Abstract
Background:
Sport specialization is associated with increased risk of overuse injuries. However, the effects of sport specialization on the risk of overuse injury may differ by specific sport and sociocultural parameters. This study aimed to determine the associations of sport participation and training volume with previous overuse injuries in Japanese male youth soccer players.
Hypothesis:
Being highly specialized before the age of 12 years and high training volume relative to participant age would be associated with a history of serious overuse injury.
Study Design:
Cross-sectional study.
Level of Evidence:
Level 3.
Methods:
Survey data were collected from male youth soccer players (seventh-ninth grade) during midseason of the 2020-2021 academic year. Participants completed a questionnaire regarding their status of sport participation (single- and multisport athletes), weekly sports volume in first- sixth grade and injury history in the previous year. Odds ratio (OR) was calculated to identify associations of specialization and training volume with a history of serious overuse injury.
Results:
A total of 841 players (mean age, 13.7 ± 0.9 years) participated; 11% reported serious overuse injury in the previous year, and 64% participated in other sports before the age of 12 years. Players who participated in other sports at <12 years old (OR, 1.79; 95% CI, 1.09-2.94; P = 0.02) and those who trained for more weekly hours than their age in fifth to sixth grade (OR, 1.70; 95% CI, 1.07-2.72; P = 0.03) had greater odds of reporting serious overuse injuries.
Conclusion:
Multisport athletes and high training volume in fifth-sixth grade were associated with previous serious overuse injuries.
Clinical Relevance:
Coaches and parents of male youth soccer players should be aware of the increased risk of serious overuse injury participating in multiple sports simultaneously, and youth athletes should adhere to age-appropriate volume recommendations.
Keywords: soccer, sport specialization, overuse injury
An increasing number of youth athletes are focusing and training year-round in a single sport from an early age. 15 Many youth athletes are specialized in a single sport due to pressure from parents and coaches, as well as the desire to become an elite athlete. 24 Sport specialization is defined as “intentional and focused participation in a single sport for a majority of the year that restricts opportunities for engagement in other sports and activities.” 2 Also, previous studies used “before the age of 12 years” as the criteria for early sport specialization.6,19,20 Recent articles have reported that prevalence rates of sport specialization vary depending on demographic factors such as athletes’ sex, age, and sport.5,27,29 In addition to these aspects, geographic/regional specific culture may impact specialization status.5,17 In terms of geographic/regional specific culture differences, although >90% of young Japanese athletes aged 13 to 18 years focused on a single sport, approximately 70% of athletes in the same age group in the United States played multiple sports.3,30
Sport specialization is associated with increased risk of overuse injuries. 4 Several studies have noted that specializing in a single sport is associated with overuse injuries in youth athletes aged 7 to 18 years.11,16,26,28 Other studies also reviewed timing of specialization, and showed that athletes who were specialized before the age of 12 to 14 years sustained more injuries than those who were specialized at, or after, the age of 12 to 14 years.1,7,20 In addition, due to underdeveloped musculoskeletal systems, high training volume is a risk factor that influences the prevalence of overuse injuries in youth athletes.14,28,31,32 To reduce the risk of overuse injury in youth sports, several studies have suggested weekly training hours less than one’s age and maintaining a ratio of 2:1 volume between organized sports and free play.12,16
The association between sport specialization and overuse injuries has been reported by several studies, but systematic reviews have concluded that the association is moderate and the evidence is limited. 4 One of the factors contributing to the limited understanding of the association between sport specialization and overuse injuries may be that these data consisted of athletes from various sports, and of different ages and sex. Furthermore, studies examining the association of sport specialization and training volume with injuries in youth soccer players are lacking, with 2 studies finding an association of sport specialization with serious injuries in female youth soccer players, 25 and another study finding no association in male youth soccer players. 9 Additional factors contributing to the lack of understanding of the association between sport specialization and injury is the inconsistent definition of “specialization.” 23 The most commonly used definition of sport specialization is the 3-point Jayanthi scale based on the participant’s answers to 3 questions 16 : “Can you pick a main sport?”, “Did you quit other sports to focus on a main sport?”, and “Do you train >8 months in a year?” However, there are shortcomings in this 3-point scale. First, this 3-point scale cannot classify late or early specialized athletes. Second, athletes who only ever participated in a single sport cannot accurately answer the questions of “Can you pick a main sport?” and “Did you quit other sports to focus on a main sport?” since they did not have a chance to pick a main sport or to quit other sports. Finally, if applying the third criterion (Do you train >8 months in a year?) of the Jayanthi scale to Japanese male youth soccer players, they are all likely to respond “yes” because they generally play their sports year-round. Therefore, it is difficult to measure sport specialization of Japanese male youth soccer players by using the above-mentioned scale.
