g. grass or wood) generate greater muscle damage in eccentric movements, such as those used in football¹⁵. This muscle damage is less when faced with the same stimulus and the greater its recovery¹⁶.

On the other hand, this physiological variable has not been studied in artificial grass fields based on its mechanical properties. The mechanical properties may be in the correct parameters. An excess above or below the recommendations could negatively influence the interaction with the athlete. For example, a soft surface prevents the risk associated with repeated impacts, but on the contrary it would be associated with an excessive increase in physiological demand, earlier fatigue and risk of injuries associated with overload¹⁷. Consequently, the ideal would be to find the optimum point of the mechanical properties. For that reason, the aim of this study was to analyse the influence of the FIFA Quality PRO certification of artificial grass pitches on the physical and physiological performance and muscle damage in football players.

Results

The two-way ANOVA test revealed differences between the quality of the surfaces and the half of the test in the variables evaluated (p < 0.05; Table 1). Results reveal a significant reduction in the sprint time of 12 m in the second half of the test compared to the first half in the FIFA Quality Pro certification field (− 0.08 s; CI 95% − 0.14 to 0.02; ES: 0.489). Similarly, the 12 m sprint times during the first half in the non-certified field were lower than the times obtained on the FIFA Quality Pro surface (− 0.10 s; CI 95% − 0.18 to 0.03; ES: 0.649). For the second half of the test, the average time of the 20 m sprints was longer on the FIFA Quality Pro surface (+ 0.13 s; CI 95% − 0.01 to 0.27; ES: 0.305). However, the average jump height was lower on the non-certified surface both in the first (− 2.00 cm; CI 95% − 3.76 to 0.23; ES: 0.297), as in the second half (− 2.01 cm; 95% CI − 4.17 to 0.16; ES: 0.274). Finally, the effort perceived by the players was higher in the first (+ 2.64; CI 95% 0.92 to 4.35; ES: 1.421) and the second half (+ 1.35; CI 95% − 0.02 to 2.72; ES: 0.637) of the test on the FIFA Quality Pro field. In this sense, the players showed a significant increase in the effort perceived in the second half of the non-certified field (+ 1.79; CI 95% 1.11 to 2.46; ES: 0.778).

The results of external load showed a greater distance travelled over the non-certified field (+ 386.01 m; CI 95% 34.93 to 737.09; ES: 0.710; Table 2). Nevertheless, the number of sprints was lower compared to the FIFA Quality Pro surface (− 8.63; CI 95% − 17.03 to − 0.22; ES: 0.843). No significant differences were found between surfaces in high-intensity distances (p > 0.05; Fig. 1), except for distance in zone 2 (− 390.39 m; CI 95% − 467.06 to − 313.71; ES: 0.91). In relation to internal load, players showed higher HR_MEAN on the non-certified field in absolute (+ 3.60 b.p.m.; CI 95% 0.31 to 6.89; ES: 0.470) and relative terms (+ 1.90%; CI 95% 0.13 to 3.67; ES: 0.490). The comparative analysis between surfaces did not show significant differences in the time spent in each of the heart rate zones (p > 0.05; Fig. 2).

The assessments made before, during and after the BEAST₉₀ showed a greater perceived effort (+ 2.32; CI 95% 0.27 to 4.39; ES: 7.093) and a lower Flicker score (− 1.49; CI 95% − 2.79 to − 0.19; ES: 0.113) at half-time in the FIFA Quality Pro field compared to the non-certified surface (p < 0.05; Table 3). Once the test was finished, players revealed lower levels of force in the upper train (+ 4.11; CI 95% 0.75 to 7.48; ES: 0.572), as well as higher concentrations of CK (+ 196.58; CI 95% 66.54 to 326.61; ES: 1.645) in the field with the non-certified surface (p < 0.05). In this field, BEAST90 test caused a significant increase in blood lactate concentration (+ 3.94; CI95% 1.15 to 6.72; ES: 1.496), as well as a reduction of the PEF (− 1.39; CI95% − 2.11 to − 0.89; ES: 0.742) compared to the baseline measurements (p < 0.05). CK (+ 105.58; CI 95% − 4.12 to 215.28; ES: 1.168) and RPE values (+ 9.77; CI 95% 8.07 to 11.62; ES: 7.093) increased in the FIFA Quality Pro field (Table 4).

