Knee and ankle injuries are soft tissue (muscle, ligaments, & tendons) injuries. Proper conditioning and strength training are vital to avoiding soft tissue injuries.
Granted there is no training or conditioning that can stop an injury from being rolled up on, tackled or blocked low at the knees, and similar incidences.
Proper conditioning means players are not as fatigued during the game which in turn sacrifices form leading players or overreach a block, make a sloppy cut, use bad form in tackling, etc. which leads to joints being over extended. Also, proper muscle balance and strength can take stress off of joints, tendons, and ligaments which hold the joints together. Muscle imbalance is a major factor in tendon & ligament injuries.
Therefor, the strength and conditioning coach and staff involved are key to avoiding injuries and keeping the team healthy.