The structure indicated is the extensor carpi ulnaris muscle of the forearm.

The extensor carpi ulnaris muscle is one of 7 muscles located in the superficial compartment of the posterior forearm. The posterior forearm consists of a superficial and a deep compartment. The superficial compartment contains 7 muscles, whereas the deep compartment contains 5 muscles.

The superficial compartment of the posterior forearm includes the following 7 muscles:

1. Brachioradialis
2. Extensor carpi radialis longus
3. Extensor carpi radialis brevis
4. Extensor digitorum
5. Extensor digiti minimi
6. Extensor carpi ulnaris
7. Anconeus

The muscles of the posterior compartment of the forearm originate from the lateral epicondyle of the humerus and the supraepicondylar ridge and they are all innervated by the radial nerve.

Origin: lateral epicondyle of humerus and posterior border of ulna

Insertion: base of 5^th metatarsal

Action: extension and abduction of wrist

Innervation: Posterior interosseous nerve

Learn more about the anatomy of the forearm muscles in this tutorial.