Is pre-season eccentric strength testing during the Nordic hamstring exercise associated with future hamstring strain injury? A systematic review and meta-analysis

Key points

1. 01Meta-analysis of 6 prospective cohort studies (1100 athletes, 156 injuries) found no significant strength difference between athletes who were later injured and those who were not
2. 02No association was found whether strength was expressed as absolute force (N), relative to body mass (N/kg), or as a between-limb asymmetry percentage
3. 03Meta-regression showed that sport played, age, height, mass, and average cohort strength did not moderate these findings
4. 04The analysis was adequately powered to detect a moderate effect size (0.50) but not small effect sizes (0.20), so a small effect cannot be ruled out
5. 05NHE-based strength testing alone provides limited clinical information for prospective injury screening

How it was conducted

Design

Systematic review and meta-analysis of prospective cohort studies

Databases searched

CINAHL, Cochrane Library, Medline Complete, Embase, Web of Science, SPORTDiscus (January 2013 to January 10, 2020)

Included studies

6 prospective cohort studies

Participants

1100 male athletes (156 prospectively injured, 944 uninjured); sports included elite soccer, Australian Football, Gaelic Football, and Rugby Union

Exposure

Pre-season eccentric knee flexor strength quantified during Nordic hamstring exercise using a field-testing device

Primary outcome

Occurrence of hamstring strain injury during follow-up periods of 3 to 10 months post-testing

What they found

• Absolute knee flexor strength: SMD -0.22 (95% CI -0.50 to 0.05) for all prospectively injured limbs vs. uninjured controls - not significant
• Absolute knee flexor strength for recurrent injuries: SMD -0.32 (95% CI -0.77 to 0.13) - not significant
• Body mass-normalised knee flexor strength: SMD -0.23 (95% CI -0.55 to 0.10) for all injured limbs - not significant
• Body mass-normalised strength for recurrent injuries: SMD -0.32 (95% CI -0.90 to 0.26) - not significant
• Between-limb asymmetry: SMD 0.01 (95% CI -0.24 to 0.25) for all injured participants - not significant
• Between-limb asymmetry for recurrent injuries: SMD 0.28 (95% CI -0.14 to 0.70) - not significant
• Meta-regression for between-limb asymmetry found a significant effect of average age (p=0.007) but no other variable was significant (p>0.24)
• Pooled mean differences corresponded to approximately -9 N (95% CI -27 to 8 N) and -0.22 N/kg (95% CI -0.54 to 0.10 N/kg) between injured and uninjured control groups
• Risk of bias was low for 5 of 6 included studies; one study had high risk of bias due to unclear outcome definition and inadequate confounder consideration

Limitations

• Only 6 studies were available, limiting the power to detect small effect sizes; an estimated 19 studies would be needed to detect effects of 0.20
• All participants were male athletes in football codes, limiting generalisability to female athletes or other sports
• Injury diagnosis methods varied across studies, ranging from MRI-confirmed to clinician-diagnosed, introducing heterogeneity in the outcome definition
• Analysis was restricted to pre-season, single time-point testing; more frequent or in-season testing, or other eccentric assessment methods, may yield different findings

Why it matters

For patients

Athletes cannot rely on a single pre-season Nordic hamstring strength test to tell them whether they are at high or low risk of a hamstring tear.

For clinicians

Eccentric knee flexor strength measured with the NHE device should not be used in isolation as an injury screening tool; a multifactorial approach incorporating muscle architecture, prior injury history, and running exposure is needed.

For readers

The NHE is a valuable training intervention for reducing hamstring injury rates, but a one-off strength measurement from the same exercise does not reliably identify who will get injured.