Even if you work out the same amount and intensity, what exercise you pick can change which part of your muscle gets bigger.
Scientific Claim
Exercise selection may be responsible for differences in regional muscle growth patterns between leg extensions and smith machine squats, even when training volume and intensity are held constant.
Original Statement
“It seems that the chosen exercises may be responsible for the differences observed in this study.”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design supports claim
Appropriate Language Strength
probability
Can suggest probability/likelihood
Assessment Explanation
The RCT design supports causal inference, but without blinding or control for technique, the claim 'may be responsible' is still too strong. 'May contribute to' better reflects uncertainty.
More Accurate Statement
“Exercise selection may contribute to differences in regional muscle growth patterns between leg extensions and smith machine squats, even when training volume and intensity are held constant.”
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 exercise selection consistently explains regional hypertrophy differences across diverse populations and protocols.
Whether exercise selection consistently explains regional hypertrophy differences across diverse populations and protocols.
What This Would Prove
Whether exercise selection consistently explains regional hypertrophy differences across diverse populations and protocols.
Ideal Study Design
A meta-analysis of 15+ RCTs comparing paired exercises (e.g., leg extension vs. squat, leg curl vs. deadlift) with matched volume/intensity, measuring regional hypertrophy via MRI/DXA in healthy adults, stratified by training status and muscle group.
Limitation: Cannot determine biomechanical mechanisms behind regional differences.
Randomized Controlled TrialLevel 1bIn EvidenceCausal role of exercise selection in regional hypertrophy under controlled conditions.
Causal role of exercise selection in regional hypertrophy under controlled conditions.
What This Would Prove
Causal role of exercise selection in regional hypertrophy under controlled conditions.
Ideal Study Design
A double-blind, crossover RCT of 40 trained men and women, each performing 8 weeks of leg extensions and 8 weeks of smith squats (randomized order, 4-week washout), with regional RF/VL growth measured via MRI, and EMG used to confirm neuromuscular activation patterns.
Limitation: Crossover design may be limited by carryover effects in highly trained individuals.
Prospective Cohort StudyLevel 2bReal-world association between habitual exercise choice and regional muscle development over time.
Real-world association between habitual exercise choice and regional muscle development over time.
What This Would Prove
Real-world association between habitual exercise choice and regional muscle development over time.
Ideal Study Design
A 3-year prospective cohort of 300 resistance-trained individuals tracking their primary lower-body exercises via digital logs, with annual MRI scans of RF and VL to assess regional hypertrophy, adjusting for nutrition, sleep, and training history.
Limitation: Cannot control for unmeasured confounders like movement form or recovery.
Evidence from Studies
Supporting (1)
The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial
The study found that doing leg extensions made different parts of the thigh muscle grow compared to doing squats—even when people worked out the same amount and intensity—proving that the type of exercise matters for where muscles grow.