Lying leg curl muscles worked

Every role here is for this technique: lying face down on the machine, the roller just above the heels, from straight knees to about 90 degrees.

What muscles does the lying leg curl work?

The prime mover of the lying leg curl is the hamstrings. They produce knee flexion. The synergists are gastrocnemius, sartorius and gracilis. Two stabilizers hold joints and segments still: gluteus maximus, and the wrist and finger flexors. The roles follow Schoenfeld et al. (2015), Ebben (2009) and Kendall et al. (2005).

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Joint actions and their prime movers

The joint actions of the lying leg curl and the prime movers that produce each
Joint action	Prime movers
Knee flexion	The hamstrings

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 lying leg curl, with what each does
Muscle	What it does here
Hamstrings	Knee flexion, all four parts The lower parts of the hamstrings are more active here than in a stiff-legged deadlift (Schoenfeld et al. 2015).

Synergists 3

Synergists in the lying leg curl, with what each does
Muscle	What it does here
Gastrocnemius	Knee flexion
Sartorius	Knee flexion
Gracilis	Knee flexion

Stabilizers 2

Stabilizers in the lying leg curl, with what each does
Muscle	What it does here
Gluteus maximus	Keeping the hips pressed into the pad
Wrist and finger flexors	Gripping the handles

Lowering and lifting

What the prime movers of the lying leg curl do in each half of a rep
Phase	What the prime movers do
Lowering	The hamstrings lengthen as the roller goes back down
Curling	They shorten to bend the knees

Where these prime movers also work

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

Sources

Schoenfeld et al. (2015), J Strength Cond Res 29(1):159
Ebben (2009), Int J Sports Physiol Perform 4(1):84
Kendall et al., Muscles: Testing and Function, 5th ed. (2005)

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.