The structure indicated is the extensor digiti minimi muscle.

The extensor digiti minimi 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 and its branches.

Origin: Lateral epicondyle of humerus

Insertion: Extensor expansion of 5^th digit at metacarpophalangeal and interphalangeal joints

Action: Extends 5^th digit. Weak extensor of wrist

Innervation: Posterior interosseous nerve

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