Return to play after a hamstring strain injury: it is time to consider natural healing

Key points

1. 01Average return to play after grade 1-2 hamstring injuries is 11-21 days, but biological muscle healing at that point is still incomplete and scar tissue is immature
2. 0225% of re-injuries occur in the first week after return to play and at the exact same location as the index injury, suggesting premature clearance
3. 03Functional recovery (normal strength and pain-free movement) precedes structural recovery - MRI abnormalities persist at return to play even when clinical tests are normal
4. 04Cross-linking of collagen type I fibers and expression of adhesion proteins, both critical for tensile strength, do not reach adequate levels until around day 21 or later
5. 05A minimum 4-week period before return to play is recommended as a first step to reduce the persistent re-injury cycle

How it was conducted

Design

Narrative review article synthesizing two domains: return-to-play criteria after hamstring injuries and biological muscle healing

Database

PubMed (searched August 2020); hand search of reference lists of previous reviews and consensus statements

RTP domain

Search yielded 110 results; 16 studies included after applying eligibility criteria (English language, human adults 19+ years, topic relevance)

Biological healing domain

Search limited to systematic reviews; 294 review articles identified, 10 fully addressed the topic of interest

Total additional studies retrieved

26 studies retrieved from PubMed across both domains

Injury focus

Grade 1 and 2 hamstring strain injuries (most common types, comprising more than 80% of all hamstring injuries)

What they found

• Average return to play after grade 1 and 2 hamstring injuries is between 11 and 21 days based on current functional RTP criteria
• Re-injury rate after hamstring injuries ranges from 12-34% across studies, indicating current RTP criteria are inadequate
• 25% of re-injuries occur in the first week after return to play and at the exact same location as the index injury (Wagensteeen et al.)
• Of 25 studies included in a systematic review by van der Horst et al. on RTP criteria, only one explicitly mentioned time as a criterion
• In a worldwide survey of 131 premier league football teams, only 12% of medical staff indicated 'other' (including time and medical imaging) as an important RTP criterion
• Type III collagen persists as the dominant collagen type at 2-3 weeks after injury - associated with lower tensile strength compared to type I collagen
• A firm attachment at newly formed myotendinous junctions cannot be formed until day 21 when integrin alpha-7-beta-1 is abundantly developed
• Scar tissue maturation (shift from type III to type I collagen ratio approaching normal tissue) does not begin until at least 3 weeks after injury
• Askling et al. found no re-injuries when players were allowed to return to play no earlier than an average of 49-86 days (note: included grade 3 injuries)
• Sonography detected fewer abnormalities than MRI at both 2 and 6 weeks after injury, and MRI has been excluded as a return-to-play criterion by a panel of international experts

Limitations

• The review is narrative rather than systematic, and no formal quality assessment or risk-of-bias analysis of included studies was performed
• Biological healing studies cited are mostly from animal models or small human studies; direct evidence in high-level athletes is scarce
• The recommended 4-week minimum is a general guideline and does not account for inter-individual variability in healing rate or differentiation between grade 1 and grade 2 injuries
• The review does not provide clinical trial evidence that enforcing a 4-week minimum actually reduces re-injury rates; this remains to be validated prospectively

Why it matters

For patients

Athletes with a hamstring strain may be cleared to play while their muscle tissue is still structurally immature, increasing re-injury risk, so waiting at least 4 weeks regardless of how the leg feels may reduce the chance of the same injury recurring.

For clinicians

Passing functional tests does not confirm that scar tissue is mechanically ready; clinicians should use the 4-week biological healing window as a minimum threshold alongside existing functional criteria before clearing athletes for return to play.

For readers

This review challenges the standard 2-3 week return-to-play timeline for hamstring strains by showing that biological tissue healing lags well behind functional recovery, and proposes integrating a minimum time criterion into clinical decision-making.