Accessory muscles, as medically defined, are duplicates of a muscle in the human body. However, accessory often refers to the muscles not usually or not directly involved in performing a task or doing physical work.
Accessory muscles that are duplicates of a muscle in the human body are usually not removed unless they interfere with normal function or pose some sort of health risk. The most common type of accessory muscle is the soleus muscle located in the ankle.
Doctors often use the involvement of accessory muscles, that is those not normally engaged in a task, as a sign of a medical issue. For example, the use of respiratory accessory muscles in the neck and chest are indicative of respiratory health issues.