Fifteen-week window for recurrent muscle strains in football: a prospective cohort of 3600 muscle strains over 23 years in professional Australian rules football

Key points

1. 01A recent (within 8 weeks) strain of the same muscle was by far the strongest risk factor, with adjusted odds ratios from 13 to 25 depending on the muscle.
2. 02Re-injury risk was highest in the first match back: 9% for hamstring, 6% for groin, 5% for quadriceps, 2% for calf.
3. 03Elevated recurrence risk lasted about 15 weeks for hamstring and quadriceps, 18 weeks for calf, and 19 weeks for groin.
4. 04Players appear to return to play while still carrying incompletely healed strains, keeping them at risk for an extended window.
5. 05A recent hamstring injury also raised the risk of a later quadriceps or calf strain.

How it was conducted

Design

Prospective cohort using the Australian Football League injury database, 1992 to 2014 (23 seasons)

Participants

3200 professional players across 272,759 player-matches

Injuries analysed

3647 muscle strains (1932 hamstring, 418 quadriceps, 458 calf, 839 groin)

Analysis

Binary logistic regression (forward stepwise) per muscle type, plus week-by-week recurrence rate plotting with log-decay trendlines

Primary outcome

Risk of muscle strain recurrence over weeks since return to play, and risk factors for each strain type

What they found

• Adjusted OR for recent (under 8 weeks) history of the same injury: hamstring 13.10 (95% CI 11.52 to 14.89), calf 13.30 (95% CI 9.62 to 18.38), quadriceps 25.19 (95% CI 18.75 to 33.84), groin 20.62 (95% CI 17.02 to 24.99).
• Adjusted OR for non-recent (over 8 weeks) history of the same injury: hamstring 3.49 (95% CI 3.15 to 3.86), calf 4.38 (95% CI 3.56 to 5.39), quadriceps 5.16 (95% CI 4.17 to 6.38), groin 3.46 (95% CI 2.97 to 4.03).
• Absolute recurrence risk in the first match back: 9% hamstring, 6% groin, 5% quadriceps, 2% calf; second match back: 4% hamstring, 4% groin, 2% quadriceps, 2% calf.
• Elevated recurrence risk persisted for 15 weeks (hamstring and quadriceps), 18 weeks (calf) and 19 weeks (groin), with log-decay fit R2 of 0.81 hamstring, 0.52 calf, 0.71 quadriceps, 0.79 groin, 0.85 all strains combined.
• Age 24 and over was an independent risk factor for calf strains (adjusted OR 1.62, 95% CI 1.32 to 1.98); recent hamstring injury raised quadriceps strain risk (adjusted OR 1.80, 95% CI 1.20 to 2.71) and calf strain risk (adjusted OR 1.75, 95% CI 1.17 to 2.60).
• Absolute single-game hamstring strain risk was about 0.2% with no history, 0.7% with a history in a prior season, 1.4% with a history before the previous 8 weeks, and 4.0% with a recent (past 8 weeks) history.
• In the substitute-rule era (2011 to 2014), hamstring (adjusted OR 0.76, 95% CI 0.67 to 0.86), groin (OR 0.78, 95% CI 0.65 to 0.93) and quadriceps (OR 0.70, 95% CI 0.53 to 0.92) strains were less likely, but calf strains were more likely (OR 1.59, 95% CI 1.30 to 1.94).

Limitations

• Injuries were defined only as those severe enough to make a player miss a match, and there was some definitional uncertainty in diagnoses made by team staff.
• The side of the injury (left or right) was not analysed, so true same-side recurrences cannot be separated.
• Observational design means associations such as concussion history appearing to 'protect' against hamstring strain likely reflect residual confounding, not cause.
• Findings come from elite male Australian rules football and may not transfer directly to other sports, levels, or recreational athletes.

Why it matters

For patients

If you are returning from a muscle strain, expect to stay at higher risk of re-injuring it for around three months, so ease back in rather than treating return to play as fully recovered.

For clinicians

Plan careful load management and monitoring for roughly 15 weeks after return to play, especially in the first match back, since recent same-muscle injury dwarfs every other risk factor.

For readers

This very large, long-running cohort quantifies how long re-injury risk lingers after a muscle strain and confirms that recent injury history is the dominant predictor.