Discussion

This research exposes the influence of the mechanical property certification of the artificial turf football fields on the psychophysiological sport performance, which is evaluated through the BEAST₉₀ test. The evaluation of sport performance based on field certification (comparing a FIFA Quality Pro and a non-certified surface) makes this study a pioneer in the scientific area. The main finding of this research was that a different mechanical behaviour between artificial turf pitches produced different psychophysiological responses in football players in a simulated game protocol (BEAST₉₀ test).

Previous studies have already demonstrated the influence of the physical and physiological response between natural and artificial turf football fields^5,18, or among different artificial turf surfaces¹⁹. However, the psychophysiological response based on the certification of the pitch had not been evidenced previously. Burillo et al.²⁰ consider it necessary to raise awareness and demonstrate quality standards to achieve sustainability (economic, physical performance and health of the athlete) in the football world. The FIFA Quality Pro surface had the highest levels of StV and FR, complying with the most demanding certification values. This aspect guarantees a less hard playing field with optimal mechanical properties for the practice of elite football¹⁹. The mechanical differences between surfaces ranged between 54.94% and 63.17%, coinciding with results of previous studies that affirm that the differences between artificial turf fields with different structures can be greater than the variation between an artificial turf field and a natural one²¹.

The inclusion of the ER values in this study provides more details about hardness and the absorption capacity of the surface¹⁹. A reduced cushioning capacity and an increase in the energy of restitution of the artificial turf surface can modify the performance of football players¹⁹, especially in high-intensity actions as the results of this study have evidenced on the external load. ER measures the energy returned to the player from the surface after an impact²². This mechanical property of the surface has been shown to be decisive in the physical performance of the players, improving the sprint times¹⁹. In this case, the uncertified field revealed a 56.78% increase in energy returned from the playing surface after an impact. This result may explain the best times in 12 m and 20 m sprints in the non-certified football field. This suggests that the percentages of FR and StV were high enough (according to current regulation) to generate an increase in the sprint times derived from the reduction of the reaction forces as a result of the partial absorption of the energy applied in FIFA Quality Pro artificial turf field²³. Certainly, the lower contact time during running on the non-certified artificial turf system²¹ and the less frequent reuse of stored elastic energy¹⁶ of the artificial turf systems with a higher damping capacity can explain the differences in the covered distance and sprint times found in the present research. However, a higher jump height was identified on the FIFA Quality Pro artificial turf field, probably because of the potentiation effect to compensate the degradation of elastic energy and lower muscle damage caused by a higher force reduction on the surface¹⁵.

This reduction in the energy returned by the surface could explain the higher RPE values found during the test in the FIFA Quality Pro in comparison to the non-certified field. This was already manifested by previous authors who demonstrated that the perceived effort is higher in comfortable and softer playgrounds, although the evidenced physiological response is usually similar^19,24. A lower muscle-sinew efficiency²⁵ or a higher hip and knee flexion²⁶ could clarify this result due to a higher energy outlay on surfaces with a higher impact reduction. However, the physiological load was slightly higher in the non-certified field (76.30 vs 74.40% HR_MEAN). This result shows that the perceived demand is more related to the difficulty in running (external load) than to the physiological responses (internal load)²⁴, since no differences were found in the blood lactate concentrations and the time spent in the different heart rate zones. The greater perceived effort on the surface with greater damping capacity was more evident at the end of the first part of the test. A lower running efficiency²⁷ or a worse use of the generated energy^28,29 could determine an early perceived fatigue in the FIFA Quality Pro field. It is possible that a higher degree of muscular engagement and activation on the softest surface may have increased the football players’ lower-limb muscle awareness and perception of fatigue³⁰.

