Epidemiology of Injuries in Young Spanish Soccer Players According to the Playing Positions

Soccer is a complex sport that involves relatively high risks of injury. The high participation rates in soccer has increased the soccer-related injuries among the youth population. There are different physiological demands between playing positions, however a limited amount of studies about the incidence of injuries in soccer players of differing ages and playing position has been published. The aim of this study was to identify the incidence, type, location and severity of injuries in young Spanish soccer players according to playing position in different age groups. There were 431 participants who were male soccer players between the ages of 7 and 23 and they were studied for a full season. All players were classified according to individual playing position: goalkeepers (GK), external defenders (ED), central defenders (CD), central midfielders (CM), external midfielders (EM) and forwards (FW), and according to age groups: younger than 9 years (U9), younger than 11 years (U11), younger than 13 years (U13), younger than 15 years (U15), younger than 18 years (U18), and younger than 23 years (U23). Frequency of type, location and severity of injuries was different according to age and playing position. Injury incidence demonstrated a growth trend as a function of age. Forwards sustained a significantly greater percentage of injuries compared with goalkeepers, central defenders, external defenders and external midfielders. In conclusion, injuries constitute a health threat and knowledge of the epidemiology of injuries in young soccer players is very important in order to be able to develop appropriate preventive measures according to playing positions and age groups.


Introduction
An increase in the physical activity of individuals has many health benefits, but a drawback of this increase is the risk of related injuries (Collard, Verhagen, Chin, Paw & van Mechelen, 2008). Soccer is considered the most popular sport in the world and his practice covers different social strata and age groups, including children and adolescents, due mainly to the perception on the part of parents that the sport is safe (Bastos, Vanderlei, Vanderlei, Júnior & Pastre, 2013;Chena, Rodríguez, Bores & Ramos-Campos, 2019;Van Beijsterveldt, van der Horst, van de Port & Backx, 2013). However, the high participation rates in soccer has increased the soccer-related injuries among the youth population (Kakavelakis, Vlazakis, Vlahakis & Charissis, 2003;Froholdt, Olsen & Bahr, 2009) causing an increase in global health care systems worldwide (Brito, Malina, Seabra, Massada, Soares, Krustrup & Rebelo, 2012). 2020, Retos, 38, 459-464 © Copyright: Federación Española de Asociaciones de Docentes de Educación Física (FEADEF) ISSN: Edición impresa: 1579-1726. Edición Web: 1988-2041 Soccer is a complex contact sport that involves relatively high risks and rates of injury. Constant exposure to repetitive actions places the integrity of bodily structures at risk, especially in cases in which growth and maturation are not yet completely developed, such as in childhood and adolescence. However, most investigators studying exposure-related injuries have focused on adult male professional soccer players (Bastos et al., 2013), where it is considered a high-risk sport (Chena et al., 2019) causing great economic losses worldwide (Brito, Malina, Seabra, Massada, Soares, Krustrup & Rebelo, 2012). In addition to age and maturity, there are other risk factors such as gender, exercise load, previous injuries, physical fitness levels, level of play and standard of training that are associated with soccer injuries (Brito et al., 2012;Pfirrmann, Herbst, Ingelfinger, Simon & Tug, 2016).
Other than the different physiological demands between playing positions, it has been reported that professional soccer players have positional differences with regard incidence of injuries (Hägglund, Waldén & Ekstrand, 2013). However, a limited amount of studies about the incidence of injuries in soccer players of differing ages and playing position has been published (Le Gall, Carling, Reilly, Vandewalle, Church & Rochcongar, 2006).
The purpose of this study was to identify the incidence, type, location and severity of injuries in young Spanish soccer players according to playing position in different age groups to generate new knowledge to develop appropriate preventive measures specific to each case.

Subject
There were 431 participants who were male soccer players between the ages of 7 and 23 and they were studied for a full season. All players had been classified according to individual playing position used in studies (Lago-Peñas et al., 2011): goalkeepers (GK), external defenders (ED), central defenders (CD), central midfielders (CM), external midfielders (EM) and forwards (FW), and according to categories as specified by the Spanish Royal Soccer Federation: younger than or equal to 9 years (U9), younger than or equal to 11 years (U11), younger than or equal to 13 years (U13), younger than or equal to 15 years (U15), younger than or equal to 18 years (U18), and younger than or equal to 23 years (U23) ( Table 1).