Fewer data examine cultural influences on sport specialization. 22 A survey of elite youth soccer players from 7 different countries showed that British youth soccer players participated in a greater variety of other sports during childhood compared with youth soccer players from other countries. 8 Internationally born athletes in the Major League Soccer (MLS) competition also specialized at a younger age than athletes born in the United States. 17 Thus, given that youth athletes in different countries do not follow the same developmental pathways, the effect of sport specialization on the risk of overuse injuries may differ depending on youth sport culture and region/country. Little is known about sport participation patterns related to sport specialization in Japanese youth soccer players, and we are unaware of any previous studies that have investigated whether or not sport sampling and high-volume training are associated with overuse injuries.
Therefore, the purpose of this study was to investigate sport participation patterns in Japanese male youth soccer players, and to determine the association of sport participation status (single- and multisport athletes) and training volume with previous serious overuse injuries. We hypothesized that athletes who are specialized to soccer before the age of 12 years and weekly training hours more than their age would be more likely to report previous serious overuse injuries.
Methods
Study Design
We used a cross-sectional study design. An online survey was distributed to male youth soccer players (seventh-ninth grade) during midseason of the 2020-2021 academic year. Both parents and participants received an information sheet describing the study, and completing the survey was deemed as obtaining parental and participant consent. This study followed the Declaration of Helsinki principles and was approved by the Ethics Review Committee on Research with Human Subjects of Waseda University.
Participants
Survey data were collected from participants between seventh and ninth grades who belonged to a youth soccer teams of the first and second divisions in the Kanto region, Japan. Under-15 soccer in Japan is organized into leagues in 9 regions, including Kanto region. Approximately 450 teams belong to the Kanto region and, out of 6 divisions, the first and second divisions consisted of 30 teams. The primary investigator contacted team personnel such as head coaches, assistant coaches, and athletic trainers via email, and 12 teams agreed to participate in the current study.
We excluded male youth soccer players who had participated in organized soccer for <1 year, below 6th grade and above 10th grade during the 2020-2021 academic year. We sent the survey recruitment to the coaches or athletic trainers of the teams that met the requirement, and the survey information sheet was distributed to potential participants through the coaches or athletic trainers. If they wished to participate, they scanned the QR code or read the URL on the information sheet and provided information about the survey, confidentiality language, and the contact address for the survey.
Questionnaire
Participants completed a self-administered and anonymous electronic questionnaire regarding demographics, sport participation status (single- and multisport athlete), weekly sports volume, and injury history in the previous year with assistance from their parent or legal guardian, if needed. Athletes reported all the organized sports they participated in during the first through sixth grade. We classified participants into 2 groups: those athletes who played soccer and had not played any other sport (single-sport athletes) and those athletes who played soccer and also had played other sports (multisport athletes) before the age of 12 years. In this study, the age of 12 years was the age of criteria for early sport specialization, in accordance with the AOSSM Early Sport Specialization Consensus Statement. 19 In addition, we asked participants to report the typical number of sessions performed per week, and hours per session spent in organized sports and free play (unorganized sports) during the first to second, third to fourth and fifth to sixth grades, and defined the product of session frequency per week and hours per session as weekly sport volume. These responses were used to classify athletes exceeding the sports volume recommendations: weekly participation hours in organized sports greater than child age, and training ratio (organized sports:free play ratio) >2:1 in each grade. Finally, athletes were asked to list up to 3 injuries sustained in the previous year for 18 different body regions (head, neck, shoulder, upper arm, elbow, forearm, wrist, hand, chest, thoracic spine, abdomen, low back, hip/groin, thigh, knee, lower leg, ankle, foot). An injury was defined as “any physical complaint sustained by an athlete that resulted from soccer training or match and led to the athletes being unable to take full part in future soccer training or match play.” For each body region, participants reported the number of days missed and the nature of the injury (acute or overuse). Serious overuse injury was defined as overuse injury >28 days of time lost. 10
Statistical Analysis
Data were summarized as means and standard deviations, frequencies and proportions (%), and odds ratios (ORs) and 95% CIs. Normality of the data distribution was determined by the Shapiro-Wilk test, and independent t test or Mann-Whitney U test was used if non-normal distribution patterns were found. In addition, chi-square tests were used for categorical variables. First, those tests were used to compare the demographic variables and the number of hours per week spent in organized sports during the first to second, third to fourth and fifth to sixth grades between single- and multisport athletes, and to identify differences in demographic variables and sports volume recommendation adherence between athletes with or without a history of serious overuse injury. Second, a binary logistic regression analysis was utilized to examine associations between variables of interest from the univariate analysis and a history of serious overuse injury, adjusting for differences in age and body mass index (BMI). Significance was set at P < 0.05, and all analyses were performed with SPSS statistical software (Version 26.0; IBM Corp).
