How to lock elbow rotation (forward kinematics) Before answering this question please consider that an 'answer' involving IK bone constraints should go in comments. In this question I am only interested in forward kinematics.

Many humanoid models use a T-pose like this:



I often end up doing custom rigs, and need to keep them simple (I really do). To avoid unrealistic bending of the elbow while animating, I want to constrain the rotation of the elbow joint.

One method (when using forward kinematics) is switch to Eulers and lock the rotation in UI, as illustrated by this picture:



However, some artists discourage the use of a straight T pose. So we often see this:



My guess is that an axis-angle rotation may do the trick in such cases, however figuring the rotation axis would require tedious calculations; also, behaviour when rotating an axis-angle-locked joint in 3D view is not very satisfying. Finally I don't really want to use something other than quaternion rotations if I can avoid it.

So, what would be a good way to constrain rotation in such cases?
You can use bone constraints to limit bones rotations (and other things) in pose mode:

Here is the wiki on rotation limiting.
The settings depend on actual bone setup and what the bone roll values are. Sometimes the bone roll needs to be also adjusted.

This will allow only rotation in local Z axis:



The range of motion can also be adjusted by setting the angle limits for appropriate axises.