An accessory muscle is a relatively rare anatomical variation where duplication of a muscle may appear anywhere in the muscular system. Treatment is not indicated unless the accessory muscle interferes with normal function.^[1] Examples are the sternalis muscle, accessory soleus muscle, extensor digitorum brevis manus and epitrochleoanconeus muscle.

An accessory muscle can also refer to a muscle that is not primarily responsible for movement but does provide assistance.^[1] Examples of such muscles are the accessory muscles of respiration where the sternocleidomastoid and the scalene muscles (anterior, middle and posterior scalene) are typically considered accessory muscles of respiration.^[2]