Background Lower extremity (LEX) strains (hamstring, quadriceps, adductor, and calf strains) are among the most common injuries in sports.

Objective Describe the incidence of LEX strains in professional American football.

Design Descriptive epidemiological study.

Setting U.S. National Football League (NFL).

Participants NFL players during the 2015–2019 seasons.

Assessment of Risk Factors Frequency was calculated by setting (game, practice, conditioning), season (offseason, preseason, regular season, postseason), and roster position. Game incidence rates were calculated for season, roster position, and play type.

Main Outcome Measurements LEX strains identified in the standardized league-wide electronic medical record (n=32 teams).

Results Across 5 seasons, 5,780 LEX strains were reported among 2,769 players (1-season risk: 26.7%, 95% CI 26.0%-27.3%), 69% (n=4,015) of which resulted in time-loss. Among time-loss strains, 58.5% were hamstring (n=2,347), 21.3% were adductor (n=855), 11.8% were calf (n=474), 8.0% were quadriceps (n=323), and 0.4% affected multiple muscles (n=16). Most were reported during preseason practices (n=1,076; 27%) and regular season games (n=1,060; 26%). Among game injuries, the highest rate was in preseason (2.9/10,000 player-plays; 95% CI 2.6–3.2). Defensive secondary players sustained the highest proportion of injuries (27%, n=1,082); but, in games, punters and kickers had the highest rate (3.8/10,000 player-plays; 95% CI 2.7–5.3). Among preseason practice strains, 72% (n=775) were reported during training camps. In games, punt plays had nearly twice the rate as kickoff plays (14.9/1,000 plays; 95% CI 13.1–17.0 vs. 7.5/1,000 plays; 95% CI 6.2–8.9, respectively) and over three times the rate as pass (4.3/1,000 plays; 95% CI 4.0–4.7) and run (2.6/1,000 plays; 95% CI 2.3–2.9) plays.

Conclusions LEX strains impact 1 in 4 NFL players each season and occur mostly during preseason practices and regular season games. The 2-week period of training camp practices comprised 19% of all time-loss strains. In games, punt plays had the highest rate.