Muscles of the upper body — origins, insertions, actions

The big rocks of upper body training

You don't need to know every muscle. You need to know the 15-20 muscles that drive every upper body exercise. Here they are with origin (where the muscle starts), insertion (where it ends), and primary action.

Chest

Pectoralis major — clavicle, sternum, ribs → humerus. Shoulder flexion, adduction, internal rotation. Trained by bench press, fly, push-up. Pectoralis minor — ribs 3-5 → coracoid process of scapula. Protracts and depresses scapula. Trained indirectly; tight pec minor causes rounded shoulders.

Back

Latissimus dorsi ("lats") — lower spine, iliac crest → humerus. Shoulder adduction, extension, internal rotation. The pulldown / pullup muscle. Trapezius — base of skull, spine → scapula and clavicle. Three sections (upper/mid/lower) with different actions. Upper traps elevate; mid retract; lower depress + upwardly rotate. Rhomboids — thoracic vertebrae → medial scapula. Retract and downwardly rotate scapula. Rowing exercises. Erector spinae (3 columns) — pelvis, sacrum → ribs and vertebrae. Spinal extension. Deadlifts, good mornings, back extensions.

Shoulders

Deltoid — three heads:

Anterior delt — clavicle → humerus. Shoulder flexion. Front raises, press.
Lateral (middle) delt — acromion → humerus. Abduction. Lateral raises.
Posterior delt — scapula spine → humerus. Extension, external rotation. Rear delt flies.

Rotator cuff (SITS — Supraspinatus, Infraspinatus, Teres minor, Subscapularis) — scapula → humerus. Stabilize the shoulder joint. Trained by face pulls, band external rotations, light cuban presses.

Arms

Biceps brachii — scapula → radius. Elbow flexion + forearm supination. Curls. Brachialis — humerus → ulna. Pure elbow flexion (no rotation). Hammer curls hit it hard. Brachioradialis — humerus → radius. Elbow flexion at neutral grip. Reverse curls. Triceps brachii — three heads:

Long head — scapula → ulna. Elbow extension + shoulder extension.
Lateral head — humerus → ulna. Elbow extension.
Medial head — humerus → ulna. Elbow extension at heavy loads.

How to use this

When a client says "I want bigger biceps," they probably want bigger UPPER ARM, which is mostly the triceps (about 2/3 of upper arm size). Coach them to balance push and pull volume — most casual lifters underdevelop the back.