The leg extension, or knee extension, is a type of strength training exercise. It’s an excellent move for strengthening your quadriceps, which are in the front of your upper legs.
Leg extensions are exercises usually done with a lever machine. You sit on a padded seat and raise a padded bar with your legs. The exercise works mainly the quadriceps muscles of the front of the thigh—the rectus femoris and the vastus muscles. You can use this exercise to build lower body strength and muscle definition as part of a strength training workout.
The leg extension targets the quadriceps, which are the large muscles of the front of the thigh. Technically, this is an "open chain kinetic" exercise, which is different from a "closed chain kinetic exercise," such as a squat.1 The difference is that in the squat, the body part you're exercising is anchored (feet on the ground), while in the leg extension, you're moving the padded bar, which means your legs aren't stationary as they work, and thus the chain of movement is open in the leg extension.
The quads are well-developed in cycling, but if your cardio is running or walking you are mostly exercising the hamstrings at the back of the thigh. In this case, you may want to develop the quads to be more in balance. Building your quads can also increase the force of kicking movements, which can be beneficial in sports such as soccer or martial arts.