Injuries in a Japanese Division I Collegiate American Football Team: A 3-Season Prospective Study

During the 2007 through 2009 seasons, a total of 289 athletes played on the Doshisha University football team (Kyoto, Japan; Japanese Division I Collegiate Football League). Of these, 97 played during 2007 (age = 20.5 ± 1.2 years, playing experience = 3.4 ± 2.1 years), 100 played during 2008 (age = 20.8 ± 1.3 years, playing experience = 3.5 ± 2.1 years), and 92 played during 2009 (age = 20.3 ± 1.5 years, playing experience = 3.6 ± 1.5 years).

Unlike collegiate football in the United States, more than half of the players started playing football at the university (55.7% of 2007 players, 65.0% of 2008 players, and 59.8% of 2009 players). Game starters by grade were sophomores (0% in 2007, 2.0% in 2008, and 9.1% in 2009), juniors (59.1% in 2007, 31.8% in 2008, and 31.8% in 2009), and seniors (40.9% in 2007, 59.1% in 2008, and 59.1% in 2009); no players started during their freshman year. For data analysis, players were grouped by position into defensive back (DB), linebacker (LB), defensive lineman (DL), offensive lineman (OL), running back (RB), tight end (TE), wide receiver (WR), or quarterback (QB). This study was approved by the Institutional Review Board of Kyoto University. Informed consent was obtained from all participants.

Japanese collegiate football consists of spring and fall seasons. The spring season runs from May to early July, and the fall season runs from September to mid-December. In the present study, data for each season were collected from the beginning of full-contact preseason practices until the end of the final game of the season, including any postseason playoff games. The preseason practices typically lasted for 1 month before the first game of the season.

Practices consisted of all types of skill training on the field, including scrimmage and position training. Field practice time was approximately 4 hours per day, 5 or 6 days per week. During the fall season of each year, 7 league games were played, with a game played every 2 weeks, plus 1 postseason game in each of 2007 and 2008. In addition, 6 exhibition games were played during the spring season of each year.

All injuries that occurred during practices or games during the 2007 through 2009 seasons were recorded by a certified athletic trainer. We defined an injury as any event that (1) occurred during a regular practice or game, (2) caused the player to seek medical care from the team physician or athletic trainer, and (3) caused a player to miss a subsequent practice or game. All fractures and dental injuries were included regardless of any time loss. Injuries were classified into 3 categories of severity: minor (time loss 1–7 days), moderate (time loss 8–21 days), and major (time loss >21 days).¹¹  The team physician made the final diagnosis in all cases and determined the return-to-play time. We defined an AE as 1 athlete participating in 1 game or practice.²²  Game and practice AEs were calculated by multiplying the average number of players participating in each game or practice by the number of games or practices.

We calculated injuries per 1000 AEs by dividing the number of injuries by the number of AEs and multiplying by 1000.²²  The practice injury rate (PIR) and game injury rate (GIR) were calculated with 95% confidence intervals (CIs).²²  The injury rate ratio (IRR) was calculated by dividing the GIR by the PIR. Injury rate ratios with 95% CIs²²  were calculated according to season, injury type, body part, severity, and mechanism. The difference between GIR and PIR was considered to be statistically significant if the 95% CI for IRR did not include 1.⁵  The IRRs for games and practices were compared among the 3 seasons using the χ²  test. The α level was set at P ≤ .05. We also calculated the numbers and percentages of injuries in practices and games according to position of player, school year, and playing experience. For all analyses, SPSS software for Windows (Japanese version 17.0; SPSS Japan Inc, Tokyo, Japan) was used.

A total of 545 injuries were reported over the 2007 through 2009 seasons. Of these, 447 (82.0%) occurred during practices and 98 (18.0%) during games (Tables 1 and 2). The overall PIR and GIR were 10.9 and 32.7, respectively. The overall IRR was 3.0 (95% CI = 2.4, 3.7), indicating that the overall GIR was higher than the overall PIR (P ≤ .05).

Sprains were the most common injuries (28.8% of all injuries), followed by strains (23.5%) and contusions (14.3%; Table 3). Sprains had the highest PIR (3.0 injuries/1000 AEs, 95% CI = 2.5, 3.5) and GIR (11.3 injuries/1000 AEs, 95% CI = 7.5, 15.1) among injury types (Table 3). Dislocation or subluxation had the highest IRR (7.7, 95% CI = 3.1, 19.1), followed by fractures, contusions, sprains, and tendinitis. The GIR was higher than the PIR for all these injury types (P < .05 for all; Table 3).

Sprains were most common at the foot and ankle (66/157 injuries, 42.0%), followed by the knee (43/157 injuries, 27.4%), shoulder and elbow, and finger (each 18/157 injuries, 11.5%). Strains were most common at the thigh (94/128 injuries, 73.4%), and contusions were most common at the knee (26/78 injuries, 33.3%).

