Biceps Brachii is a two joint muscle which performs two movements:

1.elbow flextion - as a primary muscle with both heads.

2.shoulder flextion - via the short head that crosses shoulder joint (as an assistant).

When doing preacher curls the short head is fully contracted or (almost fully) so the long head doing most of the job .

When doing seated incline curls both heads are activated to the same degree as the shoulder is fully extended, therefore personally I do seated incline curls and add a shoulder flextion at the end of the movement to fully contract the short head.

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