All subjects trained 3-4 times per week in sessions of at least 90 minutes. They also played competitions every weekend in matches that varied according to age (40 minutes for U9, 60 minutes for U11, 70 minutes for U13, 80 minutes for U15, and 90 minutes for U18 and U23). All training sessions and matches took place on second generation synthetic turf.
All participants signed a consent form and received a detailed report regarding the confidentiality of the data according to Organic Law 15/1999, dated December 13th (BOE 14-12-1999. Also, an informed consent form was presented to all the soccer players over 18 years and to the legal guardians of the underage players. An assent of soccer players under 18 was also provided along with parental consent. The participants were free to participate in this study. The study was in accordance with the European University of Atlantic´s Ethics Committee and the latest version of the Declaration of Helsinki (World Medical Association, 2001).

Procedures
The study took place following the criteria established through the consensus statement on definitions and data collection procedures of injuries for epidemiological research in soccer (Fuller et al., 2006). These variables were collected by qualified personnel through a questionnaire (Chena et al., 2019), respecting the criteria proposed by the Orchard Sports Injury Classification System (OSICS) (Orchard, Rae, Brooks, Hägglund, Til, Wales & Wood, 2010;Rae & Orchard, 2007).
The data were collected through individual interviews, performed by a single interviewer, using a reported condition inquiry addressing the occurrence of injury and its characteristics in the current season (Bastos et al., 2013). In addition to the information collected in the questionnaire, a weekly interview was performed with the medical services, injured players and the coach or physical trainer (Chena et al., 2019). As Bastos et al. (2013), a pilot study was first conducted to adjust the data acquisition procedures and test the inquiry on a population with similar characteristics to those of the present study, which confirmed the full possibility of use and fit in the proposed methodological design.

Statistical analyses
This study used descriptive procedures with no manipulation of variables nor modification of the natural course of events. It was considered a cohort study due to the longitudinal nature of the research design in which the subjects were selected by the independent variables and were followed throughout the season until a response was detected. Results were presented using a descriptive method. A comparison of proportions was performed using a Chisquare test of independence (÷ 2 ), with Bonferroni corrections of the P values to determine significant difference between groups. In addition, a Chi-square test for trend was performed for some cases in order to test linear trend within a group.
The sample was segmented according to the age groups and the play position before determining that variables did not exhibit normality as the result of a Kolmogorov-Smirnov test. A Kruskal-Wallis non-parametric test was conducted to find p in order to determine possible significant differences between categories. Subsequently, in the variables that showed significance, the test was performed for two independent samples of U Mann-Whitney. In addition, this study used Multivariate Analysis, the relationship between the incidence of typology, location and severity of injuries according to age groups and playing position. The results of the multivariate tests pos hoc found no significant differences for the set of variables studied.
Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS), with significance values set at: á<0.05; á<0.01 y á<0.001.

Results
Total exposure of all the subjects (n=431) was 86,014.78 hours; 75,904.45 hours of training and 10,110.33 hours of competition. The number of hours of training and matches per age group was different for the season ( Table 2).
The distribution of the percentage of injuries sustained per groups is illustrated in Figure 1. There was a growth trend in the percentage of injured players according to age. However, the percentage of injuries was greater among U18 players ( Figure 1A). When comparing the difference in the total number of injuries between groups, FW reported a greater frequency of injuries in comparison to the other playing position ( Figure 1B).
Frequency of injuries was different according to age and playing position. Table 3 shows the number and percentages of injuries according to individual playing position in all age groups.
The 431 soccer players from this study registered a total of 329 injuries in 86,014.78 hours of exposure, with an injury incidence of 3.82 injuries per 1,000 hours. Injury incidence demonstrated a growth trend as a function of age, showing statistically significant differences between players older than 14 (U15, U18, and U23) (Figure 2). FW were the players who suffered the most injuries compared to the other playing positions as shown in table 4. The statistical analysis showed a significant difference in the incidence of injury between goalkeepers, CD, ED and EM. FW sustained a significantly greater incidence of injuries compared with goalkeepers, CD, ED and EM (Table 4).
