Last summer I had a total shoulder replacement. I can't believe how much stronger my arm is now. Can a new joint really increase muscle strength?
There are several reasons why you may be experiencing a change in strength and function after a total shoulder replacement (TSR). The first is simply pain relief. Range of motion and strength are both decreased by pain. Before the joint replacement, it just hurts too much to move the arm or stress the joint. This makes it look like you are weaker than you really are.

During the operation, adhesions around the shoulder complex are released by the surgeon. Breaking loose these tiny points of scarring and fibrous tissue can actually free up the muscles to move more smoothly through greater motion with increased strength.

Sometimes during the procedure, tendons in the shoulder are cut and reattached in a slightly different location. The new position is designed to improve the balance and function of the joint. What you might notice from this is an increase in strength.

And finally, it's possible that your rehab program is paying off! If you've been diligent to follow your exercise program you should see a gradual improvement in motion, strength, and function.