Subtendinous bursa of triceps brachii muscle

The subtendinous bursa of the triceps brachii (a.k.a. subtendinous olecranon bursa) is a deep synovial bursa located between the triceps brachii tendon and the posterior surface of the olecranon process of the ulna, lying deep to the triceps tendon and superficial to the olecranon. It serves to reduce friction between the triceps tendon and the olecranon during elbow flexion and extension, especially in powerful or repetitive extension movements.

It is different and distinct from the intratendinous olecranon bursa (located within the collagen fibres of the triceps tendon, i.e., internal to the tendon) and the subcutaneous olecranon bursa (located between the skin and the olecranon process, i.e., most superficial).