A hamstring curl is a strength exercise that targets the three muscles running along the back of your thigh by bending your knee against resistance. It’s one of the most direct ways to isolate and strengthen the hamstrings, and it’s a staple in programs focused on leg development, knee stability, and injury prevention. You can perform it on a machine, with a resistance band, or using just your bodyweight.
The Muscles a Hamstring Curl Works
Your hamstrings are actually a group of three separate muscles. The biceps femoris sits on the outer part of the back of your thigh. The semimembranosus runs along the innermost side, and the semitendinosus sits between the other two. All three share the same basic jobs: bending your knee, extending your leg backward at the hip, and helping rotate your lower leg when the knee is bent.
A hamstring curl isolates the knee-bending function specifically. This matters because many common exercises like squats and deadlifts work the hamstrings primarily through hip extension, leaving the knee-flexion role undertrained. Research comparing the two movement patterns found that hamstring muscle activity is significantly higher during knee flexion than during hip extension, which helps explain why curls are so effective at building these muscles directly.
How to Perform a Hamstring Curl
The movement itself is simple: you start with your legs relatively straight and bend your knees to pull a resistance source toward you. On a seated machine, you sit upright with your ankles resting on top of the padded bar, then press it downward by curling your heels toward the floor. On a lying (prone) machine, you lie face down with the pad resting against the backs of your ankles and curl it upward toward your glutes.
The key form detail is keeping your pelvis stable throughout the movement. On a lying curl, the most common mistake is arching your lower back as you lift the weight. This tilts your pelvis forward and shifts tension away from the hamstrings, reducing the exercise’s effectiveness and putting unnecessary stress on your spine. Focus on pressing your hips firmly into the bench pad before you begin each rep. On a seated curl, sit with your back flat against the backrest rather than leaning forward or rocking your torso to move the weight.
Control the speed in both directions. A good tempo is about two seconds to curl the weight and two seconds to return it, with no pause or bouncing at the bottom. This keeps tension on the muscle throughout the full range of motion.
Seated vs. Lying Curl
These two variations look similar but stress the hamstrings differently. When you’re seated, your hips are bent at roughly 90 degrees, which stretches the hamstrings at the hip end before you even start curling. This means the muscle works through a longer overall length during the exercise. When you’re lying face down, your hips are nearly straight, so the hamstrings start in a shorter position.
That distinction has real consequences for muscle growth. A study published in Medicine and Science in Sports and Exercise compared the two variations over a training period and found that the seated leg curl produced greater hamstring hypertrophy than the prone version. The researchers recommended the seated curl specifically for anyone whose goal includes increasing or maintaining hamstring size. If you only have access to one type of machine, the seated version gives you a slight edge for building muscle.
Bodyweight and Free-Weight Alternatives
You don’t need a machine to train hamstring curls. The Nordic hamstring curl is a bodyweight variation where you kneel on the floor, anchor your feet under something sturdy, and slowly lower your torso toward the ground by straightening your knees. It’s brutally difficult because your hamstrings are controlling your entire bodyweight through an eccentric (lengthening) contraction. Most people can only manage a few controlled reps at first.
Other options include sliding leg curls (lying on your back with your heels on sliders or a towel, then pulling your feet toward your hips), stability ball curls, and resistance band curls. These work well for home training or as lighter accessory movements on leg day.
Sets, Reps, and Loading
For building muscle size, a protocol of 5 sets of 10 repetitions at around 70% of your one-rep max is well supported by research. Use two-minute rest periods between sets. If you’re newer to the exercise, start lighter for the first few sessions (around 50 to 60% of what you could lift for a single rep) and build to your working weight over the first week.
For general strength and endurance, 3 to 4 sets of 8 to 12 reps is a practical range. The hamstrings respond well to moderate rep ranges because they contain a mix of fast-twitch and slow-twitch muscle fibers. Training them with both heavier, lower-rep sets and lighter, higher-rep sets across your weekly program covers all bases.
Injury Prevention Benefits
Hamstring strains are among the most common injuries in sports that involve sprinting, cutting, and kicking. Eccentric hamstring training, particularly the Nordic curl, has been studied extensively for its protective effect. An umbrella review of the research found that programs including the Nordic hamstring exercise reduced hamstring injury rates by up to 51% compared to control groups. That’s a meaningful reduction for athletes in soccer, football, rugby, and track.
Strong hamstrings also protect your knees. When your quadriceps (the muscles on the front of your thigh) contract, they naturally pull your shinbone forward relative to your thighbone. If the quads act alone, this forward shear stresses the anterior cruciate ligament (ACL). But when the hamstrings fire at the same time, they pull the shinbone backward, counteracting that force and stabilizing the joint. At bent-knee positions, the hamstrings’ line of pull runs nearly parallel to the joint surface, making them especially effective at preventing the kind of tibial translation that tears ACLs. Building hamstring strength through curls directly supports this protective mechanism.
Where Hamstring Curls Fit in a Program
Hamstring curls work best as a complement to compound movements, not a replacement for them. Exercises like Romanian deadlifts and hip thrusts train the hamstrings through hip extension, working the muscles at their upper attachment near the glutes. Curls train the lower portion of the movement pattern, working the muscles closer to the knee. Combining both gives you more complete hamstring development than either approach alone.
Most programs place hamstring curls on a leg day or lower-body session, typically after squats or deadlifts. Doing your heavy compound lifts first, when you’re freshest, and finishing with curls as an isolation exercise is a logical sequence. Two to three sessions per week that include some form of hamstring work is enough for most people to see steady gains in both size and strength.