Key Muscles and Joints: Anatomy and Function

Trapezius Muscle

The trapezius muscle is a large, superficial muscle that extends from the skull to the last thoracic vertebra and laterally to the spine of the scapula. It occupies the center of the spine on both sides. Although often described as trapezoid-shaped, it is actually triangular. The appearance of a trapeze comes from the combination of the two trapezius muscles.

Sternomastoid Muscle

The sternomastoid muscle is located at all levels, running from back to front. It originates at the mastoid process of the temporal bone, immediately behind the external auditory meatus. One muscle belly terminates at the sternal manubrium, while the other terminates at the upper middle third of the clavicle, leaving a space between them. It acts as an accessory muscle of inspiration, primarily used in emergency situations or certain disease states, known as sternal tirage.

Biceps Brachii Muscle

The biceps brachii is adjacent to the coracobrachialis. Topographically located at the elbow, it plays a crucial role in shoulder joint function. Its actions depend on the fixation point:

• Elbow fixed: Acts on the shoulder girdle.
  • Short head: Coaptation, flexion, and internal rotation.
  • Long head: Coaptation in the abducted position, luxation in the anatomical position, and potential external rotation.
• Forearm free: Produces supination.
• Forearm fixed: Produces elbow flexion.

Coracobrachialis Muscle

The coracobrachialis is a long muscle more capable of rapid movements than forceful ones. It significantly affects the three axes of the scapulohumeral joint. It is located in the anterior, lower middle third of the humeral shaft. It acts as a:

• Balancer
• Coaptator
• Adductor in the anatomical position

Rectus Abdominis Muscle

The rectus abdominis is a paired muscle located on each side of the midline, forming the anterior abdominal wall. It is covered by a robust anterior fascia that enhances its tension. This muscle is unique to humans. It originates at the top of the pubis via a short tendon (2-3 cm).

Coxofemoral Joint

The coxofemoral joint, or hip joint, connects the innominate bone and the femur. It is a synovial joint of the ball-and-socket type, considered the most perfect ball-and-socket joint in the human body (along with the shoulder joint). It connects the trunk to the lower extremity. The joint is enclosed by a capsule and contains a membrane and synovial fluid. Together with the powerful surrounding muscles, it supports the body in both static and dynamic postures.