Why don't we speak of medial and lateral rotation of the forearm? And pronation and supination of the upper arm?

Question

Why don't we speak of medial and lateral rotation of the forearm?

I notice we speak of forearm supination and forearm pronation, but why not medial and lateral rotation.

It seems to me that if we take the diagram of the human with palms forward, which I understand to be the neutral position one is meant to start from.. then forearm pronation looks to me like a medial movement. So why not medial rotation?

And similarly with upper arm.. why don't we speak of pronation and supination of the upper arm? We speak of medial and lateral rotation of it..

Answer 1

Pronation and supination involve a change in relationship between the ulna and radius.

When you rotate your hand from palm ventrally (as in anatomical position) to palm facing dorsally you have moved your hand from a supinated position to a pronated one. This movement of supination to pronation occurs when the distal head of the radius rotates on the capitulum , the bone rotates along its long axis, and the proximal head of the radius flips over the ulna. (Duke Medicine)

and:

Pronation and supination are rotational motions that exist exclusively in the forearm. The mechanical bases for these movements are the existence of two forearm bones and the presence of two coupled trochoid joints—the proximal and distal radioulnar joints (PRUJ and DRUJ). (Clinical Gate)

Image source: Wikimedia, CC licence

During medial or lateral rotation of the upper arm (without additional rotation of the forearm), the humerus rotates, but the relation between the ulna and radius does not change, so there is no pronation/supination.

It is more practical to use supination/pronation for the forearm/hand, because you can have a forearm/hand in various positions where medial/lateral or internal/external does not make sense: for example, when you rotate your forearm/hand with your arm extended toward the side. Saying that, medial/lateral and internal/external rotation of the forearm is also commonly used (search: "medial rotation of the forearm"). So, you can use any of these terns according to what's more practical and less confusing in a given context.

Answer 2

Supinus and pronus are latin words which mean lying face/body/palm downw and lying face/body/palm up. The hand clearly has a front and a back, and everyone knows where the back of the hand is compared to the front. It's a very clear medical word for describing the hand. Supinator and pronator muscles are also clear groups that action the hand to a prone/supine position.

If you ask which is the front and back of your forearm, not everyone will reply the same. The upper arm doesn't have a clear front and back, so it's not the best way to describe the forearm and the upper arm.