Leg curl: how to do it, seated or lying, and how much to lift

Muscles the leg curl works

The leg curl works the hamstrings, the 4 muscles at the back of your thigh. Three of them cross both the hip and the knee. On the inner side these are the semitendinosus and the semimembranosus, and on the outer side the long head of the biceps femoris. The fourth, the short head of the biceps femoris, crosses only the knee. The gracilis on your inner thigh, the sartorius and your calves help. Your hip stays still, so the glutes do little.

Not every part works equally hard. Scans taken right after exercise (MRI) show which muscles were used:

• In 7 men, curls that bent the knee used the semitendinosus and the gracilis more than the long head and the semimembranosus.
• In 36 professional soccer players, a flywheel leg curl raised the MRI signal by 95% in the gracilis and 65% in the semitendinosus. A flywheel machine uses a spinning wheel instead of a weight stack. The short head rose 51% and the long head only 14%.
• A review drew the same line. Exercises that bend the knee use the semitendinosus and the short head more, and hip exercises the long head and the semimembranosus.

Studies that measure electrical activity with sensors on the skin (EMG) disagree. In 16 elite female handball and soccer players, a leg curl done lying on the back worked the outer hamstring harder than the inner one. In 12 trained men, the inner hamstring was more active in all 4 exercises tested, the leg curl included. EMG shows which muscles fire, not which ones grow.

Growth data agrees with the MRI scans. In the one long training study of the leg curl, the semitendinosus grew most of the 4 hamstring muscles on both machines.

How to do a leg curl

Set the machine before you add weight. These steps are for the lying leg curl:

1. Lie face down with your knees just past the end of the pad. The crease behind your knee should line up with the pivot, the round axle the lever turns on.
2. Set the ankle pad on the back of your lower legs, just above your heels.
3. Hold the handles and press your hips into the pad.
4. Start with your knees almost straight. Curl your heels towards your seat as far as you can, without lifting your hips.
5. Pause for a moment at the top. Then lower the pad under control until your knees are almost straight, with the plates only touching lightly.
6. Breathe out as you curl and breathe in as you lower.

On the seated leg curl, the same rules apply with 3 changes:

1. Sit with your back against the backrest. Move the backrest until your knees line up with the pivot.
2. Lower the thigh pad until it presses on top of your thighs, just above your knees. It holds you in the seat.
3. Curl the ankle pad down and back under the seat, as far as you can, then let it rise under control.

Release the lever or pin, move the pad, and check that it locks before you lift.

Seated or lying leg curl?

If you can only pick one, pick the seated leg curl. In the only study that compared them for growth, it built more hamstring muscle. The lying curl still worked, so doing both is fine too.

The reason is the hip. Three of the 4 hamstring muscles cross it, and sitting with your hips bent stretches them.

In that study, 20 adults new to lifting trained one leg on a seated machine, with the hip bent about 90 degrees. They trained the other leg on a lying machine, with the hip bent about 30 degrees. Each leg did 5 sets of 10 reps at 70% of its max, twice a week for 12 weeks. MRI measured muscle volume before and after.

The extra growth came only in the 3 muscles that cross the hip. The short head, which crosses only the knee, grew about the same. The sartorius grew more on the lying machine, 11.8% against 7.8%.

Strength rose about equally. The seated max went from 40.9 to 53.6 kg, and the lying max from 23.9 to 30.3 kg. The same people lifted far more seated, so never compare a seated number with a lying one. Keep the limits in mind: this is one study of 20 untrained people, on 2 different machines.

Leg curl or hip exercise? The curl trains the hamstrings at the knee. A Romanian deadlift or a back extension trains them at the hip, and the 2 grow different parts:

• In 30 male athletes over 10 weeks, the Nordic curl and a hip extension exercise both grew the semitendinosus. The Nordic curl is a kneeling exercise that bends the knee. The hip extension grew the long head more.
• In 36 untrained people over 9 weeks, the Nordic curl grew mostly the semitendinosus, by 24.3%. The stiff-leg deadlift grew mostly the semimembranosus, by 11.2%.

For the whole back of the thigh, do one curl and one hip exercise each week.

Where the advice differs

Most leg curl technique is coaching habit, not tested in studies. Where coaches disagree, the answer depends on your machine and your goal.

Common mistakes and how to fix them

Sets, reps and rest for the leg curl

There is no single right number. The table is a starting point built on the research on lifting in general. None of these studies tested the leg curl on its own.

What the studies show:

• Heavy weights build more strength. Muscle grows about as well with light or heavy weights, if the sets go close to failure.
• More weekly sets bring more muscle, up to a point. Strength gains level off sooner.
• With the same weekly sets, training a muscle more often does not add size.
• You do not need to reach failure. On a machine, a failed rep is safe: the pad just lowers. That makes the leg curl a good lift to take close to failure.
• Rest longer before heavy sets. Rests of 3 minutes built more strength and muscle than 1 minute in trained men. For muscle size, rests past 90 seconds made little further difference.

For longer answers, read how many sets and reps build muscle and how long to rest between sets.

Warm-up sets before the leg curl

One or 2 lighter sets on the stack are enough. For example, do 10 easy reps at about half your working weight, then 5 reps at about 80%. A stack moves in fixed steps, so pick the nearest pin.

Only the 80% set has some study support. In 15 trained men, a warm-up set of 5 reps at 80% of the working weight led to more total work than warm-ups at 40% or 60%. Their session included the leg press, but the clearest gain was on the bench press. Another study found that the kind of warm-up made no difference to reps.

The half-weight set and the rep counts are habit. No study shows that warm-up sets prevent injury.

After Romanian deadlifts or squats, your hamstrings are already warm. One set at about 80% is usually enough.

