When do the Ulna and Radius cross?

[thedeo85]

So I sit at my desk waiting for calls sense I work as support and am sketching in my notebook listening to Pandora. I started doing some sketches of anatomy and relised something. When does the Ulna and Radius crass each other so I know which muscle groups to define on the lower arm O.o? I did some simple base sketches trying to work it out in my head then felt my own arm as I twisted it but being a rather over weight man I could not feel vary well when this happens. Any one have any good examples of when this happens? I know the reason we cant turn our rist so fare is because of these bones crossing each other.

[bill618]

Pronation of the hand ‘crosses’ the ulna and radius.
Supination of the hand uncrosses them.

[Notophthalmus]

To add to what Bill said: If your arm is hanging by your side and your palm is facing forward (this is "standard anatomical position"), your ulna and radius are not crossed. Pronation and supination are relative to this position. When you turn (pronate) your hand so the palm faces backward, the radius crosses in front of the ulna. Supination is turning back to standard anatomical position.

The radius crosses the ulna as you type, or pitch a ball overhand (as in baseball), or climb a ladder, or do a pushup. It is halfway turned when you shake someone's hand, or hold a bottle, gun, sword, etc. The bones are parallel as you juggle, or smack someone's bottom, or pitch a ball underhand (as in softball or bowling), or uppercut someone, or demonstrate how big the fish you caught was.

[JeffX99]

When the moon is in the Seventh House.

Annnnd wait a sec Notopthalamus...did you just say "smack someone's bottom"? Nice...going to have to try that next time, "Honey? Just trying to get the feel for whether my radius and Ulna are parallel or crossing!".

But really...pretty good examples there.

[thedeo85]

Thanks for all the info guys! The examples were vary useful and the information great Love this place for Art questions when I'm drawing.

[arenhaus]

To add to what Bill said: If your arm is hanging by your side and your palm is facing forward (this is "standard anatomical position"), your ulna and radius are not crossed.

Which, interestingly, is not the natural default position. It simply allows a face-on anatomical diagram to show both radius and ulna without crossing them.

[Saraiva]

A good way to remember.

Supination is when you can hold a bowl of soup. Pronation is when your hand is prone/facing down.