Brachioradialis

Brachioradialis is a muscle located in the forearm, that acts to flex the elbow.

Attached to the distal radius and the lateral epicondyle of the humerus, it's body can be made visible in certain types of flexion.

If a person half-pronates their arm, to make a fist like they're holding a handled vessel of beer, then puts their fist under a table or desk and tries to flex at the elbow, the brachioradialis will stand out of the forearm, visible under the skin.

It is on the dorsal side of the forearm, so is innervated by the radial nerve.