Why does a wrist have 3 degrees of freedom?

[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.

 

[kuruman]

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
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

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.

 

[DaveC426913]

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.

Then
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

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.

 

[kuruman]

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.

 

[DaveE]

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.

 

[haruspex]

I easily found a source that says two:
https://www.physio-pedia.com/Wrist_and_Hand
and one that acknowledges two as a valid view:
https://www.researchgate.net/figure/Human-wrist-degrees-of-freedom-14_fig1_331847280

Btw, it's supination, not suppination.