Results
Participant Characteristics
A total of 841 male soccer players (N = 301 seventh graders, N = 296 eighth graders, and N = 244 ninth graders) completed the questionnaire. Demographic characteristics for the entire sample and the number of hours per week spent in organized sports during the first to second, third to fourth and fifth to sixth grades for single- and multisport athletes are presented in Table 1. A total of 542 (64%) players participated in other sports before the age of 12 years (multisport athletes) and the 3 most common additional other sports were swimming (N = 286, 53%), gymnastics (N = 64, 12%) and Futsal (N = 62, 11%). Multisport athletes were also more likely to have greater weekly sports volume across first to sixth grade than single-sport athletes (Table 1). With regards to age of specialization, multisport athletes were specialized in soccer (quit other sports to focus on soccer) at approximately 10 years old (mean age of 9.9 ± 1.7 years). The proportion of athletes with a training ratio >2 increased as the grade level increased. However, the proportion of athletes with weekly training volume greater than their age did not change as grade level increased (Figure 1).
Table 1.
Participant demographics a
| Variable | Total n = 841 | Sport Participation Status | P | |
|---|---|---|---|---|
| Single-Sport Athletes n = 299 | Multisport Athletes n = 542 | |||
| Age, y | 13.7 ± 0.9 | 13.7 ± 1.0 | 13.8 ± 0.9 | 0.29 |
| Mass, kg | 54.2 ± 9.5 | 52.3 ± 8.5 | 52.6 ± 8.4 | 0.80 |
| Height, m | 1.66 ± 0.08 | 1.64 ± 0.08 | 1.65 ± 0.08 | 0.15 |
| BMI, kg/m2 | 19.7 ± 2.5 | 19.2 ± 1.9 | 19.3 ± 1.9 | 0.51 |
| Weekly hours in organized sports | ||||
| First-second grade, hours/week | 6.0 ± 3.4 | 5.3 ± 3.3 | 6.4 ± 3.4 | <0.01* |
| Third-fourth grade, hours/week | 8.8 ± 4.0 | 8.0 ± 3.7 | 9.2 ± 4.1 | <0.01* |
| Fifth-sixth grade, hours/week | 10.2 ± 4.5 | 9.7 ± 4.2 | 10.5 ± 4.6 | <0.01* |
BMI, body mass index.
Values are presented as mean ± SD.
P < 0.05.
Figure 1.
Distribution of players exceeding the recommended value for sports activities first to second, third to fourth and fifth to sixth grades. (a) Training ratio >2, organized sport:free play ratio >2:1. (b) Training volume >age, weekly hours in organized sport >age.
Injury History in the Previous Year
Locations of overuse injuries are shown in Figure 2. A total of 228 (27%) players reported a total of 277 overuse injuries in the previous year. The knee (26%) was the most common site of overuse injuries, followed by low back (24%) and hip/groin (18%) for all overuse injuries. Also, 95 (11%) players reported a total of 108 serious overuse injuries in the previous year. The lower back (35%) was the most common site of serious overuse injuries, followed by the knee (22%) and hip/groin (20%). Diagnostic type of the common serious overuse injuries included fractures (55%), osteochondrosis (17%), and muscle strain (11%).
Figure 2.
Distribution of the severity of overuse injury location.
Association of Sport Sampling and Training Volume With Previous Serious Overuse Injuries
Differences between participations with or without history of serious overuse injuries are presented in Table 2. History of previous serious overuse injury was associated with multisport athletes and athletes exceeding weekly training volume by more than their age in fifth to sixth grade. Other variables (height, mass, BMI, weekly hours in organized sports, weekly hours in free play and training ratio) revealed no significant difference between players with a history of serious overuse injuries and players without a history of serious overuse injuries.
