biceps bulge; flexed biceps
Refers to the visible bulge of the biceps when flexed; often used in the phrase 力こぶを作る (make a muscle).
He flexed his biceps to show off.
Used almost exclusively in the set phrase 力こぶを入れる, meaning to put in extra effort or enthusiasm into something.
See also: 力こぶを入れる
He is putting a lot of effort into his work.