How to Draw Arms: Learn Arm Movements and Bone Proportions

This Week’s Plaster Study: Exploring Arm Structure This week’s plaster study features a girl’s bust with a resting chin pose. Bright objects are so tempting to overwork! Compared to previous plaster studies, this one offered more room for variation in the hair. 🙋

As I worked on the pose, I took the opportunity to delve into the arm structure. Let’s dive in!

1. The Composition and Functional Structure of the Arm

Take a look at the diagram on the far left to understand the distribution of arm bones.

●Upper Arm – Humerus

Location: The only bone in the upper arm, connecting the shoulder joint to the elbow joint.

Key Features:
The upper end has a rounded ball-like shape that connects to the shoulder. The lower end forms the elbow joint by connecting with the radius and ulna.

Forearm – Radius and Ulna

These bones are responsible for supporting the arm and enabling rotational movements.

●Radius:

Location: On the thumb side.Smaller at the upper end (near the elbow joint) and broader at the lower end (near the wrist joint).

Key Features:Facilitates forearm rotation (pronation and supination).

●Ulna:

Location: On the pinky side.
The upper end has an olecranon, forming the prominent bony tip of the elbow. (Refer to the groove illustration on the far right.)
Provides stability and is the main supporting bone of the elbow joint.

2. Arm Movements and Joint Mechanisms

●The Elbow Joint: Formed by the humerus, radius, and ulna.

Our arm movements include bending, straightening, and rotating. Rotational movements

Rotational movements, such as writing, riding a bike, or typing, are particularly frequent in daily life. But what allows us to make these movements?

Movement Features:

1.Bending and Straightening: Centered around the elbow joint, with the ulna providing stability and the radius enabling mobility. The small diagram on the far right illustrates how the elbow joint flexes and extends, resembling a C-shaped groove.

The elbow joint is a hinge-type joint primarily responsible for the flexion and extension of the forearm relative to the upper arm.

Flexion: When the forearm bends upward, the elbow joint reduces the angle between the humerus and the forearm, such as when lifting heavy objects or picking something up.
Extension: When the forearm straightens downward, the elbow joint opens up, forming a near-straight arm posture, such as in the motion of pushing against a wall.

The small diagram below illustrates the movement of the elbow joint, with the joint shaped like a C-shaped groove.

The way our elbow flexes and extends is similar to a cylinder rotating within a groove.

Pronation and Supination: When the palm rotates, the radius rotates around the ulna (see the middle diagram). The radius overlaps the ulna during rotation, creating an X-like shape.

Pronation: When the palm faces downward, the radius rotates and crosses over the ulna, with the palm facing down. For example, placing your palm flat on a table demonstrates pronation.

Supination: When the palm faces upward, the radius and ulna align parallel to each other, allowing the palm to face up. For instance, opening your palm to catch something is an example of supination.

3.Simplifying Bone Shapes:

To better understand their features, bones can be simplified into geometric forms. Refer to the middle diagram for simplified shapes.

●Humerus:

Resembles a cylinder with varying thickness, wider at both ends and narrowing in the middle.

●Radius and Ulna:

The radius is broader near the wrist, while the ulna is wider near the elbow.

Among these arm bone structures, which part do you find the hardest to draw? Let me know in the comments! 👋

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