How To Do the Leg Curl

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The leg curl is the exercise people do at the end of leg day when they are already finished, which is roughly the opposite of what the hamstrings deserve.

They are half of your thigh, they do most of the work when you sprint, and they are the muscle group with the most research attention of anything below the waist, for reasons that have nothing to do with aesthetics.

The machine version is simple to execute and easy to progress. What follows is how to do it properly, and what the evidence says about where it sits next to the exercise it constantly gets compared to.

What high-volume hamstring work actually changes

This is the study with the hardest numbers.

A systematic review and meta-analysis searched PubMed, Web of Science and Scopus for studies on low-volume versus high-volume Nordic hamstring exercise in soccer players, examining eccentric strength and hamstring muscle architecture. Eleven studies met inclusion criteria, with pooled effect sizes calculated as Hedges' g using a random-effects model.

High-volume interventions significantly improved eccentric hamstring strength, with an effect size of 0.77 and a confidence interval from 0.49 to 1.06.

They improved fascicle length, effect size 0.43, confidence interval 0.20 to 0.65, with zero heterogeneity between studies.

They improved muscle thickness, effect size 0.48, confidence interval 0.28 to 0.68, again with zero heterogeneity.

Effects on pennation angle were non-significant, at negative 0.16.

Fascicle length is the interesting one. That is the length of the muscle fibres themselves, measured by ultrasound, and it changed. Training did not just make the hamstrings stronger, it changed their architecture.

The zero heterogeneity on both fascicle length and thickness means the studies largely agreed with each other, which is unusual and worth noticing.

The honest caveat: this is Nordic hamstring exercise in soccer players, not the machine leg curl, and the review exists partly because there is still debate about the minimal effective dose. Take it as evidence that hard eccentric knee flexor work produces structural change, not as a claim about the machine specifically.

Set up the machine before you argue about anything else

Whether the machine is lying, seated or standing, the same setup rules apply.

The pad should sit just above your heels, on the achilles, not up on your calf. Too high and the lever is wrong and it hurts.

The axis of the machine should line up with your knee joint. Every one of these machines has an adjustment for this and almost nobody uses it. If your knee is not in line with the pivot, the resistance curve you experience is not the one the machine was built to deliver.

Hips flat against the pad on a lying curl. When your hips rise off the bench to help finish the rep, the movement has become a hip extension with a leg curl attached.

Hold the handles and stay tight. Your torso should not be contributing.

Standard coaching practice rather than a finding from the sources cited here.

Seated or lying, and why it is not arbitrary

The two versions differ in one thing that matters: where your hip is.

A seated leg curl puts the hip in flexion, which puts the hamstrings at a longer length before the rep starts. A lying curl has the hip roughly straight, so the muscle starts shorter.

This is not a trivia point. The same evidence base that produced the fascicle length findings above is built on eccentric work at long muscle lengths, and the seated version is the one that trains a longer position on the machine.

If you have both machines available, that is a real reason to prefer the seated one, or to run both.

The hip position difference is basic anatomy and standard coaching practice. No source cited here directly compared seated and lying machine curls for growth outcomes, so treat it as a well-reasoned preference rather than a proven ranking.

The movement

Curl by pulling your heels toward your backside, not by yanking the pad.

Go through the full available range. Get a genuine stretch at the bottom with the knee close to straight, and a hard squeeze at the top.

Control the eccentric deliberately. This is the entire point of the exercise. All the architecture evidence above comes from eccentric-focused work, and letting the weight drop back down turns two-thirds of your set into nothing. Take two to three seconds on the way down.

Point your toes or flex them, whichever you prefer, but keep it consistent within a set. Toes pointed reduces calf involvement, toes flexed brings the gastrocnemius in.

Cramping is normal in this exercise, particularly on the first few sets after a layoff. It is not a sign you are doing it wrong.

And stop short of the point where your hips start rising. That is your actual working range.

Also standard coaching practice, included because a guide that skipped it would be useless.

How the machine compares to the Nordic curl

Somebody measured this directly, which is more than the internet argument usually manages.

A cross-sectional study compared the Nordic hamstring curl against the conventional prone leg curl at different loads, plus a novel high-velocity variation called hamstring catches. Twenty-three recreationally active men completed the final sample, having first attended a session to establish repetition maximums for loading, at 8, 16 and 24 rep max for the leg curl and 8 and 16 rep max for the catches, and to familiarise themselves with all three exercises.

The outcomes were peak normalised electromyographic activity and the rate of electromyographic rise in the biceps femoris long head, plus knee angular velocity.

Rate of rise is the part worth flagging. That is how quickly the muscle ramps up its activity, not just how much it eventually reaches, and it is a different question from peak activation.

The study's own framing is candid about why it exists: hamstring injuries remain a major burden in football, and the Nordic hamstring exercise is poorly adopted despite its documented effects, so understanding how it compares to other hamstring curls may clarify the physiological adaptations each produces.

Twenty-three recreationally active men in a cross-sectional design measuring muscle activity, not growth or strength over time. Muscle activation is not the same thing as adaptation, and this article reports the comparison the study set up rather than its result table.

It is not the only way to load a hamstring

One more study is worth knowing about, because it tested alternatives head to head.

A randomised controlled trial, pre-registered on ClinicalTrials.gov, took 35 healthy young female footballers averaging 21 years old and randomised them to one of three interventions: the Nordic hamstring exercise, the single leg hamstring bridge, or the razor hamstring curl. Outcomes were hip extensor and knee flexor strength, including isometric knee flexor peak torque at 30 degrees of knee flexion, and sprint performance, analysed with two-way repeated measures analysis of variance across both limbs.

Three different ways of loading the same muscle group, compared properly, in a female population that most of this literature has historically under-studied.

The practical point for you is that the machine curl is one tool among several, and the hamstrings respond to hip-dominant loading as well as knee-dominant loading. Romanian deadlifts and hip-dominant work are not interchangeable with curls, and running both is more complete than arguing about which is better.

Thirty-five participants across three groups is roughly twelve per arm, which is small, and this article reports the trial's design rather than its outcome figures.

Why any of this is worth your time

The hamstrings are the muscle group where hard eccentric work has been shown to change the structure of the muscle itself. Pooled across eleven studies, high-volume work moved eccentric strength by an effect size of 0.77, fascicle length by 0.43 and thickness by 0.48, with the last two showing no disagreement between studies.

What that buys you on the machine: line the knee up with the pivot, keep the hips down, use the full range, and above all control the eccentric, because that is where the evidence lives.

Do not treat the curl as the whole hamstring plan. Knee flexion and hip extension are different jobs and you want both.

This is a training article, not medical advice. Hamstring pain, a previous strain, or anything that feels sharp during curls is a matter for a qualified professional and not something to train through on the strength of an article.