Humerus

An anatomical and biomechanical study guide focusing on the long bone of the upper arm — its bony landmarks, muscular attachments, and clinical significance.

1. Osteology of the Humerus

Divided into proximal epiphysis, diaphysis (shaft), and distal epiphysis

Region	Landmark	Description
Proximal	Head	Smooth, rounded articular surface that articulates with the glenoid fossa of the scapula.
	Anatomical Neck	Groove separating the head from the tubercles.
	Surgical Neck	Constriction below the tubercles; a common site for fractures.
	Greater Tubercle	Large, lateral projection for the insertion of three rotator cuff muscles.
	Lesser Tubercle	Smaller, anterior projection for the insertion of one rotator cuff muscle.
	Intertubercular Groove	Also known as the bicipital groove; separates the tubercles and houses the long head of the biceps tendon.
Shaft	Deltoid Tuberosity	Roughened, V-shaped area on the lateral surface for the insertion of the deltoid muscle.
Shaft	Radial (Spiral) Groove	Oblique groove on the posterior surface that marks the path of the radial nerve and deep brachial artery.
Distal	Medial Epicondyle	Prominent projection on the medial side; origin for the common flexor tendon.
	Lateral Epicondyle	Smaller projection on the lateral side; origin for the common extensor tendon.
	Capitulum	Rounded articular surface on the lateral side that articulates with the head of the radius.
	Trochlea	Spool-shaped articular surface on the medial side that articulates with the trochlear notch of the ulna.
	Olecranon Fossa	Deep depression on the posterior surface that accommodates the olecranon of the ulna during elbow extension.
	Coronoid Fossa	Smaller depression on the anterior surface that accommodates the coronoid process of the ulna during elbow flexion.

2 & 3. Muscular Attachments

Muscles originating from and inserting onto the humerus

These muscles have their proximal attachment on the humerus and typically act on the elbow or forearm.

Muscle	Origin Site	Action
Brachialis	Distal half of anterior humerus	Primary flexor of the elbow
Brachioradialis	Lateral supracondylar ridge	Elbow flexion (especially in the thumb-up, mid-prone position)
Triceps Brachii (Medial & Lateral Heads)	Posterior surface of humerus	Elbow extension
Anconeus	Lateral epicondyle	Assists in elbow extension and stabilizes the ulna during pronation
Common Flexor Tendon	Medial epicondyle	Origin for muscles that primarily perform wrist and finger flexion, and pronation
Common Extensor Tendon	Lateral epicondyle	Origin for muscles that primarily perform wrist and finger extension, and supination
Supinator	Lateral epicondyle (and ulna)	Forearm supination

These muscles originate on the axial skeleton or scapula and act primarily on the glenohumeral joint (shoulder).

Muscle	Insertion Site	Action
Supraspinatus	Greater tubercle (superior facet)	Initiates the first 15° of shoulder abduction
Infraspinatus	Greater tubercle (middle facet)	Lateral (external) rotation of the shoulder
Teres Minor	Greater tubercle (inferior facet)	Lateral (external) rotation of the shoulder
Subscapularis	Lesser tubercle	Medial (internal) rotation of the shoulder
Pectoralis Major	Lateral lip of the bicipital groove	Adduction, medial rotation, and flexion of the shoulder
Latissimus Dorsi	Floor of the bicipital groove	Adduction, medial rotation, and extension of the shoulder
Teres Major	Medial lip of the bicipital groove	Adduction and medial rotation of the shoulder
Deltoid	Deltoid tuberosity	Abduction (past 15°), flexion (anterior fibers), and extension (posterior fibers) of the shoulder
Coracobrachialis	Medial aspect of the mid-shaft	Flexion and adduction of the arm

4. Functional Mechanics & Force Couples

SITS rotator cuff mnemonic and Lady Between Two Majors diagram

The Rotator Cuff (SITS)

Comprising the SITS muscles, these insert onto the tubercles to provide dynamic stability by compressing the humeral head into the glenoid fossa.

Supraspinatus

Infraspinatus

Teres minor

Subscapularis

"Lady Between Two Majors"

A mnemonic for the bicipital groove attachments:

Lateral lip

Pectoralis Major

Floor

Latissimus Dorsi (The "Lady")

Medial lip

Teres Major

5. Clinical Kinesiology: Nerve Relationships

Critical areas where nerves contact the bone — vulnerable during fractures

Surgical Neck

Axillary Nerve

Innervated muscles: Deltoid and Teres Minor

Fracture here can lead to paralysis of the deltoid, resulting in an inability to abduct the arm.

Radial Groove

Radial Nerve

Innervated muscles: Triceps brachii and wrist/finger extensors

Fracture of the mid-shaft can cause "wrist drop" due to paralysis of the extensor muscles.

Medial Epicondyle

Ulnar Nerve

Innervated muscles: Flexor carpi ulnaris and medial half of flexor digitorum profundus

Injury here can cause numbness and tingling in the 4th and 5th digits (the "funny bone" sensation).

6. Practice Calculation: Elbow Flexion Torque

Consider the Brachialis muscle. If it generates a tension force F_m at an insertion angle θ relative to the ulna, the effective torque τ at the elbow is:

τ = r · F_m · sin(θ)

r — distance from the elbow joint axis to the insertion on the ulnar tuberosity.

θ — changes throughout the range of motion, reaching peak mechanical advantage when the elbow is at approximately 90°.