Which muscle (s) are used to extend your arms backward?

Teres major, posterior deltoid muscle and latissimus dorsi

Explanation:

The muscles that arise from the posterior part of the shoulder joint are involved in the backward movement of the forearm. The posterior deltoid muscle and latissimus dorsi are the primary muscles that bring about the extension of arm (the backward movement of the arm).

Latissimus dorsi is the triangular muscle that arises from the lower part of the spine. It serves to extend the arm and bring it down from the raised position. Teres major is the thick and rounded muscle which also serve in the extension of the arm, that is the backward movement of the arm