When the arm flexes (bends upward), which muscle contracts?

"Flexion" is a bending movement where the angle between two parts decreases. Contracting your biceps exhibits flexion, i.e. it brings your forearm closer to your upper arm and decreasing the angle between the two. So, your biceps is described as a "flexor" muscle.

Explanation:

For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. When you want to bend your elbow, your biceps muscle contracts (Figure below), and, at the same time, the triceps muscle relaxes. The biceps is the flexor, and the triceps is the extensor of your elbow joint.

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