It became very clear to me recently that I am simply not in cardiovascular shape for sparring. I'm useless in the last round. Besides TKD, I also train in aikido, which is murder on my knees, so I'm looking for some good cardio that will be a little gentler on my knees than running. Any suggestions? I've been using a stationary bike at half-hour stretches to this point, but I haven't gotten the kind of results I want yet.
or a rowing machine or cross trainer? Also check out Ross Enamait's site for some excellent conditioning drills, many of which will be much lower impact on your knees and higher intensity that running. http://rosstraining.com/blog/
The thing is here is that you are not replecating sparring conditions. How long are your rounds? If they are two minutes then ride for two minute intervals. Copy the pattern of your sparring, so for example, flat out for 30 seconds, ease off for ten seconds and go again. Try that for 3 x 2 minute rounds and you'll be as tired as you are when sparring and a lot more than the 30 minute ride.
I've had good results by doing shadow boxing, but using the techniques I learned in karate rather than the more boxing-based ones I learned in MMA. Where it gets really cool is if you take what's succesful in your sparring, and watch competition fighting, and work through the techniques you see there. Also, if you do what rob redmond calls 'isolation exercises' like these ones: http://www.24fightingchickens.com/2006/09/08/how-to-front-kick/
Be careful with skipping. I have knee trouble as well and skipping has aggravated it before. However it is an excellent exercise, if you build up slowly, 3mins a day for a week then 4mins the next(at your own place), and stretch the calf muscle thoroughly afterwards you shouldn't have any problem. As for cardio, swimming is the best. There's no impact on the joints so its nice and easy on the knees. (Just make sure to warm up the knees first) I have been using a mixture of cycling and swimming for my cardio and its working quite well for TKD sparring.