How much should you leg curl?

No study sets a standard for the leg curl. StrengthLog publishes strength tables only for the squat, the bench press, the deadlift and the overhead press. The one large table online comes from what gym app users log.

Strength Level has a table for each machine. Its lying leg curl table uses 76,852 of 358,631 logged lifts, and its seated table 125,544 of 457,839, from August 2017 to March 2026. The rest failed its checks for implausible, duplicate or automated entries. Each level is the one-rep max (1RM, the most weight you can lift once) that beats a share of its users:

Rounded, Strength Level puts the intermediate line on the lying curl at 0.75 times body weight for men and 0.6 for women. On the seated curl, it puts it at 1 times for men and 0.75 for women. The average logged lift is 63 kg for men and 37 kg for women lying, and 80 kg and 49 kg seated. Nothing checks form or range of motion.

Weight stacks differ between machines: the lever, the cam and the pulleys change how heavy the same pin feels. In the seated and lying study, the same people started at about 41 kg seated and 24 kg lying. Your numbers compare best on the same machine.

Spend your effort on picking a weight and adding to it:

1. Pick a weight you can curl 10 to 12 times with 2 or 3 reps to spare.
2. Keep it until every set reaches the top of your rep range with clean reps.
3. Then move one pin. If that jump is too big, add reps at the new weight from a lower range.

On Strength Level, 3 sets of 10 is the most logged leg curl workout on both machines. For the idea behind adding weight, read progressive overload.

Under 18? The table is for adults and sets no target for teenagers. Clean reps and steady progress matter more.

Find your leg curl max

You do not need to test a true one-rep max on the leg curl. Type a set you did, and the calculator estimates your max with 7 formulas. Sets of 10 reps or fewer give the closest estimate. For more detail, open the 1RM calculator.

On a machine, use the number to compare your own sets over time, on the same machine. Formulas are rougher on the leg curl than on bigger lifts. One study had 40 trained adults aged around 73 with low bone density. Even formulas built for each machine missed their true max by 6.9% on the leg curl, against 3.7% on the leg press.

Setting up the leg curl machine

Nothing rests on your spine, and a failed rep just lowers the pad. Most problems on the leg curl come from the setup.

The pivot. Line up the crease behind your knee with the axle. When your knee and the lever turn around different points, the pad slides along your leg as you curl.

The ankle pad. Set it just above your heels, on the back of your lower legs. Too low and it presses on your Achilles tendon. Too high and it rubs up and down your calf. That is habit, not a tested rule.

Your hips and lower back. On the lying curl, your hips lift when the weight is too heavy, and your lower back arches to finish the rep. Hold the handles and take weight off until your hips stay down. On the seated curl, the thigh pad does this job.

Cramps. The back of your thigh or your calf can cramp at the top of a hard set. A review of cramp research found no single cause and recommended gentle static stretching until the cramp lets go. Stop the set, straighten the leg and stretch it gently. If your calves cramp often, try pointing your toes.

When the leg curl hurts

Pain is a reason to change something, not to push through. Diagnosis and treatment belong to a physiotherapist or a doctor.

• Back of the knee: check that your knee lines up with the pivot and the pad sits above your heels. Use less weight and stop a little short at the top for a few weeks.
• Front of the knee or kneecap on the lying curl: move up so your kneecaps sit just past the pad, not on its edge.
• Deep in the buttock, near the sit bone: long-lasting pain there is a typical sign of a sore hamstring tendon (proximal hamstring tendinopathy). The seated curl stretches that spot, so the lying curl may sit better, though that is mechanics, not a tested result. A 2016 review found no study with random groups that tested its rehabilitation, so see a physiotherapist.
• Back of the thigh: a sudden sharp pain during a set needs a physiotherapist's check before you load the leg again. If pain stays after a cramp has let go, have it checked too.

For lasting pain, read what to do when a lift hurts.

Leg curl variations and alternatives

In Trackist's exercise library, the lying leg curl and the seated leg curl sit in one group with 4 other hamstring curls: the Nordic hamstring curl, the standing leg curl, the Swiss ball leg curl and the slider hamstring curl. The app's page for each curl lists the others as similar movements. When you swap a curl for one session or for the rest of your plan, the picker puts that group first, then other hamstring exercises, starting with those your equipment and joints allow.

How Trackist runs your leg curls

Trackist is an AI personal trainer for the gym. It writes your plan, and the lying and the seated leg curl get this from the app:

• From your second session of a leg curl, Trackist suggests what to try from the last time you logged it. When every set at your top weight reached the top of the rep range, it adds 5 kg (10 lb). Inside the range, it keeps the weight and asks for more reps. There is no suggestion in a deload week.
• When you fall short of the reps, it keeps the weight, takes about 5% off or leaves the call to you, depending on how far short you fell and how hard the sets felt.
• From your second session, it logs a personal record when you beat your heaviest weight, your most reps at a weight or your best estimated max.
• The exercise page shows 4 form cues and the muscles worked. It lists no joints, because the library marks none on the leg curl. From your second session, it adds a chart of your estimated one-rep max.
• When your knee hurts on a leg curl, you report it and rate it from 1 to 5. Trackist swaps it for another curl, such as the seated or the standing leg curl. The app has no narrower pattern for hamstring curls, so your profile stores the knee as a whole. Later plans then leave out every movement that loads the knee: squats, lunges, the leg extension, the Nordic curl, running and jumps. The leg curls stay, because the library marks no joint on them. A report of hip or lower back pain on a leg curl works the same way for that joint.
• Leg curls do not count towards your Strength Score, which Trackist works out from squats, hip hinges, presses and pulls.