To train the legs in near isolation using heavy loads while minimizing the risk of injury to the lower back.
"The leg press does allow you to use a great deal of weight while calling upon a lot of muscle mass," says MF training adviser Jason Ferruggia. And, he adds, when used for high reps with a "pumping cadence," in which you don't pause between reps, you can pump a lot of blood into the legs, priming them for growth.
Using very heavy weights while lifting through a full range of motion can put you at risk for injury. The closer your knees come to your chest, the more likely your lower back will round and lose contact with the pad. "As it is, there is a great deal of pressure on the lower back during a heavy leg press," says Ferruggia. He recommends doing nonstop pumping reps through a limited range of motion; that is, bending your knees to only about 90 degrees.
Good. If performed with caution, the leg press machine is a solid method for building solid legs.