Nordic hamstring curls reliably build eccentric hamstring strength and probably reduce hamstring injuries in male footballers, but the size of the effect is uncertain and untested in triathletes
Interpretation
The strength and muscle-architecture effect is consistent (Cuthbert 2020, 13 studies, low volume as effective as high). The injury outcome is contested: van Dyk 2019 (15 studies, 8,459 athletes) reported RR 0.49, but a reappraisal restricted to the five RCTs that randomised to the Nordic curl found RR 0.56 with a 95% CI of 0.20-1.52, and 0.59 (0.27-1.29) when updated (Impellizzeri 2021). The two key trials, 942 Danish and 579 Dutch male footballers, both showed large reductions, the largest for recurrent injuries. No trials in triathletes, runners or cyclists; few women or masters athletes; adherence is poor even in elite football (Bahr 2015).
Practical implication
A low-volume, gradually introduced Nordic curl is a reasonable option for triathletes who sprint, play other sports or have had a hamstring strain, and an optional extra for others. Injury benefit is extrapolated from male football, not shown in triathletes, and doesn't replace sensible run-load progression.
Sources
7 sources, independently verified.
Programmes including the Nordic hamstring exercise reduced hamstring injury risk: RR 0.49 (95% CI 0.32-0.74) overall; RR 0.52 (0.32-0.85) in eight randomised studies; RR 0.55 (0.34-0.89) after removing high risk-of-bias studies. Authors concluded a reduction of up to 51%. Pooled studies include multi-component programmes, not only trials randomising to the Nordic curl.
Reanalysis restricted to RCTs that randomised athletes to the Nordic hamstring exercise: only 5 of the 15 earlier studies qualified, RR 0.56 (95% CI 0.20-1.52); updated with one further RCT, RR 0.59 (0.27-1.29), parametric prediction interval 0.10-3.29. Evidence judged inconclusive and mostly at high risk of bias; at best a conditional recommendation, and only for football.
A 10-week progressive Nordic hamstring exercise programme followed by weekly in-season sessions: 15 vs 52 acute hamstring injuries; rates per 100 player-seasons 3.8 vs 13.1 overall (rate ratio 0.293), 3.1 vs 8.1 for new injuries (0.410) and 7.1 vs 45.8 for recurrent injuries (0.137). Men only, football only, cluster randomisation.
25 Nordic hamstring sessions over 13 weeks: 38 hamstring injuries in 36 of 579 players (6.2%); lower injury odds in the Nordic group (OR 0.282, 95% CI 0.110-0.721); no difference in severity; 91% compliance. Men only.
Very large within-study gains in eccentric hamstring strength for high-volume (g = 2.12) and low-volume (g = 2.28) programmes; fascicle length g >= 2.58; pennation angle reductions g >= 1.31. Low volume did not impair adaptations. Moderate to high heterogeneity. Strength and architecture outcomes, not injuries.
The Nordic hamstring programme was completed in full in only 16 of 150 club-seasons (10.7%) and partly in 9 (6%). Adherence is a major barrier even in professional settings.
26 studies: stretch-type hamstring injuries occur with extensive hip flexion and an extended knee; sprint-type injuries are most likely caused by excessive strain during eccentric contraction in the late swing phase of high-speed running.