title: Influence of Extrinsic Risk Factors on National Football League Injury Rates.
abstract: BACKGROUND
  The risk of injury associated with American football is significant, with recent reports indicating that football has one of the highest rates of all-cause injury, including concussion, of all major sports. There are limited studies examining risk factors for injuries in the National Football League (NFL).
  
  
  OBJECTIVE
  To identify risk factors for NFL concussions and musculoskeletal injuries.
  
  
  METHODS
  Case-control study; Level of evidence, 3.
  
  
  METHODS
  Injury report data were collected prospectively for each week over the 2012-2013 and 2013-2014 regular seasons for all 32 teams. Poisson regression models were used to identify the relationship between predetermined variables and the risk of the 5 most frequent injuries (knee, ankle, hamstring, shoulder, and concussion).
  
  
  RESULTS
  A total of 480 games or 960 team games (TGs) from the 2012-2013 and 2013-2014 regular seasons were included in this study. A trend to an increasing risk of concussion and TG ankle injury with decreasing mean game-day temperature was observed. The risk of TG concussion (incidence rate ratio [IRR], 2.16; 95% CI, 1.35-3.45; P = .001) and TG ankle injury (IRR, 1.48; 95% CI, 1.10-1.98; P = .01) was significantly greater for TGs played at a mean game-day temperature of ≤9.7°C (≤49.5°F) compared with a mean game-day temperature of ≥21.0°C (≥69.8°F). The risk of TG shoulder injury was significantly increased for TGs played on grass surfaces (IRR, 1.36; 95% CI, 1.02-1.81; P = .038) compared with synthetic surfaces. The risk of TG injury was not associated with time in season, altitude, time zone change prior to game, or distance traveled to a game.
  
  
  CONCLUSIONS
  This study evaluated extrinsic risk factors for injury in the NFL. A hazardous association was identified for risk of concussion and ankle injury with colder game-day temperature. Further research should be conducted to substantiate this relationship and its potential implication for injury prevention initiatives.
authors.affiliation: Department of Family & Community Medicine, St Michael's Hospital, University of Toronto, Toronto, Ontario, Canada.
authors.forename: David W
authors.initials: DW
authors.lastname: Lawrence
authors.affiliation: Toronto Rehabilitation Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Paul
authors.initials: P
authors.lastname: Comper
authors.affiliation: Faculty of Kinesiology and Physical Education, University of Toronto, Toronto, Ontario, Canada.; Keenan Research Centre for Biomedical Science, St Michael's Hospital, Toronto, Ontario, Canada.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Hutchison
keywords: NFL; National Football League; concussion; epidemiology; injury prevention; temperature