Proceedings of the First IOC World Congress on Sport SciencesColorado Springs: US Olympic Committee pp 194-195 (1989)
A PROSPECTIVE STUDY OF INJURY RATES IN A NATIONAL SAMPLE OF
AMERICAN COLLEGE FOOTBALL PLAYERS
Zemper, E.D. Exercise Research Associates of Oregon, 111 E. 16th Ave. Suite B, Eugene, Oregon 97401 USA.
The total number of injuries incurred by participants in American football each year is quite large, primarily because there are a large number of teams in this country that participate in the sport and there are a large number of players on each team (averaging approximately 100 players per team at the collegiate level). However, the rate of injuries per 1,000 athlete-exposures for football is actually less than for other sports such as wrestling, men's and women's soccer and women's gymnastics. (An athlete-exposure is one athlete taking part in one practice or competition where he or she is exposed to the possibility of being injured.) The rate per 1,000 athlete-exposures is emphasized in this study, rather than the rate per 100 players most often seen in the research literature, because it is a much more precise measure of injury rates. The use of injury rates per 100 players assumes all players participate in every practice and in every game, and that all teams have the same number of practices and games, which obviously is not the case. Use of injury rates per 1,000 athlete-exposures compensates for the fluctuating numbers of players attending practices each week, the varying numbers who actually participate in games (in most cases nowhere near the total number on the squad), and the varying numbers of practices for each team.
The data for this study were collected by the Athletic Injury Monitoring System (AIMS) (formerly known as the National Sports Injury Surveillance System), which is designed to be a longitudinal sports injury data collection system that provides analyses of national injury rates and patterns for the purpose of providing data to assist in developing ways to prevent or reduce the severity of sports injuries through rule changes, equipment modifications or changes in coaching techniques. Beginning with the 1986 season, AIMS has been collecting exposure and injury data from a stratified national sample of college football teams (stratification based on size of college and geographic location). Following specific instructions, data are collected and recorded on two basic easy-to-use data collection forms by on-site certified athletic trainers and returned to the AIMS office on a weekly basis, from the beginning of preseason practices until the final regular season or postseason game. Upon arrival at the AIMS office, each form is screened for completeness and consistency before being entered into a computer file for later analysis. In the case of incomplete or inconsistent data on any form, the individual athletic trainer is contacted for clarification. One form is used to record the number of practices and competitions each week and the number of players taking part in each. The second form is used to record data on each injury that keeps a player from participating for one day or more. During the three year period of this study, 98.9% of the weekly forms were completed and returned.
There were a total of 80 teams involved over the 1986-1987-1988 seasons (an average of 27 teams per season, or approximately 5% of the total number of college football teams), with a total of 8,325 players. During the three seasons there were 619,310 athlete-exposures (A-E), with 49,399 (8.0%) occurring in games and 569,911 (92.0%) in practice sessions. There were a total of 3,744 time-loss injuries reported. Taking into account the number of recurrent injuries and the number of players injured more than once in a given season, a total of 2,684 players (32.2% of the total population) were injured at least once during a season. The total injury rates were 45.0 per 100 players, or 6.05 injuries per 1,000 A-E. The injury rate during games was 32.11 per 1,000 A-E, while during practices it was 3.79 per 1,000 A-E. While 57.6% of the total number of injuries occurred during practices, the game injury rate was 8.5 times as high as the rate during practice. This major difference in injury rate between competition and practice is seen in most other sports that have been studied. Injuries requiring surgery accounted for 8.5% of the total reported injuries. Approximately one-half of these surgical injuries used arthroscopic surgery, allowing many of the players to return to activity before the end of the season.
The knee (19.1%), ankle (15.4%) and shoulder (10.8%) were the most frequently injured body parts, followed by the quadriceps (6.1%), head (5.8%) and neck(5.0%). The case rates were: knee - 1.15/1,000 A-E; ankle - 0.93/1,000 A-E; shoulder - 0.65/1,000 A-E; quadriceps - 0.37/1,000 A-E; head - 0.35/1,000 A-E; and neck - 0.31/1,000 A-E. The most frequent types of injuries were ligament sprains (32.1%), muscle strains (~8.6%), and contusions (15.2%), followed by fractures (6.8%) and cerebral concussions (5.4%). The case rates for these types of injuries were: sprains - 1.94/1,000 A-E; strains - 1.12/1,000 A-E; contusions - 0.92/1,000 A-E; fractures - 0.41/1,000 A-E; and concussions - 0.33/1,000 A-E. The distributions of injuries by body part and injury type were consistent with previous studies. Combining these data showed ankle sprains to be the most common football injury, followed by knee ligament sprains. The next most common injuries were other knee injuries (e.g., complete ligament tear, torn meniscus), and upper leg strains (quadriceps and hamstrings). The fifth most common football injury was cerebral concussion, despite the head being protected by a helmet.
Given the contact nature of this sport, it is to be expected that the most common mechanisms of injury involve the major football activities of blocking and tackling. Being tackled was the mechanism involved in 18.4% of the recorded injuries, blocking was the primary mechanism in 17.7% of the injuries, tackling 17.3% and being blocked 13.1% of the injuries. Interestingly, the non-contact categories of sprinting/running (9.1%), "no evidence of contact" (6.4%) and overuse/gradual onset (4.3%) were the next most common injury mechanisms. Injury rates by position played appeared to be reasonably equivalent for most positions after being adjusted for the number of players on the field at each position. Since being tackled was the most common injury mechanism, it would be expected that the running backs, who are the primary ball handlers, would have among the highest injury rates (0.43/1,000A-E). Flankers/wide receivers had the highest injury rate (0.48/1,000 A-E), possibly because they frequently are involved in high speed collisions in the open field. Injury rates for all other offensive and defensive positions ranged from 0.15/1,000 AE for defensive safeties to 0.32/1,000 A-E for defensive linebackers.
A consistent observation during the three seasons covered by this study, and in previous research by the author, was that the injury rate during games is highest during the third quarter of the game. The commonly held assumption has been that more injuries occur as players become more tired in later stages of the game. If this were true, it would be expected that the injury rate would be lowest during the first and third quarters (after players have had a chance to rest during the halftime break), and highest during the second and fourth quarters. These data show that the injury rates (per 1,000 A-E) are: first quarter - 5.06; second quarter - 8.83; third quarter - 9.92; and fourth quarter - 8.00. This pattern suggests the hypothesis that the injury rate is lowest during the first quarter when the players have had an opportunity to warm-up and stretch during the pre-game period, and is highest during the third quarter after the players have been inactive and cooled down during the halftime break. This hypothesis would only relate to injuries affected by warming-up and stretching, and detailed analysis of sprains and muscle strains during games show that they occur at the highest rate during the third quarter and at the lowest rate during the first quarter. The implication is that coaches, athletic trainers and team physicians could reduce the number of injuries during the third quarter by ensuring that players again warm-up and re-stretch after the halftime break. However, this is unlikely to occur unless it is mandated by the football rules committee.
The results obtained during the first three years of the operation of AIMS demonstrate the practicality of operating a national sports injury data collection system of this nature to provide descriptive summaries of epidemiologic data that can be used for decision-making purposes by various groups concerned with the prevention and reduction in severity of sports injuries.