The muscle underneath your bicep gets just as strong and big whether you curl with your arm in front or behind you.
Scientific Claim
The brachialis muscle increases in thickness by approximately 8–10% after 10 weeks of unilateral cable curl training, with no difference between shoulder flexed and extended positions, indicating it responds robustly to elbow flexion resistance training regardless of shoulder angle.
Original Statement
“Similar responses were observed between conditions for... brachialis (PREA: 10% [ES: 0.72], BAYE: 8% [ES: 0.65]; p = 0.911)... no significant difference was detected between biceps brachii and brachialis for PREA or BAYE.”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
definitive
Can make definitive causal claims
Assessment Explanation
The RCT design with direct ultrasound measurements and equivalence testing supports definitive causal claims about brachialis growth under these specific conditions.
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 brachialis hypertrophy is consistently similar across different elbow flexion exercises and shoulder positions in diverse populations.
Whether brachialis hypertrophy is consistently similar across different elbow flexion exercises and shoulder positions in diverse populations.
What This Would Prove
Whether brachialis hypertrophy is consistently similar across different elbow flexion exercises and shoulder positions in diverse populations.
Ideal Study Design
A systematic review and meta-analysis of all RCTs measuring brachialis thickness via ultrasound during 8–16 weeks of elbow flexion training, comparing shoulder flexed, extended, and neutral positions, with standardized training volume and resistance profiles.
Limitation: Cannot determine if brachialis responds differently to eccentric overload or longer durations.
Randomized Controlled TrialLevel 1bWhether brachialis hypertrophy is greater with higher training volumes or eccentric emphasis compared to biceps brachii.
Whether brachialis hypertrophy is greater with higher training volumes or eccentric emphasis compared to biceps brachii.
What This Would Prove
Whether brachialis hypertrophy is greater with higher training volumes or eccentric emphasis compared to biceps brachii.
Ideal Study Design
A 12-week RCT of 40 young men randomized to 3 groups: high-volume cable curls (5 sets), eccentric-focused curls (4s eccentric), or control (2 sets), measuring brachialis and biceps brachii thickness at three regions weekly via ultrasound.
Limitation: Does not test effects in women or trained individuals.
Prospective Cohort StudyLevel 2bWhether habitual use of specific curl variations correlates with brachialis size in experienced lifters.
Whether habitual use of specific curl variations correlates with brachialis size in experienced lifters.
What This Would Prove
Whether habitual use of specific curl variations correlates with brachialis size in experienced lifters.
Ideal Study Design
A 2-year prospective cohort of 100 resistance-trained individuals tracking their primary curl variation (Preacher, incline, barbell) and annual brachialis thickness via ultrasound, controlling for total volume, age, and training history.
Limitation: Cannot establish causation due to self-selection bias.
Animal Model StudyLevel 5Whether mechanical loading during elbow flexion preferentially activates brachialis vs. biceps brachii muscle fibers.
Whether mechanical loading during elbow flexion preferentially activates brachialis vs. biceps brachii muscle fibers.
What This Would Prove
Whether mechanical loading during elbow flexion preferentially activates brachialis vs. biceps brachii muscle fibers.
Ideal Study Design
A study in rats with implanted EMG electrodes and muscle biopsies during electrically induced elbow flexion at different shoulder angles, measuring fiber recruitment patterns and mTOR activation in brachialis vs. biceps brachii over 6 weeks.
Limitation: Cannot replicate human training adaptations or neural control.
Cross-Sectional StudyLevel 3bWhether elite athletes with different curl preferences have larger brachialis muscles.
Whether elite athletes with different curl preferences have larger brachialis muscles.
What This Would Prove
Whether elite athletes with different curl preferences have larger brachialis muscles.
Ideal Study Design
A cross-sectional analysis of 50 elite bodybuilders and powerlifters who specialize in either Preacher or incline curls, measuring brachialis thickness via ultrasound and comparing to biceps brachii, controlling for training age and body composition.
Limitation: Cannot determine if differences are caused by training or genetics.
Evidence from Studies
Supporting (1)
The study had people do arm curls with their shoulders in two different positions and found that their brachialis muscle got about 8–10% thicker in both cases — so shoulder position doesn’t matter, just doing the curls does.