Save the Knees!
2. Don't Lock-Out Your Knees: When performing a leg press or leg extension movement like squats or seated leg press, do not fully lock-out your knees. Locking your knee joint transfers all of the weight from the muscle to the joint. The result is un-needed stress on the knee that can lead to injury. By not fully locking your knees - you will keep the tension on the muscle and you will experience a more intense set at your current weight. If you are unable to complete your set without locking your knees, the weight is too heavy and must be lowered. *The only exception to this rule is with seated leg extensions and stiff-legged deadlifts.
3. Learn To Squat With Proper Form: So many people squat on the balls of their feet - instead of their heels. Do not let your knees come forward as you descend. Stick your butt out, and keep your back arched (Do not Round) as you descend. When you ascend, you should be pushing off the heel of your foot - not your toes. If you have trouble doing this, try squatting without any shoes on. Many athletic shoes have a very thick sole that when squatting - forces your body forward (and onto your toes).
4. Do Not Use More Weight Than You Can Handle: Make sure you are able to complete the full range of motion for any leg exercise you are performing. If you cannot - you are probably using too much weight. Lower the weight and perform the exercise correctly (you are only cheating yourself!).
5. Think About Your Knees When Performing Cardio: You may want to avoid exercises like running (whether on the treadmill or road). The impact of your feet slamming the pavement may irritate an existing knee injury. An elliptical trainer or stationary bike may be easier on your knees. If you utilize an elliptical trainer, lower the incline (height) to lessen the stress on your knees.
If you follow the tips listed above, you should be able to still train legs intensely. IF you don't suffer from knee pain - these tips should help you prevent any future problems. If you have an existing knee injury, these tips can help alleviate the pain and allow you to continue training.