in my physio for a shoulder injury which i've now completed, they found my rear deltoids have wasted to basically nothing. i need to build the rear delts up again.

i just the exercise name and the motion as i'm not able to lift any weights yet just lifing my arms maybe with a book in each arm.

http://www.exrx.net/Lists/ExList/ShouldWt.html

Scroll down to Posterior Deltoid.

My usual rear deltoid exercise (which always kills me) is the rear lateral raise with dumbbells. It should be pretty easy to do with very light weight (meaning no machine required and offers pretty good isolation).

A good exercise for overall shoulder health is the Y-T-W-L which is often done as a rehab/prehab exercise with light weight. The You Tube demo shows a bench used but a stability ball can be used as well.

You can start with just your hands and graduate to very light dumbells or anchor some light resistance tubes to the base of the bench.

The "W" and especially the "T" will hit the rear delts the best but all should help strengthen and support the rear delts.

http://www.youtube.com/watch?v=xck3d8LidME