Leg extension muscles worked

Every role here is for this technique: seated machine, back against the pad, the roller just above the ankles, from a 90 degree knee to straight.

What muscles does the leg extension work?

The prime mover of the leg extension is the quadriceps. They produce knee extension. No muscle assists as a synergist. One stabilizer holds a joint or segment still: the wrist and finger flexors. The roles follow Kendall et al. (2005), Neumann (2017) and NSCA (2016).

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Joint actions and their prime movers

The joint actions of the leg extension and the prime movers that produce each
Joint action	Prime movers
Knee extension	The quadriceps

Every muscle by role

A prime mover produces the lift’s main joint action against the load; a synergist assists that action, or adds a secondary action the lift needs; a stabilizer contracts mostly isometrically to hold a joint or a segment still. A role describes what the muscle does in the technique above, not how hard it works or how much it grows.

Prime movers 1

Prime movers in the leg extension, with what each does
Muscle	What it does here
Quadriceps	Knee extension, all four heads Rectus femoris also crosses the hip, which the seat holds bent.

Stabilizers 1

Stabilizers in the leg extension, with what each does
Muscle	What it does here
Wrist and finger flexors	Gripping the handles to stay in the seat

Lowering and lifting

What the prime movers of the leg extension do in each half of a rep
Phase	What the prime movers do
Lowering	The quadriceps lengthen as the roller comes back
Extending	They shorten to straighten the knees

Where these prime movers also work

The quadriceps: prime mover in the back squat, front squat, leg press, conventional deadlift, sumo deadlift, Bulgarian split squat and walking lunge; synergist in the barbell hip thrust and front plank.

Sources

Kendall et al., Muscles: Testing and Function, 5th ed. (2005)
Neumann, Kinesiology of the Musculoskeletal System, 3rd ed. (2017)
NSCA, Essentials of Strength Training and Conditioning, 4th ed. (2016)

The cited studies that measured muscle activity used electromyography, and a larger signal need not mean more force or more growth (Vigotsky et al. (2018), Front Physiol 8:985). The roles draw on those studies for which muscles a lift involves and how two lifts differ, never to rank one muscle against another.

See these muscles in 3D

Visualiser · Biology Muscles Worked by Exercise 3D Explorer Choose a lift and a real body lights its prime movers, synergists and stabilisers, front and back, with the same muscles in 3D. Visualiser · Biology Muscle Origin, Insertion and Action Explorer Paint a muscle’s origin and insertion on real 3D bones, or click a bone to see what attaches to it.