The major body area most frequently injured was the lower limb (328/545 injuries, 60.2%), which had a PIR of 6.6 injuries/1000 AEs (95% CI = 5.8, 7.4) and GIR of 18.7 injuries/1000 AEs (95% CI = 13.8, 23.6; Table 4). The next most common body major body area injured was the upper limb, which had a PIR of 2.4 injuries/1000 AEs (95% CI = 1.9, 2.9) and GIR of 8.7 injuries/1000 AEs (95% CI = 5.4, 12.0).

Almost half of all injuries were minor (272/545 injuries, 49.9%; Table 5). During practices, the injuries were most frequently minor (5.8 injuries/1000 AEs, 95% CI = 5.1, 6.5), and during games, the injuries were most frequently moderate (13.3 injuries/1000 AEs, 95% CI = 9.2, 17.4; Table 5). Calculation of IRRs showed that the GIR was higher than the PIR for all injury severities (P ≤ .05; Table 5).

Contact was the most common mechanism of injury in both practices (222/447 injuries, 49.7%) and games (62/98 injuries, 63.3%; Table 6). Calculation of IRRs showed that the GIR was higher than the PIR for both contact and noncontact injuries (P ≤ .05; Table 6).

Overall, the highest number of injuries occurred in RBs (104/545 injuries, 19.1%), followed by WRs (98/545 injuries, 18.0%) and LBs (90/545 injuries, 16.5%; Table 7). During practices, the highest numbers of injuries occurred in WRs (89/447 injuries, 19.9%), followed by RBs (87/447 injuries, 19.5%) and LBs (73/447 injuries, 16.3%). During games, the highest numbers of injuries occurred in DBs (19/98 injuries, 19.4%), followed by DLs (18/98 injuries, 18.4%), and RBs and LBs (each 17/98 injuries, 17.3%). Offensive units accounted for 56.5% of all injuries and defensive units for 43.5%.

The highest overall number of injuries occurred in athletes in their junior year (162/545 injuries, 29.7%). The highest number of game injuries also occurred in athletes in their junior year (39/98 injuries, 39.8%; Table 7). The highest number of practice injuries occurred in athletes in their junior and sophomore years (each 123/447 injuries, 27.5%). Analysis of injuries according to playing experience showed that the highest number of overall injuries occurred in players with 2 years of experience (118/545 injuries, 21.7%). The highest number of practice injuries also occurred in players with 2 years of experience (103/447 injuries, 23.0%). The highest number of game injuries occurred in players with 3 years of experience (27/98 injuries, 27.6%).

Our 3-year prospective observational study examined current injury patterns among players on a collegiate football team in Japan. The overall injury rate, including practice and game injuries, was higher in Japanese players (12.3 injuries/1000 AEs, 95% CI = 11.3, 13.3) than in collegiate American football players (8.6–10.5 injuries/1000 AEs).^3,8  This difference is explained by the relatively higher number of practice injuries in Japanese players.

We found that the overall GIR was higher than the PIR (P ≤ .05). This is consistent with the findings of previous studies conducted in the United States.^3,8  Although the GIR in Japan (32.7 injuries/1000 AEs) was similar to the GIR reported in the United States (35.9 injuries/1000 AEs),⁷  the PIR in Japan (10.9 injuries/1000 AEs) was higher than the PIR reported in the United States (5.8–7.0 injuries/1000 AEs).^3,8 

Practice length is not regulated in Japan, although it is strictly regulated by the NCAA in the United States.²³  It is therefore possible that the higher PIR in Japan is due to a longer average practice time (4 hours per day) than in the United States. Another possible explanation for the higher PIR is the emphasis placed on scrimmage during practices by Japanese football coaches, which may be related to the high proportion of players who start playing football during their time at the university.²⁰  This may lead to an increased number of contacts (eg, tackling or blocking), resulting in cumulative trauma, which could raise the injury rate.

American football is regarded as a high-risk sport. Many efforts in the United States have been directed at decreasing the risks by rule changes, new regulations, and the development of protective equipment.⁷  Importantly, these changes have been evidence based.⁷  However, such efforts to decrease the risks have not been made in Japan. We suggest that the Japanese PIR could be reduced if the number of scrimmages and length of practices were modified by individual coaches or by league regulations. We hope that our findings will contribute to future legislation governing safety in American football in Japan.

In our study, the major body area most frequently injured was the lower limb (60.2% of injuries). This is consistent with the results of previous studies.^3,7,11,13  Further analysis showed that the thigh and gluteal area was the most frequently injured part of the lower limb (31.7% of all lower limb injuries), followed by the knee (30.5% of all lower limb injuries) and the ankle and foot (25.3% of all lower limb injuries).

