Doing two different hamstring exercises—Nordic curls and stiff-leg deadlifts—makes you stronger at both, even if your muscles don’t grow the same way, meaning muscle growth isn’t what’s making you stronger in the other exercise.
Scientific Claim
Training with the Nordic hamstring exercise or stiff-leg deadlift for nine weeks, three times per week, produces similar increases in strength transfer between the two exercises in resistance-untrained individuals, despite differing patterns of hamstring muscle hypertrophy, indicating that strength gains in untrained movements are not dependent on muscle size changes in the trained muscles.
Original Statement
“Conversely, no difference in strength transfer was observed between the NHE group (10.7±8.5%) and the SDL group (20.7±15.0%) (p=0.06).”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
The p-value of 0.06 is not statistically significant, and effect sizes overlap substantially. While the RCT design supports causal inference, the lack of statistical significance means the conclusion should reflect probability, not certainty.
More Accurate Statement
“Training with the Nordic hamstring exercise or stiff-leg deadlift for nine weeks, three times per week, likely produces similar increases in strength transfer between the two exercises in resistance-untrained individuals, despite differing patterns of hamstring muscle hypertrophy, suggesting that strength gains in untrained movements may not depend on muscle size changes in the trained muscles.”
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Systematic Review & Meta-AnalysisLevel 1aWhether the pattern of minimal hypertrophy-dependence in strength transfer between NHE and SDL is consistent across diverse populations and training protocols.
Whether the pattern of minimal hypertrophy-dependence in strength transfer between NHE and SDL is consistent across diverse populations and training protocols.
What This Would Prove
Whether the pattern of minimal hypertrophy-dependence in strength transfer between NHE and SDL is consistent across diverse populations and training protocols.
Ideal Study Design
A meta-analysis of 15+ randomized controlled trials (n≥300 total participants) comparing strength transfer between NHE and SDL in resistance-untrained adults, with standardized 8–12 week programs, MRI-measured hamstring hypertrophy, and 1RM transfer as primary outcome, stratified by sex, age, and baseline strength.
Limitation: Cannot establish causation in individual studies, only summarize existing evidence.
Randomized Controlled TrialLevel 1bIn EvidenceWhether the observed trend in strength transfer is statistically significant with greater power and confirms independence from hypertrophy.
Whether the observed trend in strength transfer is statistically significant with greater power and confirms independence from hypertrophy.
What This Would Prove
Whether the observed trend in strength transfer is statistically significant with greater power and confirms independence from hypertrophy.
Ideal Study Design
A double-blind, placebo-controlled RCT with 100+ resistance-untrained adults randomized to NHE, SDL, or control, with pre/post MRI volumetry of SM, ST, BF, and 1RM transfer measured, using a 12-week, 3x/week protocol with standardized load progression.
Limitation: Cannot isolate neural vs. structural mechanisms underlying strength transfer.
Prospective Cohort StudyLevel 2bWhether the relationship between hypertrophy and strength transfer holds over longer durations and in trained populations.
Whether the relationship between hypertrophy and strength transfer holds over longer durations and in trained populations.
What This Would Prove
Whether the relationship between hypertrophy and strength transfer holds over longer durations and in trained populations.
Ideal Study Design
A 6-month prospective cohort of 200 resistance-trained athletes following either NHE or SDL training, with monthly MRI scans of hamstring muscles and biweekly 1RM assessments for both exercises, controlling for volume, intensity, and nutrition.
Limitation: Cannot control for confounding variables as rigorously as an RCT.
Cross-Sectional StudyLevel 3Whether individuals with greater hamstring hypertrophy consistently show less strength transfer between NHE and SDL in real-world settings.
Whether individuals with greater hamstring hypertrophy consistently show less strength transfer between NHE and SDL in real-world settings.
What This Would Prove
Whether individuals with greater hamstring hypertrophy consistently show less strength transfer between NHE and SDL in real-world settings.
Ideal Study Design
A cross-sectional analysis of 300 resistance-trained individuals with documented 6+ months of NHE or SDL training, measuring hamstring volume via MRI and strength transfer via 1RM testing of the non-trained exercise, controlling for training history and volume.
Limitation: Cannot determine direction of causality or temporal sequence.
Evidence from Studies
Supporting (1)
People who trained with one hamstring exercise got stronger at the other exercise too—even though their muscles didn’t grow the same way. This means getting stronger doesn’t always mean your muscles got bigger.