Leg curls are not inherently bad for your knees, and for most people they are actively good for knee health. The hamstrings, the muscles leg curls target, serve as one of the knee joint’s primary stabilizers, limiting the forward sliding of the shinbone that stresses ligaments like the ACL. The concern about leg curls and knees usually stems from a misunderstanding about open-chain exercises and a few narrow clinical situations where modifications matter more than avoidance.
How Your Hamstrings Protect the Knee
The worry about leg curls and knees makes more sense once you understand what hamstrings actually do beyond bending your leg. When your hamstrings contract, they pull the tibia (shinbone) backward, directly counteracting the forward shear forces that strain the ACL. In biomechanical studies, hamstring force significantly reduced both anterior translation and internal rotation of the tibia, while reversing the direction of the shear force acting on the shinbone during weight-bearing knee flexion.1PubMed. Hamstrings cocontraction reduces internal rotation, anterior translation, and anterior cruciate ligament load in weight-bearing flexion In simpler terms, strong hamstrings act like a seatbelt for your knee, pulling the joint into a safer position every time you land from a jump, decelerate during a sprint, or change direction.
This protective effect has been confirmed in cadaver studies as well. Research applying simulated hamstring tension during impact loading found that the hamstring muscles are well positioned to limit both anterior tibial translation and ACL strain during the knee flexion phase of a jump landing.2PubMed Central. Effect of varying hamstring tension on anterior cruciate ligament strain during in vitro impulsive knee flexion and compression loading Beyond raw strength, hamstring stiffness itself correlates with less forward sliding of the tibia, even in people with intact ACLs.3The Knee. Influence of hamstring stiffness on anterior tibial translation after anterior cruciate ligament rupture So the question isn’t really whether you should train your hamstrings for knee health. The question is whether the leg curl machine is a safe way to do it.
Why Leg Curls Get Blamed
The source of the “leg curls are bad for your knees” myth usually traces to a broader suspicion of open kinetic chain exercises. An open-chain exercise is one where your foot moves freely through space, as in a leg curl or leg extension. A closed-chain exercise is one where your foot stays planted, as in a squat or lunge. In certain rehabilitation circles, especially for patellofemoral pain (pain around the kneecap), open-chain quadriceps exercises like the leg extension earned a reputation for increasing stress behind the patella. That caution then got extended, somewhat unfairly, to the leg curl.
The distinction matters, though. Research on patellofemoral pain syndrome suggests patients may tolerate closed-chain exercises better than open-chain ones for quadriceps strengthening specifically.4Europe PMC / Springer. How can we strengthen the quadriceps femoris in patients with patellofemoral pain syndrome? That finding applies to the leg extension, where the quadriceps pull directly on the patellar tendon, dragging the kneecap against its groove. During a leg curl, the hamstrings are doing the work, and the forces on the patellofemoral joint are quite different. The kneecap isn’t being loaded in the same way. So the advice to be cautious with open-chain exercises near the knee was never really about leg curls at all. It was about leg extensions, and it got generalized.
If you have an existing knee condition like osteoarthritis, patellar tendinopathy, or a recent ACL repair, the exercise itself isn’t the enemy. The load, range of motion, and timing within a rehab program matter far more than whether your foot is planted or free.
Leg Curls After ACL Surgery
One of the clearest tests of whether leg curls are safe for knees is how they perform in ACL rehabilitation, where the knee is at its most vulnerable. Traditionally, surgeons and physical therapists delayed open-chain hamstring exercises for weeks or months after ACL reconstruction, out of caution. More recent evidence is pushing that timeline earlier.
A retrospective study looking at early open-chain hamstring exercises after ACL reconstruction found that patients who started leg curls early showed significant improvements in isometric hamstring strength by 12 weeks, with large effect sizes at multiple testing angles.5PubMed Central. Early Open Kinetic Chain Hamstring Exercise After ACL Reconstruction: A Retrospective Safety and Efficacy Study The study reported no safety concerns, suggesting that the knee tolerates leg curls well even during the healing window after major ligament surgery. That is about as strong an endorsement as you can get: if a freshly reconstructed knee can handle them, a healthy knee almost certainly can.
