The prone leg curl primarily works your hamstrings, the group of three muscles running along the back of your thigh. It also recruits your calf muscles to a meaningful degree, and how much depends on what you do with your feet during the movement. Here’s a closer look at every muscle involved and how to get the most out of this exercise.
Primary Muscles: The Hamstrings
Your hamstrings are actually three separate muscles: the biceps femoris (on the outer back of your thigh), the semitendinosus, and the semimembranosus (both on the inner back). All three work together to bend your knee against resistance during the prone leg curl. Because you’re lying face down with your hips pinned to the bench, the exercise isolates knee flexion almost entirely, making the hamstrings do the vast majority of the work without much help from your glutes or lower back.
This is one of the key advantages of the prone position. During other posterior chain exercises like Romanian deadlifts or hip extensions, your glutes and spinal erectors fire heavily alongside the hamstrings. Research on prone leg movements shows the hamstrings typically activate first, with the gluteus maximus firing roughly 350 milliseconds later, well after the main work has begun. The fixed, face-down position essentially takes the glutes out of the equation as prime movers, letting you load the hamstrings more directly.
Secondary Muscles: Calves and Lower Leg
Your calf muscles play a bigger role than most people realize. The gastrocnemius, the large diamond-shaped muscle of your calf, crosses both the knee joint and the ankle joint. That means it assists with knee flexion during every rep. The soleus, the deeper calf muscle underneath, also contributes, though its role shifts depending on your ankle position.
Research from the National Strength and Conditioning Association found that gastrocnemius activation increases significantly when your ankle is pulled up toward your shin (dorsiflexed) during the curl. The soleus, on the other hand, is more active when your toes are pointed (plantarflexed), regardless of knee angle. This means your foot position isn’t just a comfort preference. It changes which muscles share the load with your hamstrings.
How Foot Position Changes Muscle Recruitment
You can use ankle position as a simple tool to shift emphasis during the exercise:
- Toes pulled toward your shins (dorsiflexion): This stretches the gastrocnemius and forces it to contribute more to knee flexion. You’ll feel your calves working harder alongside your hamstrings. Some people find this makes the exercise slightly harder at the same weight.
- Toes pointed away (plantarflexion): This shortens the gastrocnemius, reducing its ability to assist with knee flexion. More of the demand shifts onto the hamstrings alone, which can be useful if your goal is maximum hamstring isolation. The soleus picks up more activity in this position, but it doesn’t cross the knee, so it isn’t helping with the curl itself.
If you want to bias the exercise toward pure hamstring work, pointing your toes is the better choice. If you want your calves involved as well, pull your toes up.
What the Prone Curl Doesn’t Work
Because your hips stay flat on the pad throughout the movement, the prone leg curl produces very little glute activation compared to exercises like hip thrusts or deadlift variations. Your lower back muscles (erector spinae) act as stabilizers to keep your pelvis from lifting off the bench, but they aren’t generating force in a meaningful way. And your quadriceps, on the front of your thigh, are completely uninvolved since they perform the opposite action (knee extension).
This narrow focus is both the strength and limitation of the exercise. It’s excellent for isolating the hamstrings, but it shouldn’t be your only posterior chain movement if balanced leg development is the goal.
Proper Setup on the Machine
Getting the pad and body position right matters for both effectiveness and joint safety. Lie face down and adjust the calf pad so it sits just above your Achilles tendon, right where the fleshy part of your calf meets the tendon. If the pad is too high (mid-calf), you lose leverage. If it’s too low (on the heel), it puts unnecessary pressure on the ankle joint.
Your knees should align with or sit just past the edge of the bench so they can flex freely without the bench digging into your kneecaps. Most machines have a pivot point marked or built into the frame. Line your knee joint up with that axis. Start each rep with your legs fully extended but not locked, and curl the weight up by bending at the knees until your shins are roughly perpendicular to the floor or slightly beyond. Lower under control. Letting the weight drop quickly robs your hamstrings of the eccentric (lowering) portion, which is where a large share of the muscle-building stimulus happens.
Prone Curl vs. Seated Leg Curl
Both machines target the same muscles, but the hip position changes things. On the seated leg curl, your hips are flexed at roughly 90 degrees, which stretches the hamstrings across the hip joint before you even start curling. This puts the hamstrings in a longer starting position and typically produces a stronger stretch sensation. On the prone curl, your hips are extended (straight), so the hamstrings start in a relatively shortened position at the hip.
In practical terms, the seated curl tends to produce more muscle damage and soreness because of that stretched position, while the prone curl lets most people handle slightly heavier loads with less discomfort. Neither is objectively better. Using both over time gives your hamstrings stimulus across different muscle lengths, which supports more complete development.