Anatomy Basics: Metacarpals and the thumb


Hand drawing in color pencil
Continuing on with the structures of the hand we are moving onto a new group of bones, the metacarpals, to  focus on a few of the movements the thumb. 

Thumbs tend be a challenge for many people and understanding the thumb's connection to the hand and how the thumb moves can be helpful in resolving many issues when drawing the hand.

Today, we are going to look at the role that the metacarpals play structurally in the hand and how the lower part of the thumb moves based on the connection the metacarpal makes to the wrist bones.

Let's take a look.



Metacarpals

The metacarpals are the bones that make up the palm of the hand. They are the connection between the carpal bones of the wrist and the bones of the fingers and thumb, phalanges. These bones provide the structural support for the palm.

The four metacarpal bones that connect the wrist to the fingers have a very limited range of movement. They are tightly fixed into place at their base to the carpal bones. However, there is some movement between the carpal and the metacarpal bones as there is some gliding between the two groups. 

When drawing the hand, I would just consider the connection between these four metacarpals and the carpal bones fixed and not worry much about the movement between the bones here.

At the other end, the joints between the metacarpals and the finger bones work like a hinge allowing each finger to bend towards the palm.

Moving the thumb

The metacarpal bone that connects the thumb to the wrist has the most range of movement among all the metacarpals. Unlike the others, this one creates a saddle style joint with a carpal bone. This joint allows for the flexion, extension, adduction, and abduction of the thumb at the base of the hand.

To the right is an illustrated example of adduction of the thumb. This means the thumb moves towards the center of the hand, with the metacarpal pivoting at its base. The reverse of this action is abduction, simply meaning it moves away from the palm of the hand. 

Now here is an illustrated example of flexion of the thumb at its base. The tip of thumb moves from its location at the side of the hand out in front of it.
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The reverse of this would be extension, moving the tip of the thumb back to the side of the hand.

Combining these two types of the movements, flexion and adduction, and you have part of the action that allows for gripping.

Using your hand as reference, practice drawing the hand with the thumb in different positions based on the movements described above. This will show you how the thumb moves and allow you to see the forms that are created within the hand as it moves. Take note of the location of where the bend occurs at the base of the thumb, as this will show the location of the joint between the metacarpal and carpal bone. As you move your thumb around, also note what the range of motion that is available with the thumb.