Interestingly, standard ACL rehab programs often underload the hamstrings compared to the quadriceps. When researchers evaluated electromyographic activity during bodyweight exercises used in a validated ACL rehab protocol, hamstring muscle activity stayed relatively low across rehabilitation phases, while quadriceps activity increased steadily. Only one exercise, the Cook hip lift, pushed hamstring activity above 60% of maximum.6PubMed Central. Electromyography Evaluation of Bodyweight Exercise Progression in a Validated Anterior Cruciate Ligament Injury Rehabilitation Program: A Cross-Sectional Study That makes a case for adding dedicated hamstring work like leg curls to rehab programs rather than avoiding it, because the standard exercises aren’t doing enough to rebuild those muscles.
Seated Versus Prone Leg Curls
Most gyms offer two flavors of leg curl machine: seated (where you sit upright and curl the pad down toward the floor) and prone (where you lie face-down and curl the pad up toward your glutes). The differences between these two aren’t just about comfort. They change what the hamstrings are doing at a muscular level, and that can matter for people with knee sensitivity.
When you sit upright, your hip is flexed, which stretches the hamstrings across both the hip and knee joints simultaneously. That longer muscle length tends to recruit more of the biarticular hamstring muscles, the ones that cross both joints. A study comparing the two found that seated leg curls produced greater overall hamstring growth: about 14% increase in whole hamstring volume compared to about 9% for prone.7PubMed Central. Greater Hamstrings Muscle Hypertrophy but Similar Damage Protection after Training at Long versus Short Muscle Lengths The difference was driven by the two-joint muscles; growth in the single-joint muscles was similar between variations.
For knee-specific considerations, the hip position also affects torque production. Research examining concurrent ankle and hip positions found that a flexed hip paired with a dorsiflexed ankle produced the greatest hamstring torque, while an extended hip with a plantarflexed ankle decreased torque output.8PubMed Central. The Effect of Concurrent Ankle and Hip Positions on Hamstring Function in Athletes This means the seated leg curl, with its flexed hip, puts the hamstrings in a mechanically stronger position. For people with knee issues who need to build hamstring strength at lower absolute loads, this can be useful: you can challenge the muscle more with less weight on the stack, which means less compressive force through the joint.
On the other hand, if you find the deep knee flexion at the bottom of a seated leg curl uncomfortable, prone curls keep the hamstrings shorter and may feel less aggressive around the knee. Neither version is objectively “safer” than the other for a healthy knee. The choice depends on comfort and goals.
Does Ankle Position Matter?
You’ll hear trainers say to point your toes (plantarflex) or pull your toes toward your shin (dorsiflex) during leg curls to shift the muscle emphasis. There’s a kernel of biomechanical truth here, but the practical implications are smaller than people think.
From a pure physics standpoint, dorsiflexing the ankle stretches the gastrocnemius, the calf muscle that also crosses the back of the knee. When stretched, the gastrocnemius sits on a more favorable part of its force-length curve, meaning it can contribute more force to knee flexion. This was demonstrated in both experiments and computer simulations: the moment produced at the knee with the ankle dorsiflexed was significantly higher than with the ankle plantarflexed at all tested speeds.9Journal of Applied Biomechanics. Examining the Role of the Gastrocnemius during the Leg Curl Exercise Another study confirmed that gastrocnemius activation was greatest when the knee was at 90 degrees of flexion and the ankle was maximally dorsiflexed.10The Journal of Strength & Conditioning Research. Different Knee and Ankle Positions Affect Force and Muscle Activation During Prone Leg Curl in Trained Subjects
But here’s where it gets less clear-cut. A randomized trial that actually tracked hamstring outcomes over 10 weeks of prone leg curl training found no significant differences between ankle positions for hamstring EMG activity, muscle thickness gains, or strength improvements.11PubMed. Myoelectric activity and improvements in strength and hypertrophy are unaffected by the ankle position during prone leg curl exercise The takeaway: while ankle position changes how much the calf contributes to the movement in a single rep, it doesn’t meaningfully change what the hamstrings get out of the exercise over weeks and months of training. For people with knee concerns, the practical advice is simple. Use whatever ankle position feels natural and comfortable. If you get cramping in your calves, try relaxing the foot. Don’t overthink it.
