If you want to make the front part of your thigh (rectus femoris) bigger, leg extensions work better than leg presses — after 12 weeks, leg extensions made it grow 12% more than leg presses did.
Scientific Claim
In untrained adults, knee extension training is associated with significantly greater hypertrophy of the rectus femoris muscle (+13.2%) compared to leg press training (+1.1%) after 12 weeks of training at 70% one-repetition maximum, twice weekly.
Original Statement
“Rectus femoris volume gains were greater for KE than LP (+13.2% vs. +1.1%, 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 randomization or control for confounders. The verb 'builds' implies causation, but only association can be claimed. The finding is specific to untrained adults under this protocol.
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 preferential rectus femoris hypertrophy with knee extension is reproducible across populations, training volumes, and equipment types.
Whether the preferential rectus femoris hypertrophy with knee extension is reproducible across populations, training volumes, and equipment types.
What This Would Prove
Whether the preferential rectus femoris hypertrophy with knee extension is reproducible across populations, training volumes, and equipment types.
Ideal Study Design
A meta-analysis of all RCTs comparing rectus femoris hypertrophy via MRI after knee extension vs. leg press in untrained adults, using standardized protocols (70% 1RM, 5×10, 2x/week, 10–16 weeks), with subgroup analyses by sex, age, and baseline strength.
Limitation: Cannot determine if the effect is due to mechanical tension, metabolic stress, or neural adaptation.
Randomized Controlled TrialLevel 1bWhether knee extension causally induces greater rectus femoris growth than leg press in a controlled setting.
Whether knee extension causally induces greater rectus femoris growth than leg press in a controlled setting.
What This Would Prove
Whether knee extension causally induces greater rectus femoris growth than leg press in a controlled setting.
Ideal Study Design
A double-blind, randomized crossover RCT with 40+ untrained adults, each performing 12 weeks of knee extension and leg press (70% 1RM, 5×10, 2x/week) in random order, with MRI-measured rectus femoris volume as primary outcome, and EMG-controlled for muscle activation.
Limitation: Cannot determine if the effect persists beyond 12 weeks or in trained individuals.
Prospective Cohort StudyLevel 2bWhether individuals who regularly perform knee extension in real-world settings show greater rectus femoris size over time compared to those who use only compound movements.
Whether individuals who regularly perform knee extension in real-world settings show greater rectus femoris size over time compared to those who use only compound movements.
What This Would Prove
Whether individuals who regularly perform knee extension in real-world settings show greater rectus femoris size over time compared to those who use only compound movements.
Ideal Study Design
A 2-year prospective cohort study of 150+ gym-goers who self-select either knee extension or leg press as their primary quadriceps exercise, with annual MRI scans of rectus femoris volume, controlling for total training volume, nutrition, and other exercises.
Limitation: Cannot rule out selection bias or confounding from other training variables.
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 knee extensions made the rectus femoris muscle grow much more than doing leg presses — exactly what the claim says.