Final answer:
The common action of four of the five muscles in the posterior compartment of the forearm is extension, involving the extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, and extensor digiti minimi.
Step-by-step explanation:
Four of the five muscles that we study in the posterior compartment of the forearm that move the wrist or the digits have the action of extension. These muscles are the extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, and the extensor digiti minimi. Along with the extensor carpi ulnaris, they originate on the humerus and are responsible for extending the wrist and fingers, facilitating movements such as releasing a grip or spreading the fingers apart.