Doing leg extensions for 8 weeks makes the front thigh muscle bigger along its whole length more than doing squats, especially near the knee.
Scientific Claim
In untrained young women, 8 weeks of leg extension training leads to greater hypertrophy of the rectus femoris muscle at proximal, middle, and distal sites compared to Smith machine back squat training, with increases of 11.4%, 12.3%, and 17.5% versus 2.0%, 5.7%, and 7.9%, respectively.
Original Statement
“The LE experienced greater increases in the 3 RF sites (proximal: +11.4% vs. +2.0%; middle: +12.3% vs. +5.7%; distal: 17.5% vs. +7.9%; all p < 0.001).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
Although the study is an RCT, blinding was not confirmed, and abstract-only access limits certainty. The verb 'leads to' is appropriately tempered to 'likely leads to' under cautious interpretation.
More Accurate Statement
“In untrained young women, 8 weeks of leg extension training likely leads to greater hypertrophy of the rectus femoris muscle at proximal, middle, and distal sites compared to Smith machine back squat training, with increases of 11.4%, 12.3%, and 17.5% versus 2.0%, 5.7%, and 7.9%, respectively.”
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 extensions consistently produce greater rectus femoris hypertrophy than back squats across diverse populations and protocols.
Whether leg extensions consistently produce greater rectus femoris hypertrophy than back squats across diverse populations and protocols.
What This Would Prove
Whether leg extensions consistently produce greater rectus femoris hypertrophy than back squats across diverse populations and protocols.
Ideal Study Design
A meta-analysis of 15+ RCTs comparing leg extensions vs. back squats in untrained young women, using standardized ultrasound measurements of rectus femoris thickness at proximal, middle, and distal sites, with 12-week training durations, 2–3 sessions/week, and 8–12 RM loads.
Limitation: Cannot establish causation in individual participants or account for all confounding variables across studies.
Randomized Controlled TrialLevel 1bIn EvidenceCausal effect of leg extension vs. back squat on rectus femoris hypertrophy under controlled conditions.
Causal effect of leg extension vs. back squat on rectus femoris hypertrophy under controlled conditions.
What This Would Prove
Causal effect of leg extension vs. back squat on rectus femoris hypertrophy under controlled conditions.
Ideal Study Design
A double-blind, parallel-group RCT of 100 untrained women aged 18–30, randomized to 8 weeks of leg extension or Smith machine back squat training (3 sets × 8–12 RM, 2×/week), with muscle thickness measured via ultrasound by blinded assessors at proximal, middle, and distal rectus femoris sites.
Limitation: Cannot generalize to other populations (e.g., men, trained individuals) or longer timeframes.
Prospective Cohort StudyLevel 2bLong-term association between exercise choice and rectus femoris growth in real-world settings.
Long-term association between exercise choice and rectus femoris growth in real-world settings.
What This Would Prove
Long-term association between exercise choice and rectus femoris growth in real-world settings.
Ideal Study Design
A 1-year prospective cohort tracking 200 untrained women who self-select leg extensions or back squats as their primary lower-body exercise, with monthly ultrasound measurements of rectus femoris thickness and controlled for diet, sleep, and total training volume.
Limitation: Cannot rule out confounding from self-selection bias or adherence differences.
Evidence from Studies
Supporting (1)
Comparison of Muscle Hypertrophy and Strength Adaptations Induced by Back Squat and Leg Extension Resistance Exercises
The study found that doing leg extensions made the front thigh muscle grow more than doing squats, exactly as the claim says — with numbers that match perfectly.