How to Leg extension

What it is

A seated machine exercise where you extend your knees against a padded lever, isolating the quadriceps. It loads the quads — including through the lengthened position — without involving the hips or requiring balance.

Muscles worked

Primary: Quadriceps. Secondary: Hip flexors.

How to perform it

Sit in the machine with your back against the pad and knees aligned with the machine's pivot.
Set the shin pad to rest just above your ankles and hold the handles.
Extend your knees to raise the pad until your legs are nearly straight, squeezing the quads.
Lower under control to a full stretch without letting the weight stack crash down.

Common form faults

Using momentum to swing the weight up. Slow down and control both directions; reduce the load so the quads, not a heave, do the work.
Only doing the top half of the movement. Lower to a full stretch each rep — the lengthened portion is a key part of the stimulus.

Programming

A useful quad-isolation accessory, especially for adding volume or bringing up the quads. Typically 10–20 reps; a good movement to take close to failure safely.

Frequently asked questions

Are leg extensions bad for the knees?

For most healthy people used sensibly — controlled reps, reasonable loads — they're a fine quad-building exercise. If you have specific knee issues, moderate the load and range and see how your knees respond, or get individual advice.

Do I need them if I squat?

Not essential, but they add focused quad volume that compounds don't isolate as directly. They're a helpful accessory if quad size is a priority.