Replying to Flex and knee/ligement injury

I recently picked up some full tilt seth pros and they have a 10 flex tongue for a kid that weighs 130/140 they are pretty stiff so I was going to pick up some 4 flexs. I once herd a stiffer boot causing more acl and mcl and other knee problems but wasn't sure so I looked into this a little and did not dig up much. Obviously your binding din matters the most but does anything else contribute to these injuries? Just wanted too know the opinion of ns NS and do any of you have and tricks to strengthen your acl, mcl, achilles, etc.