A muscle of the elbow. It is also known as the "subanconeus muscle", as it lies below the anconeus muscle. It is classified as a muscle of the posterior brachium. More specifically, it is described "as originating from the deep part of the medial head of the triceps brachii and inserting into the posterior aspect of the joint capsule of the elbow."

Source: Tubbs RS, Oakes WJ, Salter EG. The subanconeus muscle. Folia Morphol (Warsz). 2006 Feb;65(1):22-5. PMID: 16783731.