When you move your arm backward, it stretches your biceps because the muscle runs over your shoulder joint — and when you move your arm forward, the biceps gets shorter.
Mechanism
Synthesis from 2 studies
When you move your arm back, the biceps gets stretched because it connects your shoulder to your elbow — like a rope pulling tight. When you move your arm forward, the rope goes slack and gets shorter. This is just physics, not biology — the muscle changes length because of how it's attached.
Most probable mechanism
When you move your arm backward, the biceps muscle gets pulled tight because it connects from your shoulder to your elbow. This stretch makes the muscle stiffer, like pulling on a rubber band. When you move your arm forward, the muscle relaxes and gets shorter.
Shoulder extension increases the distance between the biceps brachii's proximal attachments at the scapula and its distal insertion at the radius, elongating the muscle fibers across the glenohumeral joint.
Muscle elongation increases passive tension in elastic components including titin, connective tissue, and fascia, which is reflected as increased shear modulus.
Shoulder flexion reduces the distance between the biceps' proximal and distal attachments, decreasing muscle length and passive tension.
Evidence from Studies
Supporting (2)
Community contributions welcome
Biceps brachii regional shear modulus following elbow flexion exercises at different muscle lengths.
Contradicting (0)
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Gold Standard Evidence Needed
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