i've torn both my acl's. once playing soccer, other time with football. both were non-contact and happened when i was basically just running. also, both times i had been heavily lifting weights. my hamstrings were easily in the 60-80% range of my quads and i'm very flexible and stretch before everything i do. i couldn't understand why either happened.
i personally believe that over-lifting and fatigue caused both of mine. i could feel my legs beginning to give out on me. i think that when i over worked my muscles that they began to become less responsive and couldn't keep up with the cutting and sprinting i was doing. i think 2 hour long lifting sessions, plus 2 hours of cutting on the field for football is a little overkill.
i have another theory that going repeatedly below 90% while doing squats also hurt me. its rediculous that the coaches here expect you to drop down to the point your thighs are parallel to the ground. your basically sitting on your ankle and it puts tons of stress on your knee. both times i tore mine i had just started doing squats again, i will never do them again
i'm not learned in the medical field, but my dad is the director of physical medicine at the hospital where i live and i've spent over a year in rehab so i've talked to a bunch of people about this subject and basically they tell me its the right combination of wrong things. just be careful your son does not repeat them on the other leg and as long as he does rehab and stays dedicated he'll be fine. eventually he'll be better off than he was before
I did a particial tear in high school from falling. Then ~ 2 years ago completly tore it by twisting. Now it is a lot easier for a women than a man to tear it but I would think it was from the football practice not the weight lifting because i did a lot of weight lifting before and after the surgery and the dr was really pleased with my recovery and said to keep it up. I am so sorry about your son. My surgery went really good, but when I tore it that was extremly PAINFUL!!!!!