3D Deltoid Muscle

📍 Overview

The deltoid is a large, triangular muscle that caps the shoulder. It's responsible for arm abduction (lifting arm sideways) and gives the shoulder its rounded contour. Named for its resemblance to the Greek letter Δ (delta).

🔴 Three Heads

The deltoid has three distinct parts, each with different origins and functions:

Anterior (Front)

Origin: Lateral 1/3 of clavicle (collarbone)
Action: Flexion, medial rotation of arm
Example: Raising arm forward, front raises

Lateral (Middle)

Origin: Acromion process of scapula
Action: Arm abduction (lifting sideways)
Example: Lateral raises, jumping jacks

Posterior (Rear)

Origin: Spine of scapula (shoulder blade)
Action: Extension, lateral rotation of arm
Example: Pulling arm backward, reverse flyes

📌 Common Insertion

All three heads converge and insert on the deltoid tuberosity — a rough triangular area on the lateral (outer) surface of the humerus (upper arm bone), about halfway down.

🧠 Innervation

Axillary nerve (C5-C6) — branches from the brachial plexus. Damage to this nerve (e.g., shoulder dislocation) can cause deltoid weakness and numbness over the lateral shoulder.

🩸 Blood Supply

Primarily the posterior circumflex humeral artery (from axillary artery), with contributions from the thoracoacromial artery and deltoid branch of the profunda brachii.

⚡ Training Tips

Overhead Press: Hits all 3 heads, emphasis on anterior
Lateral Raises: Isolates lateral head
Face Pulls / Reverse Flyes: Targets posterior head
Front Raises: Isolates anterior head

💡 The lateral head is hardest to develop — most pressing movements emphasize anterior. Add lateral raises!

🎯 Fun Facts

The deltoid is a common intramuscular injection site (vaccines!)
It's one of the few muscles that can abduct the arm past 15° — the supraspinatus initiates the first 15°
Well-developed deltoids create the "V-taper" physique
The muscle can generate up to 20% of total shoulder abduction force