The majority of injury sites were found in the lower extremities (83.59%). The thigh area was the most affected     .82 a Significant differences with goalkeepers; b Significant differences with external defenders; c Significant differences with central defenders; d Significant differences with central midfielders; e Significant differences with external midfielders; f Significant differences with forwards (p<.05) Π Significant differences with U23; φ Significant differences with U18; # Significant differences with U15; & Significant differences with U13; $ Significant differences with U11; * Significant differences with U9 (p<.001) and knees and ankles were the joints where most injuries occurred, with variable distribution according to the playing position and the age groups. There was a growth trend in the incidence of muscle-tendinous injuries in the players according to age. The majority of observed injuries were muscle-tendinous and joint injuries, totalling 47.11% and 34.95% respectively (155 and 115 injuries of the 329 incidences). The incidence of muscle-tendinous injuries was significantly higher in U15, U18 and U23 soccer players compared to U9, U11 and U13 players. The incidence of joint injuries was significantly higher in U23 while U18 players showed significantly more contusion injuries than in the other categories (Table 5). Results showed that 23.10% of injuries were minor, 34.95% were mild, 27.36% were moderate, and 14.59% were severe.
lthough the statistical analysis found differences in the incidence of injuries according to the typology, location and severity according to the age group, these differences were not found according to positional roles ( Table 6).
The FW showed more joint injuries in U9 (0.84 joint injuries per 1,000 hours of exposure) and the majority were located in the knee and ankle. GK from U11 suffered 0.27 joint injuries per 1,000 hours of exposure which were minor and were located mainly in the ankle. However, MD from U11 showed more muscle and tendon injuries. The incidence of bone injuries was higher in U13 and the CD were the players who showed most injuries of this typology. Muscle and tendon injuries were significantly more frequent in U23, U18 and U15. FW from U23 were the players who suffered more joint and muscle-tendinous injuries with 1.52 and 1.8 injuries per each 1,000 hours respectively. The majority of these injuries were located in the ankle and the back thigh and caused between 3 and 28 days for time loss injury. The EM from U18 showed more joint injuries located in the ankle, while the CD suffered more muscle and tendon injuries. Muscle injuries were distributed between the different positions in U15, but the ED and forward were the players who most injuries suffered in the front thigh while the MD and EM showed more injuries in the back thigh.

Discussion
Injuries constitute a health threat, especially in soccer, where it is considered a high-risk sport (Bastos et al., 2013). Knowledge of the epidemiology of injuries in young soccer players is very important in order to be able to develop appropriate preventive measures (Chena et al., 2019). According to Pfirrmann et al. (2016) the epidemiologic information provides a composite picture of injury prevalence and incidences and can enable researchers to detect possible susceptibilities to injury in different age groups and across different performance levels. Considering that only 15% of the studies refer to youth soccer (Volpi & Bisciotti, 2015) and there is very little information about Spanish soccer players, the aim of this study was to describe the epidemiology of injuries in young Spanish soccer players according to playing position in of all age groups.
Variations in definitions and methodologies have created differences in the results and conclusions obtained from studies of soccer injuries. This has made interstudy comparisons difficult (Fuller et al., 2006). Thus, the definitions of injury, incidence and severity used in the present study follow those employed in other epidemiologic studies on elite soccer (Chena et al., 2019;Van Beijsterveldt et al., 2013;Froholdt et al., 2009;Brito et al., 2012;Le Gall et al., 2006). The epidemiological variables were collected by qualified personnel through a questionnaire (Chena et al., 2019), respecting the criteria proposed by OSICS (Orchard et al., 2010;Rae & Orchard, 2007) as used in other studies (Chena et al., 2019;Brito et al., 2012;Le Gall et al., 2006). Soccer seems to be considered a safe sport in young players (Dvorak, 2009) if the variables and the risks of injury is properly monitored (Brito et al., 2012;Dahlström, Backe, Ekberg, Janson & Timpka, 2012).
