Hamstring Strain Is Associated with Increased Subsequent Nonhamstring Lower-Extremity Injury in National Football League Players: A Matched Cohort Study

Objective: To evaluate whether hamstring strain (HS) is associated with subsequent nonhamstring lower-extremity injury (NHLEI) after return to play (RTP) in National Football League (NFL) players and whether RTP timing influences risk. Design: Retrospective matched cohort study. Setting: Publicly available NFL injury reports from the 2014 to 2024 regular seasons. Participants: NFL players sustaining HS were matched 1:1 to controls by season, week, position, age, and workload. A total of 1236 HS episodes were matched to 1236 controls. Assessment of Risk Factors: HS status and RTP timing. Main Outcome Measures: NHLEI within 30, 60, and 90 days after RTP. Results: HS was associated with increased NHLEI risk at 30 days (hazard ratio [HR], 1.28; P = 0.002), 60 days (HR, 1.34; P < 0.001), and 90 days (HR, 1.39; P < 0.001). In adjusted analyses (584 pairs), HS remained associated with increased injury risk at all time points (adjusted HRs, 1.38-1.40; all P ≤ 0.003). Players returning within 0 to 7 days had higher subsequent injury risk than those missing 7 to 14 days (HR, 0.72; P = 0.027) or ≥14 days (HR, 0.75; P = 0.019). HS was associated with increased hazards of foot/ankle, knee, groin, and quadriceps injuries. Conclusions: NFL players sustaining HS had increased subsequent NHLEI hazard after RTP, with highest risk among earlier RTP. These findings support further study of RTP timing and post-RTP injury prevention. Clinical Relevance: RTP after HS may represent a period of elevated lower-extremity injury vulnerability in NFL players.