Pain-free versus pain-threshold rehabilitation following acute hamstring strain injury: a randomized controlled trial

Key points

1. 01Return to play clearance time was similar between groups: median 15 days (pain-free) vs 17 days (pain-threshold), a non-significant difference (p = 0.37)
2. 02Pain-threshold rehabilitation produced 15% greater recovery of isometric knee flexor strength at 90/90 hip/knee flexion at return to play and at two-month follow-up
3. 03Biceps femoris long head fascicle length was 0.91 cm better maintained at two-month follow-up in the pain-threshold group
4. 04No adverse events (re-injuries) occurred during rehabilitation in either group; re-injury rates at six months were similar (2 in each group)
5. 05Fear of movement decreased significantly in both groups with no difference between them

How it was conducted

Design

Single-centre double-blind randomised controlled trial

Participants

43 men with acute hamstring strain injury, aged 18-40, randomised within 7 days of injury

Groups

Pain-free (n=22, NRS 0 during exercise) vs pain-threshold (n=21, NRS up to 4/10 during exercise)

Intervention

Standardised twice-weekly supervised protocol of hamstring strengthening (including Nordic hamstring exercise and eccentric slider) and progressive running until return to play criteria met

Primary outcome

Days from injury to return to play clearance

Follow-up

Two-month clinical reassessment and six-month re-injury monitoring after return to play clearance

What they found

• Median time to return to play clearance: 15 days (95% CI 13-17) in pain-free group vs 17 days (95% CI 11-24) in pain-threshold group; hazard ratio 0.75 (95% CI 0.40-1.40), p = 0.37
• Isometric knee flexor strength at 90/90 improved by 35% (95% CI 26-44) in pain-free and 49% (95% CI 36-63) in pain-threshold group from baseline to return to play; difference of 15% (95% CI 1-28) in favour of pain-threshold
• At two-month follow-up, isometric knee flexor strength at 90/90 remained 15% greater (95% CI 1-29) in the pain-threshold group
• BFlh fascicle length improved from baseline to return to play by 1.70 cm (95% CI 1.33-2.08) in pain-free and 1.95 cm (95% CI 1.41-2.48) in pain-threshold; no significant between-group difference at return to play (95% CI -0.29 to 0.78)
• At two-month follow-up, BFlh fascicle length improvement from baseline was 0.91 cm greater (95% CI 0.34-1.48) in the pain-threshold group
• Fear of movement (Tampa Scale for Kinesiophobia) reduced by -7 points (95% CI -5 to -9) in pain-free and -8 points (95% CI -5 to -11) in pain-threshold; between-group difference non-significant
• Six-month re-injury hazard ratio: 1.05 (95% CI 0.14-7.47), p = 1; 2 re-injuries in each group

Limitations

• Small sample size (n=43) limits power to detect differences in re-injury rates
• All participants were male; results may not apply to female athletes
• Acute hamstring strain was confirmed by clinical assessment only, without MRI, so injury grading is limited
• Return to full sporting activity was not measured, only return to play clearance criteria, reducing external validity

Why it matters

For patients

Athletes recovering from a hamstring strain do not need to wait for complete pain resolution before progressing rehabilitation exercises; working through mild discomfort (up to 4/10) appears safe and may result in better muscle strength recovery.

For clinicians

Pain-threshold guidance (allowing up to 4/10 NRS during exercise) can replace strict pain-free rules in hamstring strain rehab without increasing re-injury risk, and appears to improve knee flexor strength and muscle architecture outcomes at two months post-clearance.

For readers

This is the first RCT to directly compare pain-free versus pain-threshold hamstring rehabilitation; findings challenge long-standing pain-avoidance guidelines that lacked a scientific basis.