I'm 5'1 and 115 lbs. I used to weight 90 lbs. I finally gained back the weight I lost, I'm very happy with my weight but lost the definition I had. I have a fast metabolisim so if I work out too much I lose the weight. I just want to tone up again is there anyway to do exercises that keep they weight on just look fit. Plus I have a bad back to I can't do any heavy lifting so weight lifting is out for now.

This is what I do every night and it just makes me look better

2 sets of

100 sit ups
30 press ups
50 arm curls with elbow out a bit to utilise shoulder muscle

For toning up, circuit machines (weight machines) work well.

You can use free weights without too much stress on your back. Simple curls while sitting, squats with weights held at your sides, etc.. For toning you don't need anything heavy.

You can tone without the use of weights as well... simply use your own body... walk while periodically tightening your stomach muscles and relaxing them, squats, tricep dips using a sturdy chair, desk, or table, etc..

How old are you? Male or female?

25 female

How about just walking, biking, stationary bike, exercise bands... swimming, they have a lot of low impact exercise that you can do, 15-30 minutes of any movement that works musles in general will help tone you, depends on what your wanting to tone.