# Why does a wrist have 3 degrees of freedom?

• Alex8932
In summary: Thanks for the summary.In summary, the other two movements make use of the wrist and can't be done without it so I see why they are considered to be degrees of freedom of the wrist. Radial/ulnar is a rotation about the z-axis Flexion/extension is a rotation about the x-axis Pronation/suppination is a rotation about the y-axis.
Alex8932
Homework Statement
Why does a wrist have 3 degrees of freedom?
Relevant Equations
The image at this link (https://i.stack.imgur.com/pcSjn.png) illustrates them but pronation can be done even without the wrist present so I don't understand why it is considered to be a degree of freedom of the wrist. During pronation the whole wrist itself is moving.
The other two movements make use of the wrist and can't be done without it so I see why they are considered to be degrees of freedom of the wrist.

PeroK
It has three degrees of freedom because it can be rotated about three mutually perpendicular axes.

If you label axes such that
z is perpendicular to the palm on the front of the right hand
x is along the outstretched thumb
y is along the four fingers

Then
Flexion/extension is a rotation about the x-axis
Pronation/suppination is a rotation about the y-axis

This idea of three mutually perpendicular axes is used in one version of the right hand rule which is implemented without rotation about any of the axes.

Lnewqban
At first I was skeptical, but after some thought I see your point.

Pronation is accomplished by the rotation of the radius and ulna about a common lengthwise axis. I'm not sure if the wrist joint itself makes any active movement; it seems to just go along for the ride.

"During pronation, the distal end of the radius rotates around the ulna from its position on the lateral side of the wrist to the medial side of the wrist. This action turns the hand, wrist, and forearm almost 180 degrees..."
https://www.innerbody.com/image/musc03.html#:~:text=During%20pronation%2C%20the%20distal%20end,the%20position%20of%20the%20arm.
kuruman said:
Then
Flexion/extension is a rotation about the x-axis
Pronation/suppination is a rotation about the y-axis

This idea of three mutually perpendicular axes is used in one version of the right hand rule which is implemented without rotation about any of the axes.
View attachment 322289
I don't see how this applies to the OP's question. It doesn't seem to restrict itself to the wrist joint.

PeroK
DaveC426913 said:
I don't see how this applies to the OP's question. It doesn't seem to restrict itself to the wrist joint.
Perhaps the degrees of freedom of the wrist are defined by the ability of the hand to rotate about the three axes. This article says in the abstract that the wrist has 3 dof. However, there seems to be no consensus because this article in section "Joints of the Wrist and Hand" says "The wrist has two degrees of freedom[11], although some say three degrees of freedom because they include the movements of pronation and supination[8], which occur at the the radioulnar joint. The radioulnar joint is often referred to as a joint of the forearm but it is this articulation that gives the wrist more freedom of movement. The true joints of the wrist and hand are listed in the table below[11]."

So I guess the answer to OP's question "Why does a wrist have 3 degrees of freedom?" the answer seems to be, "Because some people have counted 3 whereas others have counted 2." I am no expert on the subject, so I will shut up at this point.

Lnewqban, DaveC426913, gmax137 and 2 others
Alex8932 said:
Homework Statement:: Why does a wrist have 3 degrees of freedom?
Relevant Equations:: The image at this link (https://i.stack.imgur.com/pcSjn.png) illustrates them but pronation can be done even without the wrist present so I don't understand why it is considered to be a degree of freedom of the wrist. During pronation the whole wrist itself is moving.

The other two movements make use of the wrist and can't be done without it so I see why they are considered to be degrees of freedom of the wrist.
Umm... Why not?
OK, then evolution, that's why. The people with these wrists made more babies that lived to replicate. Different for octopuses and zebras though.

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