The leg extension machine is one of the worst pieces of equipment at the gym. I have to admit that I once used this machine but now I grimace every time I see someone kicking their legs into the air. This machine is a poor choice to strengthen your quads, because this exercise puts the most stress on your knee joint of all quad exercises out there. The long lever arm of the weight on your shin generates extremely high pressures between the kneecap and the femur. Over time, repeated large forces are compressing the knee cap onto the femur, can lead to degeneration of cartilage of the patello-femoral joint (imagine kneeling on the ground on your kneecaps, then having someone jump on your back!). Also, very high tensile stresses are placed on the patellar tendon, which may lead to patellar tendonitis (aka. Jumper’s knee).
Research done by Lutz, et al. in the Journal of Bone and Joint Surgery compared open (foot is free) vs. closed (foot is fixed) kinetic chain ex’s and determined that closed chain exercises placed significantly less shearing force on the knee joint than opened chained knee extension. Since this is an open chained exercise, stress is placed on the ACL due to high shearing forces on the knee joint. The leg extension machine does isolate the quadriceps muscle, but does not strengthen any other muscles in the leg, nor does it challenge your balance, coordination, or functional strength. When in life do you actually have to forcefully extend you knees against greater than 40+ pounds of resistance? Maybe if you’re a soccer player!
Here are some good alternative exercises to strengthen your quads and legs:
Squats: With proper form, squats can functionally strengthen the quads along with the gluts and other leg muscles, as well as challenge your balance and coordination.
Lunges: With proper form, lunges are an excellent choice to further challenge your dynamic balance while still safely strengthening your legs in a functional way
Leg press: Similar to squats, the leg press lets you push much more weight without pressure on your back or neck. For even more of a challenge, try a single leg press!