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 the volume of the gluteus maximus and adductor magnus 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 but does not confirm randomization, blinding, or control group status. Without confirmed RCT design, causal language is inappropriate. The observed differences are associations.
More Accurate Statement
“Leg press training is associated with increased volume of the gluteus maximus and adductor magnus 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 produces greater hypertrophy in gluteus maximus and adductor magnus than single-joint knee-dominant exercises across populations.
Whether leg press consistently produces greater hypertrophy in gluteus maximus and adductor magnus than single-joint knee-dominant exercises across populations.
What This Would Prove
Whether leg press consistently produces greater hypertrophy in gluteus maximus and adductor magnus than single-joint knee-dominant exercises across populations.
Ideal Study Design
A meta-analysis of all RCTs comparing leg press to knee extension or other isolated quadriceps exercises in untrained adults, using MRI or DXA to measure gluteus maximus and adductor magnus volume changes after 8–16 weeks of matched volume training.
Limitation: Cannot determine if differences are due to mechanical tension or neural adaptation.
Randomized Controlled TrialLevel 1bIn EvidenceWhether leg press causally induces greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
Whether leg press causally induces greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
What This Would Prove
Whether leg press causally induces greater hypertrophy in gluteus maximus and adductor magnus than knee extension.
Ideal Study Design
A double-blind, randomized RCT with 60+ untrained adults (age 20–35), assigned to either leg press or knee extension training (70% 1RM, 5×10, 2x/week for 12 weeks), with MRI-measured changes in gluteus maximus and adductor magnus as primary outcomes, and matched total training volume.
Limitation: Does not reflect real-world training variability or long-term adherence.
Prospective Cohort StudyLevel 2bWhether habitual leg press use in the general population correlates with larger gluteus maximus and adductor magnus size.
Whether habitual leg press use in the general population correlates with larger gluteus maximus and adductor magnus size.
What This Would Prove
Whether habitual leg press use in the general population correlates with larger gluteus maximus and adductor magnus size.
Ideal Study Design
A 2-year prospective cohort of 300+ gym users (age 18–45) who report using leg press ≥2x/week vs. those who avoid it, with baseline and follow-up MRI measurements of gluteus maximus and adductor magnus volume, controlling for total lower-body training volume and diet.
Limitation: Cannot rule out confounding from other exercises or recovery habits.
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.