Ive been training on and off for years, and seriously for about 4 months. Ive avoided any serious leg work because I have a torn ACL in my left knee, making it extremely unstable. It requires massive amounts of tape to go skiing. My knee has no problem with leg extensions, hamstring curls, or running but i know if i load up some serious weight and do any kind of squatting or pressing motion it'll blow out and I'll eat the bar. I was able to squat close to 350 lbs pre injury about 9 years ago, and i'd like some suggestions as to alternative but effective exercises. Thanks.