An interesting finding warranting further research was that knee and ankle injuries (predominantly sprains) had higher GIRs, whereas thigh and gluteal injuries (predominantly strains) had higher PIRs. Although specifically analyzing these injuries was beyond the scope of our study, these findings could help to identify appropriate conditioning exercises to prevent and reduce injuries to those body parts.

Most injuries in this study were minor (49.9% of all injuries), which is similar to the findings of previous studies (48.6%–64.5% of all injuries).^5,8  Comparison of IRRs showed that the GIR was highest for moderate injuries, whereas the PIR was highest for mild injuries. This observation suggests that higher intensity and speed during games may explain the more severe injuries. However, Dick et al⁷  pointed out that football coaches have more influence during practices than during games, which could increase the use of proper techniques during practices (eg, safe tackling and blocking). This could explain why minor injuries were the most frequent during practices.

Our study showed that the leading cause of injury was contact (284 of all injuries, 52.1%). Contact was the mechanism in 49.7% of practice versus 63.3% of game injuries. In comparison, Dick et al⁷  and Shanker et al⁸  reported higher percentages of injuries due to contact in the United States, especially during games (82.4%–82.9%). This difference may be explained by the superior physical characteristics (body mass, stature, power, and strength) of players in the United States,²⁴  which are potentially linked to higher forces on impact. For contact injuries, the GIR was approximately 3.8 times higher than the PIR. This may have been because of increased contact and higher intensity and speed during games, resulting in higher impact forces. Another possibility is that, as discussed previously,⁷  football coaches have more control during practices than during games, resulting in a lower injury risk during practices.

Our analysis demonstrated that the highest numbers of injuries occurred in RBs, WRs, and LBs. This is consistent with US injury data.^3,7–9,13  The high numbers of injuries occurring in RBs and WRs may be explained by the requirements of these positions, which emphasize running and ball carrying.¹²  The high numbers of injuries occurring in LBs may be because they often tackle offensive players moving at high rates of speed.

Our analysis according to offensive and defensive units showed that higher numbers of injuries occurred in offensive units than defensive units, which is consistent with the results of a previous study.⁸  This can be explained by the presence of RBs and WRs in the offensive units, as well as OLs, who sustained the fourth highest number of injuries. We examined 3 categories of mechanism of injury (contact, noncontact, and other or unknown), but further study is needed to analyze more specific mechanisms in offensive and defensive units such as tackling, blocking, or being blocked, to help clarify the reasons for the differences in injuries between the units.

As most athletes who started playing football during their time at Doshisha University did so during their junior or senior years, higher frequencies of contact during games and practices may have contributed to the higher numbers of injuries during those years. Based on our observations, these players may be more likely to be exposed to repeated and cumulative body trauma, which has also been observed in previous studies.^10,13 

The highest numbers of injuries during practices occurred in players with 1 or 2 years of experience. This observation may be important for identifying underlying causes of injury related to the playing experience. To decrease the high numbers of injuries in these experienced players, coaches and athletic trainers should work together to carefully watch and analyze their injury patterns and trends and identify appropriate types of practice.

Our study has the following limitations. First, we only examined 3 categories of mechanism of injury (contact, no contact, and other or unknown), and practices were not classified into types (eg, scrimmages, conditioning, helmet use), which prevented in-depth analysis of the mechanisms and practice types underlying the injuries. Examination of additional mechanisms, such as tackling, blocking, or sprinting, and classification of practice types would have strengthened the study. Second, we studied only 1 poorly ranked team, whereas the injury data of most studies in the United States were collected from multiple teams and a variety of conferences and divisions.^1,3,5,7,8  This limits the ability to generalize our findings to higher-ranked teams or different conferences or divisions in Japan. Injury data were collected from only 1 team in this study because Japan has no injury-reporting system such as the National Athletic Injury/Illness Reporting System (NAIRS),¹¹  and few certified athletic trainers work full time at schools and keep injury records. Information regarding injuries is usually not available to the public or shared among institutions. Future authors should examine a number of teams across a variety of conferences and divisions. Finally, we compared PIRs in Japan with those in the United States despite the different practice styles used by coaches in these countries, which may affect PIRs.

This is the first study to analyze American football injuries per 1000 AEs over a 3-year period in Japan. We found that the GIR was higher than the PIR, which is consistent with research conducted in the United States. A unique finding of this study was that the PIR was higher in our Japanese team than the PIR reported in the United States. This may be explained by the particular style of practices in Japan. Furthermore, lower limb injuries comprised 60% of all injuries, with knee injuries and ankle and foot injuries the most frequent during games and thigh and gluteus injuries the most frequent during practices. To develop an effective strategy to decrease football injuries in Japan, further research is required, especially regarding mechanisms of injury, injuries according to types of practices, injury-prevention interventions, and injury patterns and trends in schools, divisions, and leagues.