ORCHARD, J. W., and J. W. POWELL. Risk of Knee and Ankle Sprains under Various Weather Conditions in American Football. Med. Sci. Sports Exerc., Vol. 35, No. 7, pp. 1118-1123, 2003.
Purpose: Previous studies have found conflicting relationships between type of playing surface and injury in American football but have not taken into account possible variations in the surface conditions of outdoor stadiums due to changing weather.
Methods: A total of 5910 National Football League team games between 1989 and 1998 inclusive were studied to determine associations between knee and ankle sprains, playing surface, and the weather conditions on the day of the game.
Results: There was reduced risk of significant ankle sprains (at least 7-d time loss) for games in natural grass stadiums compared with domes (indoor stadiums using AstroTurf) (RR 0.69, 95% CI 0.58-0.83). There was also reduced risk of significant knee sprains on grass compared with domes (RR 0.77, 95% CI 0.66-0.91), although most of this reduction was related to cold and wet weather on grass (RR 0.66, 95% CI 0.47-0.93 compared with hot and dry weather on grass). In open (outdoor) AstroTurf stadiums, cold weather was associated with a lower risk of significant ankle sprains (RR 0.68, 95% CI 0.51-0.91), significant knee sprains (RR 0.60, 95% CI 0.47-0.77) and ACL injuries (RR 0.50, 95% CI 0.31-0.81) compared with hot weather in the same stadiums. Weather did not have any significant effects on the injury risk in domes. The ACL incidence rate was lower during the later (cooler) months of the season in open stadiums (both AstroTurf and natural grass) but not in domes.
Conclusion: Cold weather is associated with lower knee and ankle injury risk in outdoor stadiums (both natural grass and AstroTurf), probably because of reduced shoe-surface traction.
The National Football League (NFL) has been conducting its injury surveillance system since 1980, which is thought to be longer than any other professional sporting competition in the world (20,22). Risk factors for injury can be divided into intrinsic (player-related) and extrinsic (environment-related). Many of the documented intrinsic risk factors for injury are nonreversible, such as player age, sex, and past history of injury. A potential extrinsic risk factor of interest within the sports medicine community has been the playing surface (1,3,5-7,9,11,12,14,19,20,22,26,27). Injury causation is complex, with potential interactions between surface type and other extrinsic (e.g., weather, type of play) and intrinsic (e.g., player position, age, injury history, and shoe selection) variables.
Many studies in the 1970s and 1980s compared the injury rates between artificial surfaces, such as AstroTurf (Southwest Recreation Industries, Leander, TX), and natural grass. (In this study AstroTurf and artificial turf will be used interchangeably, as this report covers a time period when AstroTurf was the only type of artificial surface used in the NFL.) There have been conflicting conclusions from the results of these studies. Many studies have found that artificial turf and grass have a similar overall injury rate (12,14,20-22,25,26). Injuries to the foot and ankle have been reported as slightly more common on AstroTurf than grass (3,20-22,26). Injuries to the knee have been reported as either not related to the playing surface or slightly more common on artificial turf (3,20-22,26). The relative risk for injuries on AstroTurf compared with natural grass has varied when the data were controlled for player position and type of play (22).
Surface characteristics (such as hardness) of AstroTurf and particularly natural grass are quite variable (11). Natural grass varies widely in soil moisture content, soil type, and species of turfgrass. Previous studies that have compared injury rates on natural grass and artificial turf have generally not assessed the condition of either surface, which could be a reason for the conflicting conclusions from these studies.
The aim of this study was to describe the association between weather conditions and knee and ankle sprains for natural grass and AstroTurf surfaces in the NFL for the 10 seasons 1989-1998, as weather conditions are responsible for some of the variation in surface characteristics in outdoor stadiums.