Working your biceps with your arms stretched back might grow the upper part of the muscle a bit more than doing curls with your arms already in front of you.
Mechanism
Synthesis from 4 studies
Training the biceps with the arm stretched back might stretch the top part of the muscle more, which could lead to a little more growth there — but most studies show that whether you train with your arm in front or behind, the muscle grows about the same overall. The real driver of growth is...
Most probable mechanism
When the biceps is stretched while under load, the muscle fibers near the top get pulled tighter and longer than usual, which may cause more tiny damage and signaling that leads to more muscle growth in that area over time.
Muscle fibers in the proximal and mid regions of the biceps brachii experience greater passive stretch and mechanical strain during resistance exercise when the shoulder is extended compared to when it is flexed.
Increased mechanical strain in these regions elevates intracellular tension, activating signaling pathways associated with protein synthesis and muscle remodeling.
Regional differences in muscle fiber recruitment and sarcomere strain distribution lead to localized increases in myofibrillar protein accretion, particularly in proximal regions.
Less supported by current evidence, but not ruled out
When the biceps is trained in either stretched or shortened positions, the overall load and muscle activation remain similar, leading to uniform growth throughout the muscle without regional preference.
Muscle activation levels and total mechanical load are comparable between shoulder extended and flexed positions during elbow flexion exercises.
Systemic anabolic signaling and muscle protein synthesis rates are not significantly different between training positions, resulting in uniform hypertrophy.
Evidence from Studies
Supporting (1)
Community contributions welcome
Distinct muscle growth and strength adaptations after preacher and incline biceps curls
Contradicting (3)
Community contributions welcome
The effects of shoulder extension angle on elbow flexor hypertrophy in the cable curl exercise
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.