One concern is that they are assymetrical. I already have mild scoliosis (lateral assymetry of a spine), and don't want to increase it. Swimming would probably work at least as well as tennis, actually, if I wanted to do it more often and with more intensity, and maybe better form. But I don't.

Really? I'd be a bit surprised, actually. They might work endurance, but strength? Don't forget that I'm a girl, too. It's not as easy for us to grow muscles.

I used to hate gyms. But it helped when I outlined an actual training regiment and started recording progress. The records are inspiring.