If you want to get stronger at squats, doing squats is way better than doing leg extensions — but if you want to get stronger at leg extensions, both exercises work about the same.
Scientific Claim
In untrained young women, 8 weeks of Smith machine back squat training leads to greater improvements in back squat 3RM strength (+46.7%) compared to leg extension training (+21.3%), while both exercises produce similar gains in leg extension 3RM strength (+19.8% vs. +23.4%).
Original Statement
“Smith machine back squat induced greater increases in 3RM-SQ (+46.7 vs. 21.3%; p < 0.001), but no between-group difference was observed in 3RM-LE increases (SQ = +19.8% vs. LE = +23.4%; p = 0.824).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
RCT design supports causal claims, but lack of blinding and abstract-only access require cautious language. 'Leads to' is appropriately softened to 'likely leads to'.
More Accurate Statement
“In untrained young women, 8 weeks of Smith machine back squat training likely leads to greater improvements in back squat 3RM strength (+46.7%) compared to leg extension training (+21.3%), while both exercises are likely similarly effective for increasing leg extension 3RM strength (+19.8% vs. +23.4%).”
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 training with a specific movement consistently produces greater strength gains in that movement compared to alternative exercises.
Whether training with a specific movement consistently produces greater strength gains in that movement compared to alternative exercises.
What This Would Prove
Whether training with a specific movement consistently produces greater strength gains in that movement compared to alternative exercises.
Ideal Study Design
A meta-analysis of 20+ RCTs comparing compound vs. isolation exercises for strength gains in untrained young women, using standardized 3RM testing for squat and leg extension, with matched volume and intensity.
Limitation: Cannot account for individual biomechanical or neuromuscular variability across studies.
Randomized Controlled TrialLevel 1bIn EvidenceCausal effect of back squat vs. leg extension on exercise-specific strength gains.
Causal effect of back squat vs. leg extension on exercise-specific strength gains.
What This Would Prove
Causal effect of back squat vs. leg extension on exercise-specific strength gains.
Ideal Study Design
A double-blind, parallel-group RCT of 100 untrained women aged 18–30, randomized to 8 weeks of Smith machine back squat or leg extension training (3×8–12 RM, 2×/week), with blinded 3RM testing for both exercises at baseline and post-intervention.
Limitation: Limited to Smith machine squats; may not generalize to free-weight squats or other populations.
Prospective Cohort StudyLevel 2bReal-world association between exercise choice and strength gains in squat and leg extension movements.
Real-world association between exercise choice and strength gains in squat and leg extension movements.
What This Would Prove
Real-world association between exercise choice and strength gains in squat and leg extension movements.
Ideal Study Design
A 1-year prospective cohort of 200 untrained women who self-select either squats or leg extensions as their primary lower-body exercise, with monthly 3RM testing for both movements and controlled for total training volume and nutrition.
Limitation: Self-selection bias may influence adherence and motivation, confounding strength outcomes.
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 squats made women much stronger at squats, while doing leg extensions didn’t help squats much — but both exercises made them about equally stronger at leg extensions.