Biceps

#Function

Biceps brachii has a long head and a short head, both crossing the elbow and the shoulder. Together they flex the elbow and supinate the wrist (rotate the palm upward) — biceps are a supination muscle as much as a flexion one, which is why grip orientation changes recruitment.

Brachialis lies underneath biceps, crosses only the elbow, and is a strong elbow flexor in its own right — it does not care about wrist rotation, since it never touches the wrist.

#Origin and insertion

The long head originates at the supraglenoid tubercle, just above the shoulder socket — it crosses the shoulder joint, which the short head, originating at the coracoid process, does much less. Both converge to insert at the radial tuberosity, near the elbow.

#Why shoulder position changes which head is trained

Because the long head crosses the shoulder, shoulder extension lengthens it — pulling the arm behind the body stretches the long head at its shoulder end, on top of whatever elbow-driven stretch is happening. An incline curl, with the shoulder extended by the angle of the bench, puts the long head at a genuinely longer position than a standing curl does.

The short head, not crossing the shoulder, does not get this additional stretch from shoulder position — its length is governed almost entirely by elbow angle.

Practically: an incline dumbbell curl biases the long head via added shoulder-extension stretch. A preacher curl, with the shoulder flexed and fixed against the pad, is closer to the opposite end — hardest near full elbow extension, minimal shoulder contribution. Running both across a week trains the muscle across a wider range than either alone.

#Exercise selection

Source tier: the two-joint action of the long head is standard anatomy. The shoulder-position-changes-stretch argument follows directly from that anatomy and is well-supported, not speculative.