I bet you couldn't. Why? It's impossible.
While everyone knows what that phrase is demanding, the technical wording of it means nothing. Here's why:
- Joints flex and extend. Muscles contract and relax. The act of flexing is something that your elbow can do, but your biceps can't. Going through flexion of a joint simply means shortening the joint angle. When you want to show people your lovely biceps, you contract those muscles, which brings your elbow from an obtuse to a right or an acute angle.
- Each arm has two bicep heads that contract together. You cannot contract a single bicep. When flexing your elbow or performing another function of the biceps (supination of the hand, shoulder flexion), both heads contract together. Therefore, you will always contract your biceps...plural form.
- The brachialis is the muscle that mainly flexes the elbow. Your biceps do help in this joint action, but they have a small role. The brachialis is also why there is a lump between your elbow and shoulder when your elbow is flexed. It is a large muscle that sits below part of the biceps and connects from the upper arm to the forearm.