Different leg exercises make different parts of your thigh muscles grow bigger—leg extensions grow all parts of one muscle, while squats mainly grow the middle part of another.
Scientific Claim
Exercise selection influences regional muscle hypertrophy in the rectus femoris and vastus lateralis, with leg extensions leading to growth in all three regions (proximal, central, distal) of the rectus femoris, while smith machine squats primarily stimulate central growth of the vastus lateralis.
Original Statement
“Results show that the three regions of RF grew significantly in the participants of the LEG group (p < 0.05), while only the central region of VL grew significantly in the SMTH group (p < 0.05).”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
Although the RCT design supports causal inference, the abstract lacks blinding details, introducing potential bias. The language 'influences' is too definitive; 'may influence' better reflects uncertainty.
More Accurate Statement
“Exercise selection may influence regional muscle hypertrophy in the rectus femoris and vastus lateralis, with leg extensions potentially leading to growth in all three regions of the rectus femoris, while smith machine squats may primarily stimulate central growth of the vastus lateralis.”
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 regional hypertrophy patterns are consistent across multiple RCTs using different populations, equipment, and protocols.
Whether the observed regional hypertrophy patterns are consistent across multiple RCTs using different populations, equipment, and protocols.
What This Would Prove
Whether the observed regional hypertrophy patterns are consistent across multiple RCTs using different populations, equipment, and protocols.
Ideal Study Design
A meta-analysis of 10+ high-quality RCTs (n≥50 per study) comparing leg extensions vs. squats in healthy adults aged 18–40, with standardized training volume (3–4 sets × 8–12 reps, 70–80% 1RM), 8–12 weeks duration, and regional muscle growth measured via MRI or DXA at proximal, central, and distal sites of RF and VL.
Limitation: Cannot establish mechanisms or isolate effects of individual variables like movement pattern or joint torque.
Randomized Controlled TrialLevel 1bIn EvidenceCausal effect of specific exercises on regional muscle growth under controlled, blinded conditions.
Causal effect of specific exercises on regional muscle growth under controlled, blinded conditions.
What This Would Prove
Causal effect of specific exercises on regional muscle growth under controlled, blinded conditions.
Ideal Study Design
A double-blind, placebo-controlled RCT of 100 healthy adults aged 20–35, randomized to 12 weeks of leg extensions or smith machine squats (matched volume/intensity), with participants blinded to hypothesis, assessors blinded to group assignment, and regional hypertrophy measured via serial MRI with standardized slice positioning and segmentation.
Limitation: Cannot blind participants to exercise type, limiting blinding validity.
Prospective Cohort StudyLevel 2bLong-term association between habitual exercise selection and regional muscle development in real-world settings.
Long-term association between habitual exercise selection and regional muscle development in real-world settings.
What This Would Prove
Long-term association between habitual exercise selection and regional muscle development in real-world settings.
Ideal Study Design
A 2-year prospective cohort of 200 resistance-trained individuals tracking their primary lower-body exercises (e.g., squats, leg extensions, lunges) and measuring regional RF/VL hypertrophy via annual DXA scans, adjusting for training history, nutrition, and genetics.
Limitation: Cannot control for confounding variables like training technique or recovery habits.
Case-Control StudyLevel 3bWhether individuals with disproportionate regional hypertrophy have a history of specific exercise preferences.
Whether individuals with disproportionate regional hypertrophy have a history of specific exercise preferences.
What This Would Prove
Whether individuals with disproportionate regional hypertrophy have a history of specific exercise preferences.
Ideal Study Design
A case-control study comparing 50 individuals with pronounced distal RF hypertrophy to 50 matched controls without it, retrospectively analyzing their 2+ years of training logs for frequency and type of leg exercises performed.
Limitation: Prone to recall bias and cannot establish temporal sequence.
Evidence from Studies
Supporting (1)
The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial
This study found that doing leg extensions makes the whole front thigh muscle grow, but doing squats only makes the middle part of the outer thigh muscle grow — just like the claim said.