How wide is your treadmill?

If it's thin you may be overcompensating with your right leg so you stay on the machine. This would put extra strain on it. You propably don't even realise you're doing it. When you run next try and see if you're doing something different with your right leg...

I find Tae-Bo is the best form of cardio. After about 20 minutes of training, I'm sweating like a pig...it's quite intense. Next I'd have to say rowing, it uses a lot of muscle groups to perform the exercise so it will tone your entire body in the process. Also a point I'd like to add is that any fo...