Doing leg presses and leg extensions for 12 weeks both make your main thigh muscles bigger in about the same way, but only leg extensions make the front part of your thigh (rectus femoris) grow significantly.
Scientific Claim
In untrained adults, 12 weeks of leg press training produces similar increases in overall quadriceps femoris and vasti muscle volumes as knee extension training, but does not significantly increase rectus femoris volume, whereas knee extension does.
Original Statement
“Muscle volumes of the individual and whole QF significantly increased in both conditions (P ≤ 0.026), except for the rectus femoris in the LP condition (P = 0.379). Rectus femoris volume gains were greater for KE than LP (+13.2% vs. +1.1%, P ≤ 0.001), but gains in the vasti muscles (+5.0–7.2% vs. +4.4–6.2%) and whole QF (+7.1% vs. +4.9%) were comparable between conditions (P ≥ 0.319).”
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 measurements, but does not explicitly state randomization, blinding, or control group status. Without confirmed RCT design, causal language is inappropriate. The observed differences are associations, not proven effects.
More Accurate Statement
“In untrained adults, 12 weeks of leg press training is associated with similar increases in overall quadriceps femoris and vasti muscle volumes as knee extension training, but is not associated with significant rectus femoris volume gain, whereas knee extension is.”
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 differential hypertrophic effects of leg press vs. knee extension on rectus femoris and vasti muscles are consistent across multiple RCTs in untrained populations.
Whether the differential hypertrophic effects of leg press vs. knee extension on rectus femoris and vasti muscles are consistent across multiple RCTs in untrained populations.
What This Would Prove
Whether the differential hypertrophic effects of leg press vs. knee extension on rectus femoris and vasti muscles are consistent across multiple RCTs in untrained populations.
Ideal Study Design
A meta-analysis of at least 5 high-quality RCTs (n≥50 per group) comparing 10–16 weeks of supervised leg press vs. knee extension training (70–80% 1RM, 3–5 sets, 2–3x/week) in healthy untrained adults aged 18–40, using MRI-measured muscle volume changes in rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius as primary outcomes.
Limitation: Cannot establish causation in individual studies, only summarizes existing associations.
Randomized Controlled TrialLevel 1bIn EvidenceWhether leg press and knee extension cause different patterns of quadriceps muscle hypertrophy in untrained adults.
Whether leg press and knee extension cause different patterns of quadriceps muscle hypertrophy in untrained adults.
What This Would Prove
Whether leg press and knee extension cause different patterns of quadriceps muscle hypertrophy in untrained adults.
Ideal Study Design
A double-blind, randomized, crossover RCT with 40+ untrained adults (18–40 years, no prior resistance training) performing 12 weeks of leg press (one leg) and knee extension (contralateral leg) at 70% 1RM, 5 sets × 10 reps, 2x/week, with MRI-measured muscle volume changes in all four QF heads as primary outcome, and surface EMG to confirm neuromuscular activation patterns.
Limitation: Cannot generalize to trained individuals or different training volumes/frequencies.
Prospective Cohort StudyLevel 2bWhether long-term use of leg press vs. knee extension is associated with differential quadriceps muscle development in real-world settings.
Whether long-term use of leg press vs. knee extension is associated with differential quadriceps muscle development in real-world settings.
What This Would Prove
Whether long-term use of leg press vs. knee extension is associated with differential quadriceps muscle development in real-world settings.
Ideal Study Design
A 1-year prospective cohort study tracking 200+ untrained adults who self-select either leg press or knee extension as their primary quadriceps exercise, with baseline and follow-up MRI measurements of QF muscle volumes, controlling for total training volume, diet, and activity levels.
Limitation: Cannot rule out confounding by self-selection bias or adherence differences.
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 both leg press and knee extension make the main thigh muscles bigger in similar ways, but only knee extension makes the front part of the thigh (rectus femoris) grow — just like the claim said.