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 increases gluteus maximus and adductor magnus muscle volumes in untrained adults, while knee extension does 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 describes a within-subject comparison with MRI outcomes, but does not confirm RCT design elements (randomization, blinding). Without confirmed experimental control, causal language is unjustified.
More Accurate Statement
“Leg press training is associated with increased gluteus maximus and adductor magnus muscle volumes in untrained adults, 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 induces hypertrophy in gluteus maximus and adductor magnus across populations and training protocols.
Whether leg press consistently induces hypertrophy in gluteus maximus and adductor magnus across populations and training protocols.
What This Would Prove
Whether leg press consistently induces hypertrophy in gluteus maximus and adductor magnus across populations and training protocols.
Ideal Study Design
A meta-analysis of all RCTs comparing leg press and isolated knee extension in untrained adults, using MRI-measured changes in gluteus maximus and adductor magnus volumes as primary outcomes, with standardized volume (≥10 sets/week, 70% 1RM, 12+ weeks), controlling for sex, baseline muscle mass, and training frequency.
Limitation: Cannot determine if effects are due to mechanical tension, metabolic stress, or muscle activation patterns.
Randomized Controlled TrialLevel 1bWhether 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 RCT with 60+ untrained adults assigned to leg press or knee extension (70% 1RM, 5×10, 2x/week for 12 weeks), with MRI-measured changes in gluteus maximus and adductor magnus as primary outcomes, and control for total training volume and diet.
Limitation: Does not reflect long-term adherence or real-world training variability.
Prospective Cohort StudyLevel 2bWhether habitual leg press use predicts greater gluteus maximus and adductor magnus size in free-living populations.
Whether habitual leg press use predicts greater gluteus maximus and adductor magnus size in free-living populations.
What This Would Prove
Whether habitual leg press use predicts greater gluteus maximus and adductor magnus size in free-living populations.
Ideal Study Design
A 2-year prospective cohort study of 200+ adults who regularly perform leg press or knee extension as their primary lower-body exercise, with annual MRI scans of gluteus maximus and adductor magnus volumes, controlling for total resistance training volume, age, sex, and activity level.
Limitation: Cannot control for technique differences or confounding exercises (e.g., squats, deadlifts).
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 makes your butt and inner thigh muscles bigger, but doing knee extensions doesn’t — which is exactly what the claim says.