Similar high-intensity actions (speeds over 21 km h⁻¹; accelerations and decelerations) were found on both surfaces. However, higher total distances travelled in the non-certified field were found. In addition, the HR_MEAN was significantly higher in this field both in absolute (b.p.m.) and relative (%) values. In the same vein, Sánchez-Sánchez et al.³¹ showed that the hardest artificial turf surfaces (with less normative adjustment) produce certain improvements in some performance parameters, such as total distance travelled, and as in our study, they found no differences in heart rate, blood lactate or high-intensity accelerations among surfaces with better or worst adjustment to FIFA regulations. As stated by Nédélec et al.¹⁸ the hardest playing surfaces do not produce any extra load of muscular work. Thereby, these actions with greater physical demands could obtain the same values regardless of the type or the normative adjustment of the surface. However, the non-certified field revealed a higher heterogeneity between the first and the second half of the test in the physical performance, probably due to a higher muscle damage on this surface^15,16.

Regarding the cortical response, we found how in the middle test the rate of perceived exertion in the FIFA Quality Pro was higher than on the non-certified field, nevertheless the cortical arousal was not in line with perceived exertion, since on the FIFA Quality Pro field the cortical arousal remained close to the basal values and in the non-certified presented a decreased value. This result showed the higher negative impact of the non-certified field at cortical level³², but after the test, the cortical response was similar in both fields, showing the necessity for future research to explain this effect. Linked to the cortical, muscular control and strength manifestation we found how the FIFA Quality Pro field allows players to maintain strength of muscle not directly implied in the activity (HIS) and respiratory muscles (spirometry variables), and a tendency to increase strength manifestation of muscle directly implied in the activity (horizontal jump test), while this tendency was not shown in the non-certification field. The maintained or even increased strength manifestation could be explained by the increased sympathetic modulation due to the eliciting characteristic of the BEAST₉₀ test³³, the lower blood lactate concentration, as well as the lower muscular destruction (CK) evaluated in the FIFA Quality Pro field³⁴. In this context, the lower CK values reported in the FIFA Quality Pro field highlighted the lower muscular impact of this field in the players’ muscle system, a fundamental fact to allow them a shorter recovery period after training and competitions as well as a decreased injury risk³⁵. According to previous studies, softer and accredited artificial turf fields can reduce the impact received during the test and protect against injuries associated with the impacts compared to harder artificial turf surfaces¹⁷.

This study has several limitations. Firstly, many of the significant differences found based on the p values (p < 0.05) include values above and below 0 in the CI 95%. In these cases, the upper or lower value is very close to 0, however, the results may be considered with caution and may be contrasted with larger samples in future studies. Secondly, the data have been obtained with amateur football players, therefore, they cannot be replicated in professional football.

Conclusions

In conclusion, FIFA Quality Pro fields, in comparison with fields that do not comply with the surface performance parameters, could reduce the impact on the muscle, musculoskeletal strain and therefore, improve the training stimulus. A higher force reduction and lower energy restitution diminished the muscle damage of the players, which could accelerate the recovery process after the training sessions and matches, and protect more protection against injuries compared to non-certified harder artificial turf surfaces, especially at a time in the season where a reduced musculoskeletal loading would be necessary (i.e. congested competition schedule). These insights can provide the opportunity to maximize the efficiency of the training sessions and reduce the risk of injury during the season. A strategy to be followed by coaches and players who practice football on non-certified artificial turf pitches could be to perform the plyometric and eccentric exercises characteristic of soccer on sand, since this surface has lower restitution energy and greater force reduction⁶.

Material and methods

Author contributions

Conceptualization, J.L.F., D.V.R. and J.S.S.; methodology, J.G.U and V.J.C. formal analysis, J.G.U. and J.S.S.; investigation, A.H.M., J.L.F. and D.V.R.; resources, J.L.F. and J.S.S..; writing—original draft preparation, J.L.F., J.G.U., D.V.R., A.H.M., V.J.C.; writing—review and editing, J.S.S. and L.G.; supervision, J.L.F.; project administration, J.S.S.; All authors have read and agreed to the published version of the manuscript.

Competing interests

The authors declare no competing interests.