Leg presses make your butt and inner thigh muscles bigger, but leg extensions don’t affect those muscles at all.
Scientific Claim
Leg press training is associated with significant increases in gluteus maximus and adductor magnus muscle volume, while knee extension training is not.
Original Statement
“LP, but not KE, increased volumes of the gluteus maximus (+15.4%) and the adductor magnus (+6.2%) (P ≤ 0.001).”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design cannot support claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The abstract does not confirm RCT design elements (randomization, blinding), so causal language like 'increases' is inappropriate. The observed differences are associations from a within-subject comparison.
More Accurate Statement
“Leg press training is associated with significant increases in gluteus maximus and adductor magnus muscle volume, while knee extension training is not associated with changes in these 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 leg press consistently produces greater hypertrophy in gluteus maximus and adductor magnus than knee extension across multiple studies.
Whether leg press consistently produces greater hypertrophy in gluteus maximus and adductor magnus than knee extension across multiple studies.
What This Would Prove
Whether leg press consistently produces greater hypertrophy in gluteus maximus and adductor magnus than knee extension across multiple studies.
Ideal Study Design
A meta-analysis of at least 6 RCTs comparing leg press and knee extension training (≥10 weeks, 70–80% 1RM, 2–3x/week) in untrained adults, using MRI or DXA to measure gluteus maximus and adductor magnus volume changes as primary outcomes.
Limitation: Cannot determine if differences are due to exercise mechanics or training volume differences.
Randomized Controlled TrialLevel 1bIn EvidenceWhether leg press causes greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
Whether leg press causes greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
What This Would Prove
Whether leg press causes greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
Ideal Study Design
A double-blind, randomized, crossover RCT with 30+ untrained adults performing 12 weeks of leg press and knee extension (counterbalanced order, 70% 1RM, 5×10, 2x/week), with MRI-measured gluteus maximus and adductor magnus volume changes as primary outcomes, and EMG to confirm activation patterns.
Limitation: Limited to untrained individuals; may not reflect effects in trained athletes.
Prospective Cohort StudyLevel 2bWhether habitual leg press use is associated with larger gluteus maximus and adductor magnus size compared to habitual knee extension use.
Whether habitual leg press use is associated with larger gluteus maximus and adductor magnus size compared to habitual knee extension use.
What This Would Prove
Whether habitual leg press use is associated with larger gluteus maximus and adductor magnus size compared to habitual knee extension use.
Ideal Study Design
A 1-year prospective cohort study of 150+ adults who regularly use either leg press or knee extension as their primary lower-body exercise, with baseline and follow-up MRI measurements of gluteus maximus and adductor magnus volume, controlling for total weekly volume, diet, and activity.
Limitation: Cannot control for self-selection bias or differences in form and intensity.
Evidence from Studies
Supporting (1)
Hypertrophic Effects of Single- versus Multi-Joint Exercise: A Direct Comparison Between Knee Extension and Leg Press.
The study found that doing leg presses made your butt and inner thigh muscles bigger, but doing knee extensions didn’t — which is exactly what the claim says.