Data Collection

All injury report and injured reserve data were collected prospectively for each week over the 2012-2013 and 2013-2014 regular seasons for all 32 teams. An injury was defined as any reported injury that was not documented on the injury report 1 week immediately prior to the index injury week. A concussion was defined as any injury reported as a concussion or a head injury. The 5 most prevalent injuries in the NFL were previously documented and include knee, ankle, hamstring, and shoulder injuries as well as concussions.²⁰ Injury reports do not identify the situational context (ie, game vs practice) in which the injury occurred; however, previous reports have documented that the majority of injuries during a football and NFL season occur during games.^7,22,24 Therefore, all injuries were assumed to occur during games for data analysis and modeling.

Additional situational and contextual data for each team-game (TG) were collected retrospectively to identify factors for an increased risk of TG injury. A TG was defined as a team exposure to a game and, consequently, for each game played there are 2 TGs. The included variables were selected based on previous reports of their association with hazardous outcomes in other sports and levels of play. The predictor variables included the following: game altitude,^26,37 playing surface material,^{4,16,30,31,40} time in season,^21,29,39 time zone change prior to a game,³⁵ distance traveled to game,¹⁴ air temperature,^29–31,34 game outcome,¹¹ game importance,²⁵ and time between games.¹⁰

The following scheduling variables were collected for each TG:

The number of days between the index TG and the previous TG were classified as a short week (TGs played <5 days after the previous TG), a normal week (6-8 days), or a long week (10-15 days).

The distanced traveled to a TG was calculated manually using the Google Maps “measure distance” function. The distance a team traveled for a TG was defined as the distance between the center of their home stadium to the center of the stadium in which the TG was played. TGs were tertiled based on distance traveled, resulting in TGs with no distance traveled (0 km [0 miles]), a short distance traveled (108.28-1009.14 km [67.28-627.05 miles]), and a long distance traveled (≥1012.52 km [≥629.15 miles]).

The time-in-season describes the week in which the TG was played and was analyzed in quartiles (weeks 1-4, 5-8, 9-12, and 13-16).

Time zone (TZ) change prior to a TG describes the absolute change in TZ between a team’s home TZ and the TZ in which the game was played. The change in TZ was categorized as ≥3 TZ compared with ≤2 TZ to examine the maximal change in TZs (and greater) a team may experience when playing within the United States.

Information pertaining to game-specific data was collected and included the following:

The outcome (win, loss, or tie) of a TG, in which TGs resulting in a win or tie were compared with a loss.

The identification of TGs played against divisional opponents compared with TGs not played against divisional opponents.

The playing surface type (grass, artificial turf, or a hybrid grass/turf surface) available on each stadium’s official website. Both turf and hybrid surfaces were amalgamated and compared with natural grass surfaces during the modeling analysis.

Meteorological data were also collected for each TG and included the following:

The mean game-day temperature was retrospectively collected for each game from an online meteorological archive (wunderground.com). The mean game-day temperature was quartiled for analysis resulting in TGs played at ≥21.0°C (69.8°F), 16.9°C to 20.8°C (62.4°F to 69.4°F), 10.0°C to 16.7°C (50.0°F to 62.1°F), and ≤9.7°C (≤49.5°F). For all indoor stadiums and stadiums with retractable roofs, a mean game-day temperature of 21.1°C (70°F) was assumed, representing “room” temperature.

The altitude of each stadium was obtained manually from an elevation-approximation map (http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm), which measures elevation based on coordinates relative to sea level. The center of each stadium was used as the point of reference for elevation data. The altitude of the games played were quartiled, resulting in TGs played at ≤5.18 m (16.99 feet), 5.87 to 121.02 m (19.26-397.05 feet), 142.03 to 216.34 m (465.98-709.78 feet), and ≥220.59 m (≥723.72 feet) above sea level. Altitude cut-offs in each quartile were similar to previous reports.²⁶