The results from this study show a growth trend in the percentage of injured players according to the age with statistically significant differences in the incidence of injuries in players older than 14 compared to youngers players. This event coincided with other publications (Froholdt et al., 2009), however, Brito et al. (2012 considered he overall incidence of injury and incidence of training and match injuries in sub- .14 a Significant differences with U23; b significant differences with U18; c significant differences with U15; d significant differences with U13; e significant differences with U11; f significant differences with U9 (p<.001) 28 U23 GK 0.00 0.14 0.00 0.14 0.00 0.00 0.14 0.00 0.14 0.00 0.00 0.14 ED 0.00 0.41 0.69 0.00 0.14 0.14 0.00 0.41 0.55 0.41 0.14 0.00 CD 0.14 0.14 0.69 0.00 0.00 0.28 0.00 0.14 0.55 0.14 0.41 0.00 CM 0.00 0.41 0.55 0.00 0.14 0.00 0.00 0.28 0.14 0.14 0.41 0.28 EM 0.00 0.41 0.69 0.14 0.14 0.55 0.14 0.28 0.41 0.41 0.41 0.00 FW 0.28 1.52 1.80 0.00 0.14 0.83 0.14 1.38 0.97 1.11 1.24 0.41 elite youth male soccer players did not increase with age. When Le Gall et al. (2006) compared the age groups, also observed that the injury rate was highest in the youngest. On the other hand, an older study showed that in young players, more injuries occurred in the 14-to 16-year-old players than in the 16 to 18-year-old players (Peterson, Junge, Chomiak, Graf-Baumann & Dvorak, 2000). This might be explained by weaknesses in techniques and tactics as well as in muscle strength, endurance, and coordination in the less experienced, younger players. Considering the importance of soccer injuries today, the results found in this study could have benefited from the methodological evolution that Spanish soccer has had in the last years. However, other studies have shown that the injury rate among players aged 16 years or older approaches that of adult players (Froholdt et al., 2009) as shown in this study. Observing the differences found in the incidence of injuries, soccer can be considered a safer sport for players under 12 years of age (Froholdt et al., 2009). Le Gall et al. (2006) showed a total of 4.8 injuries per 1000 hours of exposure time through a cohort study (Prevalence), while our data reported of 3.82 injuries per 1000 hours of exposure time. However, other studies showed a less injuries incidence in youth soccer (Brito et al., 2012).
Attending to playing position, there are little information about young soccer players and injuries, because most of the studies don't show detailed information between 6-23 years. Specific physiological demands exist for different playing positions (Bradley et al., 2009;Di Salvo et al., 2007;Lago-Peñas et al., 2011). According to physiological and tactical demands, it is common to observe that training methods in soccer increasingly require greater specialization. Recent literature has reported that coaches prefer the preparation of their players through the specific methods to maximize training adaptations (Impellizzeri, Rampinini, Coutts, Sassi & Marcora, 2004). In such cases, intensive, specific demands in sports are a potential risk for the occurrence of injuries (Brito et al., 2012;Pfirrmann et al., 2016). However, in spite of the distribution of the percentage of injuries was different according to playing position, only GKfrom U11 and FW from U15, U18 and U23 showed significant differences in the injury incidence. Cloke et al. (2012) reported different results of this study, reporting that midfield players and defenders were the most at-risk groups. However, Dauty and Collon (2011) found no difference in injury incidence or injury severity according to playing position.
The percentage of injuries in relation to playing role was relatively consistent across all the age groups and few significant differences were found (Dauty & Collon;2011). Le Gall et al. (2006) found no significant differences between the role and age group. In spite of various studies have looked at the influence of playing position on injury incidence (Hägglund et al., 2013;Le Gall et al., 2006), the comparisons are difficult because of differing study design and the fact that youth players may not yet have settled into a definitive positional role (Le Gall et al., 2006). Thus, early specialization through training is more directed towards the specific sport than towards the positional role. The coach chooses tactical strategies and training methodology in the training stages. Each player agrees with the coach on a choice of playing position at the beginning of the season, but this decision can change during the season and players often find themselves playing in a different position. Hence, accurate statistical data analysis and interpretation are not possible (Le Gall et al., 2006).
