Increased risk of knee injury (chondromalacia) occurs during knee extension activities. The patella becomes laterally displaced with the pull of the vastus lateralis. This patella tracking problem can produce wear on the inferior patellar surface. Greater pain is usually experienced during leg extension activities in which the knee is a greater than a 20 to 30 degree angle. Avoiding full range of motion (i.e. not locking out) during Quadricep exercise may not allow the Vastus Medialis to be fully strengthened since it is more fully activated at these final degrees of knee extension.
Examples of affected exercises:
Leg Press
Squat
Leg Extension
Example preventative / corrective exercises:
Single Leg Extensions (last 20 degrees of extension)
Leg Press (last 20 degrees of extension)