The seated knee extension, ah yes. Usually it is next to the abductor/adductor (seated in/out motions for the legs), and the leg curl. With this arrangement, you can do a leg circuit and hit all of those trouble areas on your legs. Sounds wonderful! You get to work those leg muscles all while sitting down! Unfortunately, your knees will agree to disagree. The legs are not made to move forces while in an unnatural position such as a seated leg extension in which you raise your lower legs up from a 90 degree angle. Knee mechanics work best in a weight bearing position, such as a squat. Powers et al1 described the tibia, or lower leg bone, moving forward during a leg extension. This puts unnecessary stress on the ACL. During a squat, the tibia moves slightly back, much more natural on the knee including the ACL. During a leg extension, the patella (kneecap), moves laterally over the thigh bone. While squatting down, the thigh bone externally rotates under the patella. Basically, during a squat, the right parts move at the right time, and in a leg extension these patterns are biomechanically not correct. The hamstrings play a large role in stabilizing the knee. In a seated position, however, the hamstrings are not taught and loaded, therefore they cannot be active in knee stabilization. Stresses in the cartilage of the kneecap are also unnatural. The kneecap is thickest in the middle and can accept more force. However, in a leg extension machine, these forces tend to stress the thinner parts of the cartilage.
The take home message is this: seated knee extensions are not a natural loading pattern on the knee, and the negatives outweigh the positives. The quads get plenty of work during squats, as does every other lower body muscle. You can skip all of those isolating machines and just stick to squats. This will be much more effective in terms of safety, translation to “real” strength and function, as well as toning and all of that good stuff. One good set of squats will certainly burn more calories than all of those little leg machines together. A squat demands all of your muscles to work together, including the core. If you get bored with squats, you are uninformed. Many strongmen and longtime lifters have yet to be bored with squat variations because there are so many options. Front, back, single leg, dumbbell, barbell, body weight, split, hack, box, half, deep, quarter, jump squats…..just to name a few different types. Deadlifts, step-ups, and lunges all are nice options for the lower body as well. Each of these has multiple variations. In certain instances, the leg extension may be used.
Some therapists and coaches use it with very light weight for range of motion, or use it to strengthen certain joint angles (no movement, just hold in certain positions), which is helpful for someone with foot problems as a way to keep the leg muscles active without being weight bearing. However, unless instructed by a professional, skip past the machines and learn some proper squatting techniques. Body weight squats alone can do wonders for strength, endurance, and toning. Enjoy squat progressions, not just by adding weight but changing the movement. I guarantee it will keep you busy and get you more results!
1 Powers et al (2003) Patellofemoral kinematics during weight-bearing and non-weight-bearing knee extension in persons with lateral subluxation of the patella: a preliminary study.