The GK are the most specialized players because their requirements are different compared to other playing positions. However, in spite of GK from U9 and U11 reported a higher frequency of injuries, only GK from U11 showed significant differences in the injury incidence. This difference may suggest that GK acquire injuries prevention skills as they mature (Le Gall et al., 2006).
The results from the present study showed that the total number of injuries sustained per player was greatest in FW followed by midfielders and EM. Most of the injuries observed were muscular and joint injuries, as other studies showed (Bastos et al., 2013;Chena et al., 2019;Kakavelakis et al., 2003;Froholdt et al., 2009;Brito et al., 2012;Pfirmann et al., 2016;Le Gall et al., 2006, Peterson et al., 2000. FW form U9 and goalkeeper from U11 sustained a greater percentage of joint injuries. Age and lack of athletic experience in these players may explain the frequency of this type of injury, because the joint laxity and diminished neuromuscular control in specific athletic movements could increase the risk of this type of accident (Bastos et al., 2013).
Muscular and tendinous injuries that increased with the age. This result that also occurred in previous research (Bastos et al., 2013;Froholdt et al., 2009;Brito et al., 2012). Most of muscle-tendinous injuries were in hip/adductor, front thigh and back thigh in players of U15. This finding could be due to growth in muscle and tendon units arises as a response to earlier bone growth, which in turn causes rigidity in muscles and tendons, especially in cases where they cross two joints. Midfielders from U18 and FW from U23 reported greater number of these injuries and the back thigh was the most affected area. Hamstring strain is the most common single injury in elite soccer (Dauty & Collon, 2011). Although previous studies did not find significant results according to age soccer (Dauty & Collon, 2011). Askling (2011), showed that there are at least two distinctly different types of acute hamstring strains, one occurring during high-speed running and the other during movements leading to extensive lengthening of the hamstrings. This event could be a demonstration that older soccer players show similar epidemiological characteristics to adult players.
Although no difference was found according to playing positions on injuries severity as in other studies (Dauty & Collon, 2011), results of this study showed that severe injuries were less common in young soccer players compared with adult players (Pfirrmann et al., 2016). Older players in the training categories suffered a higher percentage of severe injuries, as other studies had previously shown (Brito et al., 2012). GK from U13 showed more frequency of severe injuries as a result of injuries located in wrist, knee and ankle. Le Gall et al. (2006) observed that GK had significantly more upper body and hand injuries, however, our results showed no significant differences. Midfielders from U18 suffered a higher frequency of moderate injuries and FW from U18 and U23 reported more severe injuries. This finding could be due to the possible discrepancy between the coach-intended intensity and the actual training intensity perceived by the player in this age groups. Brink et al. (2014) showed that under 19 players reported a higher intensity and training load than the coach intended.
The U13 GK showed most severe injuries as result of injuries to the wrist, knee and ankle. For other authors, professional GK have significantly more upper body and hand injuries compared to outfield players (Le Gall et al., 2006;Dauty & Collon, 2011), however, in this study found no significant difference between the playing position players about this. Midfielders from U18 reported a greater frequency of muscle injuries in comparison to the other playing position. FW and midfielders from U23 had more injuries. These results were also observed in other studies (Cloke, Moore, Shah, Rushton, Shirley & Deehan, 2012) and could be in keeping with the view that greater intensity activity and distance covered during the practice are met with a higher risk of musculoskeletal injury.
Therefore, we conclude that the injury ratio is more balanced in elite youth participants (Pfirrmann et al., 2016). Whereas the epidemiological results reported in youth soccer did not indicate a higher incidence of injury than other youth sports, the results suggested that soccer is a safe physical activity for the youth (Van Beijsterveldt et al., 2013;Froholdt et al., 2009;Brito et al., 2012;Dvorak, 2009). Nevertheless, there is agreement on the need to use a surveillance system to monitor the injuries and to understand the magnitude of the problem before undertaking the corresponding preventive strategies (Dvorak, 2009;Cloke et al., 2012;Dauty & Collon, 2011). Age groups and player position influenced the normalized risk of injury. Although many significant differences were not found with respect to the playing position as in other studies, these results reinforce the case for injury prevention training specific position.