Maya Tutorial: Resting Elbow or Knee

This is how to set up a rig for resting elbows(or knees) on a surface while moving the hands and/or body, without shortening and/or lengthening the upper and lower arms (or legs).

It’s much simpler than it might seem.  Spend a bit of time early to save a lot of time time later counter-animating elbows or knees.

Thanks to Katrín for demonstrating the need for this, and to Max for inspiring the solution and proving it works.  Here's Max's proof of concept: https://vimeo.com/204762414

1.
Create > Locator
to Create Locator1.

2.
Create > Locator (again)
to Create Locator2.

3.
Select Locator1, then Locator2.
Keyboard command Ctrl + p to Parent Locator2 under Locator1.

4.
Make sure rig's joint's are visible.

5.
Select Elbow Pole Vector control, then Locator1.
Constrain > Parent
to Parent Constrain rig's Elbow Pole Vector control under Locator1.

6.
With ‘Snap to points’ turned on (hold v on keyboard), translate/snap Locator1 to Elbow Joint.

7.
With ‘Snap to points’ turned on (again hold v on keyboard), translate/snap Locator2 to Wrist IK joint.

8.
Turn On Elbow Pole Vector's Elbow Snap in the Channel Box.

9.
Optionally, instead of snapping Locator2 to the Wrist joint, Parent Constrain Locator2 under the Wrist joint.
Select Locator2, then Wrist joint.
Constrain > Parent
Then Delete History and Freeze Transforms of Locator 2.  This will make Locator2's default relationship to Locator1 have Translates and Rotates of 0.  Make sure the Parent Constraint from Locator2 under the Wrist IK control is deleted as well.

10.
Select Wrist IK control, then Locator2.
Constrain > Parent
to Parent Constrain Wrist IK control under Locator2.  This is the opposite of the temporary Parent Constraint in the optional step, and should be done regardless of doing or not doing the optional step.

11.
Select Elbow joint and Shoulder joint to assign Distance Tool to measure their distance.
Create > Measure Tools > Distance Tool

12.
To 'translate' Wrist IK control, rotate Locator1.  This will move Locator2 and the Wrist IK control.  Set keys on Locator1 for this, not Locator2 or the Wrist IK control.

13.
To rotate the Wrist IK control, rotate Locator2.  Set keys on Locator2, not the Wrist IK control.

14.
If/when you need to move any part of the body that affects the location of the Shoulder joint, refer to the Distance Tool’s measurement to minimize the variation in distance from Shoulder to Elbow.  For example, the character is sitting in a chair, resting their elbow on a table.  If you move the COG control (Center Of Gravity) up, this increases the distance from the shoulder to the elbow.  You can counter animate the upper body and shoulder controls to tilt back toward the elbow to decrease that distance.