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Leg Extension

Also called: Machine leg extension

Primary muscles: Quadriceps femoris

Joints / actions: Knee joint (tibiofemoral joint) (extension)

Plane: sagittal

Equipment: cable-machine

What it is

Seated at a machine, straightening the knees against resistance to lift a padded lever.

Purpose

Isolates the quadriceps for knee extension without involving the hip joint.

Starting position

Sit in the machine with the back supported, shins behind the padded lever, knees bent to about 90°.

Step by step

  1. Extend the knees to straighten the legs, lifting the pad in a controlled arc.
  2. Pause briefly at the top without locking out forcefully.
  3. Lower back down under control to the starting position.

Breathing

Exhale as you extend the knees; inhale as you lower.

Coaching cues

"Smooth and controlled — squeeze the quads at the top, don't let the weight drop."

Common mistakes

Using momentum (swinging the weight up); locking the knees out harshly; lowering too fast.

Easier regression

Reduce load or range of motion (partial reps in a pain-free arc).

Harder progression

Increase load or add a pause at the top of the range.

Alternatives

Sissy squat, wall sit (isometric quad work).

Who may benefit

Anyone wanting to isolate quad strength, including as an accessory to squats or for rehabilitation-style strengthening.

Who should modify or avoid it

Those with anterior knee pain (e.g., patellofemoral discomfort) should reduce range/load or consult the Knees region's safety guidance.

Why this exercise works

Because it is a single-joint (knee-only) movement, it isolates the quadriceps directly, which is useful both for building quad size/strength and for identifying/addressing side-to-side strength differences.

⚠ Safety note

Use a resistance and range of motion that stays comfortable at the kneecap; sharp anterior knee pain is a signal to reduce load or range.

Sources

“The quadriceps femoris is the sole prime mover of knee extension.”

Anatomy and Physiology 2e — OpenStax, Rice University · Ch. 11.6 Appendicular Muscles of the Pelvic Girdle and Lower Limbs

Peer-reviewed textbook · License: CC BY-NC-SA 4.0 · Last checked 2026-07-19

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