Tore my ACL playing Ultimate when I was 25 and had it reconstructed, along with 3 stitches to the meniscus, when I was 26. I was told unlimited biking 30 days post-op, and took that seriously.

I stopped doing stop-and-cut sports and pretty much stuck to cycling, including track racing, and the biking has helped keep my leg stable and the knee smooth inside-- it actually gets more achy if I don't ride regularly (which isn't very much)-- I took some time off track racing (but still rode all the time), but am back at it full blast (at the age of 40) for the past couple years, and I haven't even noticed any kind of strain that might reinjure it. I've put strain on it skiing, and doing yoga, but cycling has been great, including the track.