I think Leighway and all the rest of you have hit on the solution--a lightweight machine does not have to cost a fortune. I got a $90 Brother at Walmart that sews just fine for my classes (I've even free motion quilted on it, but prefer not to as no speed regulator).
I also think Leighway is right--we all need to be moving those parts more! I bought a DVD for excercises you can do either standing or sitting as a start. Plus I'm still rehabbing a new knee so do water therapy 2X a week and weights at home---you don't have to buy hand weights--even canned food or a large sack of rice make good weights. You might be surprised!