Doing bicep curls with your arm in front of your body or behind your body gives you the same muscle growth and strength gains if you're using the same amount of weight and doing the same number of reps.
Scientific Claim
Ten weeks of unilateral cable curl training with the shoulder either flexed at 50° or extended at 50° produces equivalent increases in biceps brachii and brachialis muscle thickness (6–9% relative gain) and one-repetition maximum strength (28–37% improvement) in young, untrained men, indicating that shoulder joint position does not differentially affect hypertrophy or strength outcomes when resistance profiles are matched.
Original Statement
“Both conditions showed significant increases in muscle thickness (p < 0.05) with no significant differences between them (p > 0.05)... Similarly, significant improvements in maximum strength were observed (p < 0.05), with equivalent results between conditions...”
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 study is a high-quality RCT with randomization, within-subject design, and direct measurement of outcomes, allowing definitive causal claims within the studied population and interventions.
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 equivalence of hypertrophy and strength gains between shoulder flexed and extended elbow flexion exercises holds across diverse populations, training durations, and resistance profiles.
Whether the equivalence of hypertrophy and strength gains between shoulder flexed and extended elbow flexion exercises holds across diverse populations, training durations, and resistance profiles.
What This Would Prove
Whether the equivalence of hypertrophy and strength gains between shoulder flexed and extended elbow flexion exercises holds across diverse populations, training durations, and resistance profiles.
Ideal Study Design
A systematic review and meta-analysis of all RCTs comparing shoulder flexed (e.g., Preacher) vs. extended (e.g., incline) cable or dumbbell curls in healthy adults, with standardized ultrasound measurements of biceps brachii and brachialis thickness at proximal, mid, and distal regions, and 1RM strength as primary outcomes, including studies of 8–16 weeks duration and sample sizes ≥15 per group.
Limitation: Cannot establish causation for individual studies, only summarizes existing evidence with potential for publication bias.
Randomized Controlled TrialLevel 1bIn EvidenceWhether the equivalence observed in untrained men extends to trained individuals, women, or longer training durations (e.g., 16–24 weeks).
Whether the equivalence observed in untrained men extends to trained individuals, women, or longer training durations (e.g., 16–24 weeks).
What This Would Prove
Whether the equivalence observed in untrained men extends to trained individuals, women, or longer training durations (e.g., 16–24 weeks).
Ideal Study Design
A double-blind, randomized crossover RCT of 40 participants (20 men, 20 women; aged 20–40; half trained, half untrained) performing 16 weeks of unilateral cable curls with shoulder flexed vs. extended, matched for volume and resistance profile, with pre/post B-mode ultrasound of biceps and brachialis at three regions and 1RM testing, controlling for protein intake and activity levels.
Limitation: Limited to controlled lab conditions; may not reflect real-world training variability.
Prospective Cohort StudyLevel 2bWhether long-term adherence to one shoulder position over years leads to differential muscle architecture or functional outcomes in real-world populations.
Whether long-term adherence to one shoulder position over years leads to differential muscle architecture or functional outcomes in real-world populations.
What This Would Prove
Whether long-term adherence to one shoulder position over years leads to differential muscle architecture or functional outcomes in real-world populations.
Ideal Study Design
A 5-year prospective cohort tracking 200 resistance-trained adults who self-select either shoulder-flexed or shoulder-extended curl variations, measuring annual changes in muscle thickness via ultrasound, strength via 1RM, and functional performance, adjusting for training volume, diet, and genetics.
Limitation: Cannot control for confounding variables like training technique or nutrition consistency.
Randomized Controlled TrialLevel 1bWhether altering eccentric tempo (e.g., 4s) during shoulder-extended curls enhances biceps hypertrophy compared to standard tempo.
Whether altering eccentric tempo (e.g., 4s) during shoulder-extended curls enhances biceps hypertrophy compared to standard tempo.
What This Would Prove
Whether altering eccentric tempo (e.g., 4s) during shoulder-extended curls enhances biceps hypertrophy compared to standard tempo.
Ideal Study Design
A 12-week RCT comparing 30 young men performing shoulder-extended cable curls with 1:1s vs. 4:1s eccentric tempo, matched for load and volume, measuring regional biceps brachii thickness via ultrasound and 1RM strength, with muscle biopsies to assess mTOR signaling and sarcomere remodeling.
Limitation: Does not address whether this effect generalizes to other populations or exercises.
Cross-Sectional StudyLevel 3bWhether habitual use of one shoulder position correlates with regional muscle thickness differences in experienced lifters.
Whether habitual use of one shoulder position correlates with regional muscle thickness differences in experienced lifters.
What This Would Prove
Whether habitual use of one shoulder position correlates with regional muscle thickness differences in experienced lifters.
Ideal Study Design
A cross-sectional analysis of 100 experienced lifters (5+ years training) who primarily use either Preacher or incline curls, measuring biceps and brachialis thickness at three regions via ultrasound and comparing patterns of regional hypertrophy, controlling for total training volume and history.
Limitation: Cannot determine causation; only identifies associations.
Evidence from Studies
Supporting (1)
The study found that doing bicep curls with your arm in front of you or behind you made your arms just as strong and muscular — as long as the weight felt the same. So, the position of your shoulder doesn’t matter for results.