Lateral Arm Muscles
The lateral aspect of the upper arm does not contain distinct muscle bellies, but rather is defined by the lateral intermuscular septum (LIS) and the muscles that attach to or border this fascial structure 1.
Primary Anatomical Structures
The lateral intermuscular septum serves as the key anatomical landmark separating the anterior and posterior compartments of the arm 1. This fibrous structure (mean thickness 1.0 mm) extends from the deltoid insertion distally to the lateral epicondyle of the humerus, where it becomes confluent with the annular ligament of the radius 1.
Muscles Associated with the Lateral Arm Region
The lateral arm region is bordered by muscles from both compartments:
Anterior compartment muscles:
- Biceps brachii - lies anteriorly and can be palpated on the inner surface between the biceps and triceps bodies 2
- Brachialis - deep to the biceps, with fibers attaching to the lateral intermuscular septum 3
- Brachioradialis - originates from the lateral supracondylar ridge and lateral intermuscular septum 3
Posterior compartment muscles:
- Triceps brachii (particularly the lateral head) - forms the posterior border 4
- Extensor carpi radialis brevis - partially arises from the LIS 1
Superior muscles:
- Deltoid - the deep and posterior portion of the deltoid tendon is confluent with the superior aspect of the LIS 1
Functional Myofascial Continuity
The lateral arm participates in a lateral arm myofascial chain consisting of: trapezius → deltoideus → lateral intermuscular septum/brachialis → brachioradialis 3. This structural continuity has implications for force transmission and may be relevant for understanding musculoskeletal disorders affecting this region.
Clinical Relevance
The radial nerve travels through a defect (mean diameter 1 cm) in the lateral intermuscular septum, with the posterior antebrachial cutaneous nerve traveling through a tunnel within the LIS in 85% of cases 1. This anatomical relationship is critical during surgical approaches to the lateral arm.