After having major knee surgery over 20 years ago when I had a trauma-related injury I have to admit I am probably more careful than most how I treat my knees…and very aware of the results if I overdo things. The key factor I have learned by experience is to keep away from exercises that place direct stress on the knee joints and keep to doing exercises that share the loading between the three major lower limb structures – these are the hips, knees and ankle joints.
By putting more direct stress on the knees, where the movement treats the knees as a fulcrum such as any variation of a leg extension exercise you are more likely to injure it. By the same token, if you over-flex the knee joints, such as very deep squatting or very high step up movements, you place a greater strain on your knees, so best keep away from those exercises as well. Admittedly as an Olympic Weight Lifter I did deep squatting movements and always had patella tendinitis after a few years in the sport, which resulted in some calcification of these tensions over the knee. There is more likelihood of rupturing these tendons in this case. Hence my injury.
Leg Extendions place undue stress on the knees as the movement is centered around the knee joints and is also an unnatural movement in that it is not a movement you do in everyday life or in any sport. Stick to exercises where you can see a relationship to natural movements.
So the best exercises to do are exercises as depicted in the leading picture, a Split Squat or Medium Depth Step Ups (where the knee doesn’t bend much more than 90 degrees) or Step Down movements such as the Pistol Squat or the Double Leg Squat Down movement such as depicted in the picture below.
This has got the added safety factor of having a chair beneath the posterior, particularly if you are concerned with the lack of stability of your knees. The knees are just above 90 degrees bent in this picture and in most movements in most sports and in everyday movements, the knees don’t bend much more than this. Obviously if you feel secure enough you can Squat lower than this and you don’t need much more resistance than bodyweight, but if you feel the need to add more resistance either do a pistol Squat and/or hold a set of dumbbells in your hands. Start with higher movements or in the case of Pistol Squats, you can hold onto an object for stability and as an initial progression.
The take home message is to do movements that share the loading over several joints and stay away from movements that place direct pressure on your knees.