Doing leg presses and leg extensions for 12 weeks both make your main thigh muscles bigger about the same amount, but leg presses also make your butt and inner thigh muscles grow more, while leg extensions make the front part of your thigh (rectus femoris) grow much more.
Scientific Claim
In untrained adults, 12 weeks of leg press training at 70% one-repetition maximum, performed twice weekly, is associated with similar increases in total quadriceps femoris volume (+4.9%) as knee extension training (+7.1%), but leg press produces significantly greater hypertrophy in the gluteus maximus (+15.4%) and adductor magnus (+6.2%), while knee extension produces significantly greater rectus femoris hypertrophy (+13.2% vs. +1.1%).
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). 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 design but does not explicitly state randomization, blinding, or control for confounders. Causal language like 'produces' or 'increases' is inappropriate; only association can be claimed. The findings are limited to this specific sample and protocol.
More Accurate Statement
“In untrained adults, 12 weeks of leg press training at 70% one-repetition maximum, performed twice weekly, is associated with similar increases in total quadriceps femoris volume (+4.9%) as knee extension training (+7.1%), but leg press is associated with significantly greater hypertrophy in the gluteus maximus (+15.4%) and adductor magnus (+6.2%), while knee extension is associated with significantly greater rectus femoris hypertrophy (+13.2% vs. +1.1%).”
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 observed muscle-specific hypertrophy differences between leg press and knee extension are consistent across diverse populations, training volumes, and durations.
Whether the observed muscle-specific hypertrophy differences between leg press and knee extension are consistent across diverse populations, training volumes, and durations.
What This Would Prove
Whether the observed muscle-specific hypertrophy differences between leg press and knee extension are consistent across diverse populations, training volumes, and durations.
Ideal Study Design
A meta-analysis of all randomized controlled trials comparing leg press and knee extension in untrained or recreationally active adults, using MRI-measured muscle volume changes in the quadriceps, gluteus maximus, and adductor magnus as primary outcomes, with standardized training protocols (70% 1RM, 5 sets × 10 reps, 2x/week for 10–16 weeks) and controlled for protein intake and activity levels.
Limitation: Cannot establish causation in individual studies, only summarizes existing associations.
Randomized Controlled TrialLevel 1bWhether the observed muscle-specific hypertrophy differences are causally linked to the exercise modality in a controlled setting.
Whether the observed muscle-specific hypertrophy differences are causally linked to the exercise modality in a controlled setting.
What This Would Prove
Whether the observed muscle-specific hypertrophy differences are causally linked to the exercise modality in a controlled setting.
Ideal Study Design
A double-blind, randomized crossover RCT with 50+ untrained adults, each performing 12 weeks of leg press and knee extension (70% 1RM, 5×10, 2x/week) in random order, with MRI-measured muscle volume changes in 17 lower-limb muscles as primary outcome, and dietary and activity logs controlled.
Limitation: Cannot generalize to trained individuals or different training intensities without additional trials.
Prospective Cohort StudyLevel 2bWhether long-term use of leg press vs. knee extension in real-world settings is associated with differential muscle growth patterns in diverse populations.
Whether long-term use of leg press vs. knee extension in real-world settings is associated with differential muscle growth patterns in diverse populations.
What This Would Prove
Whether long-term use of leg press vs. knee extension in real-world settings is associated with differential muscle growth patterns in diverse populations.
Ideal Study Design
A 2-year prospective cohort study tracking 200+ healthy adults who self-select leg press or knee extension as their primary quadriceps exercise, with annual MRI scans of lower-limb muscles, controlling for training volume, nutrition, and other resistance exercises.
Limitation: Cannot rule out confounding from self-selection bias or unmeasured lifestyle factors.
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 leg press and knee extension both made the front thigh muscles bigger about the same amount, but leg press also made your butt and inner thigh muscles grow more, while knee extension made one specific thigh muscle (rectus femoris) grow much more — just like the claim said.