Choose your weight and sit on the leg extension machine.
Make the necessary adjustments so that the backs of your thighs are fully supported by the seat and your back is fully supported by the back rest.
Hook your feet under the foot pads of the lever with the pads resting just above your ankles.
Grasp the side handles for support.
Exhale as you lift the lever forwards and upwards by straightening your legs until they are fully extended.
Hold for a count of two.
Inhale as you return the lever to the starting position by bending your knees.
Comments and tips
Adjust the back support and seat to accommodate full range of motion.
Position your knees at the same axis as the lever fulcrum.
As you raise the padded lever, do not allow your hips to curl up; keep your thighs in firm contact with the seat.
Do not go too heavy because it can be bad for your knees.
The leg extension exercise mainly develops your lower quads. To put more emphasis on your upper quads, as you raise your legs, point your toes forward (see video).
The exercise can also be performed with one leg at a time.
The leg extension should not be your primary quad exercise. There are far better options. The barbell squat, for example, builds your quads equally well, as well as simultaneously builds and strengthens many other muscle groups, strengthens your core, and helps you to develop functional strength. Use the leg extension only to warm up your quads, pre-exhaust them before squats, or flush them at the end of a leg workout.