Table 2.
Differences between participants with or without history of serious overuse injuries a
| Serious Overuse Injury History in the Previous Year | |||
|---|---|---|---|
| Variable | No (n = 746) | Yes (n = 95) | P |
| Age, y | 13.7 ± 0.9 | 13.8 ± 0.9 | 0.32 |
| Mass, kg | 52.4 ± 8.4 | 53.7 ± 8.8 | 0.16 |
| Height, m | 1.65 ± 0.08 | 1.66 ± 0.08 | 0.24 |
| BMI, kg/m2 | 19.2 ± 1.9 | 19.4 ± 2.0 | 0.25 |
| Weekly hours in organized sports | |||
| First-second grade, hours/week | 6.0 ± 3.5 | 5.6 ± 3.0 | 0.26 |
| Third-fourth grade, hours/week | 8.8 ± 4.0 | 8.7 ± 3.6 | 0.85 |
| Fifth-sixth grade, hours/week | 10.1 ± 4.4 | 10.8 ± 5.1 | 0.16 |
| Weekly hours in free play | |||
| First-second grade, hours/week | 6.5 ± 5.3 | 5.6 ± 4.5 | 0.05 |
| Third-fourth grade, hours/week | 5.8 ± 5.2 | 4.9 ± 4.9 | 0.14 |
| Fifth-sixth grade, hours/week | 5.6 ± 5.3 | 4.9 ± 5.2 | 0.19 |
| Sport participation | 0.01* | ||
| Single-sport athlete | 276 (92.3) | 23 (7.7) | |
| Multisport athlete | 470 (86.7) | 72 (13.2) | |
| Training ratio (organized sport: free play ratio) >2:1 | |||
| First-second grade | 0.19 | ||
| No | 635 (89.3) | 76 (10.7) | |
| Yes | 111 (85.4) | 19 (14.6) | |
| Third-fourth grade | 0.10 | ||
| No | 440 (88.7) | 56 (11.3) | |
| Yes | 306 (88.7) | 39 (11.3) | |
| Fifth-sixth grade | 0.36 | ||
| No | 383 (89.7) | 44 (10.3) | |
| Yes | 363 (87.7) | 51 (12.3) | |
| Weekly hours in organized sports > age | |||
| First-second grade | 0.22 | ||
| No | 579 (88.0) | 79 (12.0) | |
| Yes | 167 (91.3) | 16 (8.7) | |
| Third-fourth grade | 0.54 | ||
| No | 535 (88.3) | 71 (11.7) | |
| Yes | 211 (89.8) | 24 (10.2) | |
| Fifth-sixth grade | 0.02* | ||
| No | 583 (90.1) | 64 (9.9) | |
| Yes | 163 (84.0) | 31 (16.0) | |
BMI, body mass index.
Values are presented as mean ± SD.
P < 0.05.
The OR from binary logistic regression analysis is presented in Table 3. After accounting for age and BMI, logistic regression analysis showed that players who participated in other sports before the age of 12 years (OR=1.79; 95% CI, 1.09−2.94; p=0.02) and those who trained in weekly hours over their age for 5th-6th grade (OR=1.70; 95% CI, 1.07−2.72; p=0.03) had greater odds of reporting serious low back and lower extremity overuse injury.
Table 3.
OR for sport specialization before the age of 12 years and serious overuse injury history in the previous year a
| Serious Overuse Injury History in the Previous Year | ||
|---|---|---|
| OR (95% CI) | P | |
| Age | 1.07 (0.84-1.37) | 0.57 |
| BMI | 1.07 (0.95-1.20) | 0.24 |
| Sport participatioins, single-sport athlete | - | |
| Sport participation, multisport athlete | 1.79 (1.09-2.94) | 0.02* |
| Weekly hours in organized sports > age at fifth-sixth grade - No | - | |
| Weekly hours in organized sports > age at fifth-sixth grade - Yes | 1.70 (1.07-2.72) | 0.03* |
BMI, body mass index; OR, odds ratio.
Values are presented as mean ± SD.
P < 0.05.
Discussion
To our knowledge, this is the first study to examine the association of sport specialization and training volume with previous serious overuse injuries in Japanese male youth soccer players. This study demonstrated that playing multiple sports was associated with greater likelihood of serious overuse injury in Japanese male youth soccer players. The odds of reporting a previous serious overuse injury were also higher in youth soccer players whose weekly training hours were more than their age in fifth-sixth grades compared with players whose weekly training hours were less than their age.
