Can someone tell me about the physical requirements for a physical therapy assistant?

I would love to train for this job and I think I would be good at it. But I am almost 50 and not physically strong. I have some shoulder pain from bone impingement, although the doctor says it is mild and I'm about to go for PT and get strengthening exercises. Also I wear lightweight knee braces (sleeves) for my kneecaps, which are new but seem to be working very well. However, would these existing problems be aggravated by the PTA work? Also, how much weight do I need to be able to lift? Would I be lifting patients in and out of wheelchairs, and if so, could I get help? Is there a minimum amount of weight I have to demonstrate that I can lift? What other physical strength/dexterity requirements are there for the job? Thank you!