The most important finding of this study was that playing multiple sports, rather than a single sport, was associated with a history of serious overuse injury, contrary to our hypothesis and previous studies.16,25 In a retrospective study of approximately 1200 youth athletes aged 7 to 18 years in various sports, single-sport athletes had an increased odds of serious overuse injury compared with multisport athletes. 16 In a prospective cohort study of 351 female youth soccer players aged 12 to 15 years, those who played other sports in addition to soccer had a 61% decreased odds of lower extremity overuse injury compared with those who played only soccer. 25 However, in a retrospective study of approximately 2000 male youth soccer players aged 7 to 17 years, single-sport athletes had similar odds of lower extremity overuse injury as multisport athletes. 9
Taken together, the present and previous findings suggest that the effect of early sport specialization on overuse injury risk depends on multiple factors, including sports, sex, age, and sociocultural factors. Moreover, one of the main reasons why the findings of this study were not consistent with the results of previous studies may be the difference in the concept of multisport between Japan and some other countries. A multisport athlete is typically considered an athlete who participates in multiple sports in a year (Figure 3a). 18 However, since participation in most sports is generally year-round in Japan, multisport athletes in Japan perform multiple sports year-round (Figure 3b). As a result, the overall training volume of Japanese youth athletes is high (Table 1), which, consequently, gives little time for recovery. This may have led to increased risk of serious overuse injuries. From a clinical perspective, athletic trainers, coaches, and parents should be aware of serious overuse injury risk associated with sport participation and training volume. Furthermore, adequate rest needs to be provided for youth athletes, especially for multisport athletes in Japan.
Figure 3.
Multisport models in North America and Japan. (a) Multisport model commonly used in North America in middle and high schools. A different sport is performed in each season throughout a year. (b) Multisport model in Japan. Although the number of sports is 2, each sport requires year-long commitment, which increases overall training volume.
Consistent with our hypothesis and the findings of others,13,16,21,28 this study found that athletes with weekly training hours over their age for fifth to sixth grade had greater odds of reporting serious overuse injury. Interestingly, despite using different methodologies (e.g., different sports, sex, and ages), the present and previous studies all came to similar conclusions. Synthesizing all evidence including the results of the current study, this evidence is valuable and practical: training for a greater number of hours than one’s age increases the risk of overuse injury, regardless of sport, sex, age, and sociocultural factors. Youth athletes, parents, and coaches should foster this principle and plan weekly training volume within one’s age to minimize the risk of overuse injury.
Limitation
This study has several limitations. Because we used a cross-sectional design, our data relied heavily on the recall ability of participants with regards to sports activities and injury history and it was not possible to establish causality between previous injury and early sport specialization. To minimize recall bias, we encouraged the parents to assist their children in completing the questionnaire, if needed. However, we acknowledge that identifying the long-term effects of childhood sports activities and training volume on overuse injury requires a longitudinal study design. This sample was also relatively small and included only male youth soccer players from the Kanto region of Japan. However, the survey response rate was 77.0% (841/1092). In addition, it was not possible to determine whether sex of the soccer players, sociocultural parameters, or some combination of both were responsible for the associations with overuse injuries because the sample of soccer players in this study consisted of all male athletes. Including female athletes and additional factors such as sociocultural and sport-specific aspects is warranted. Our study also did not use a definition of early sport specialization that includes year-around (>8 months per year) participation in a single sport. This definition is most commonly used, but was not included in our questionnaire because a preliminary survey of several team coaching staffs revealed that most youth soccer players participate in soccer for >8 months of the year.
Conclusion
Our study found that participating in other sports before the age of 12 years (multi-sport athletes) and high training volume for fifth to sixth grade was associated with previous serious overuse injuries in male, Japanese youth soccer players. The study findings contradict the common notion of sport specialization, which may be unique to sociocultural influences in the study population. Future longitudinal studies are warranted to determine whether risk of overuse injuries related to sport specialization depends on sociocultural influence, athlete’s sex, chosen sports, and competitive level.
Acknowledgments
The authors are grateful for the participation of youth soccer teams in the Kanto region, Japan.
Footnotes
The authors report no potential conflicts of interest in the development and publication of this article.
ORCID iD: Norikazu Hirose 
https://orcid.org/0000-0001-7212-6057
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