The Hamstring-to-Quadriceps Ratio
A common argument in favor of leg curls is that you need to keep your hamstring-to-quadriceps strength ratio in balance to protect the knee. The logic sounds intuitive: if your quads are much stronger than your hamstrings, the forward pull on the tibia dominates, and your ACL picks up the slack. This idea has driven training recommendations for decades, and leg curls are the most obvious way to boost the hamstring side of the equation.
The evidence, though, is messier than the clean narrative suggests. A systematic review examining whether the hamstring-to-quadriceps ratio is useful for predicting ACL and hamstring injuries concluded there is very limited evidence that this ratio works as an independent risk factor, regardless of how the ratio is calculated.12PubMed Central. Is hamstrings-to-quadriceps torque ratio useful for predicting anterior cruciate ligament and hamstring injuries? A systematic and critical review Plenty of athletes with “good” ratios get hurt, and plenty with “bad” ratios never do.
That said, the picture isn’t entirely blank. A prospective study of male American football players found that those with a lower hamstring-to-quadriceps ratio were more likely to sustain noncontact ACL injuries.13Asia-Pacific Journal of Sports Medicine, Arthroscopy, Rehabilitation and Technology. Lower hamstring to quadriceps muscle strength ratio and lower body weight as factors associated with noncontact anterior cruciate ligament injury in male American football players Meanwhile, a study of 574 patients after ACL reconstruction initially found that higher ratios were linked to fewer second ACL injuries, but the association disappeared once other factors like age, sex, graft type, and sport level were accounted for.14PubMed Central. No Association Between Hamstrings-to-Quadriceps Strength Ratio and Second ACL Injuries After Accounting for Prognostic Factors
What does this mean for you? The ratio by itself is probably a poor screening tool. But that doesn’t mean hamstring strength is unimportant for knee health. The direct biomechanical evidence for hamstrings protecting the ACL is strong. The weakness is in trying to reduce that protection to a single number. Training your hamstrings through leg curls is still a sound strategy for knee health; just don’t obsess over hitting a magic ratio on an isokinetic machine.
When Leg Curls Might Genuinely Bother Your Knees
While the general answer is reassuring, there are a few situations where leg curls can cause knee discomfort, and knowing these helps you adjust instead of abandoning the exercise entirely.
- Popliteal irritation: The pad on some machines sits right behind the ankle, and as you curl deep into flexion, the structures at the back of the knee get compressed. If you feel pinching or pressure behind the knee at full flexion, limiting the range of motion by a few degrees often solves it.
- Baker’s cyst or effusion: If your knee is already swollen, the deep flexion at the end range of a leg curl can mechanically compress fluid in the joint capsule, causing pain. This is a timing issue, not a reason to swear off leg curls forever. Reduce the range and load until the swelling subsides.
- Machine pad placement: Some older or poorly designed machines place the resistance pad too high up on the calf, close to the knee. This creates a shearing lever arm at the joint rather than a clean rotational force. Adjusting the pad so it sits just above the ankle bones fixes this.
- Too much weight, too soon: This applies to every exercise, but people tend to ego-load the leg curl because the movement feels simple. Jerky, momentum-driven reps increase the forces transmitted through the knee during the direction change at the bottom and top of the movement.
None of these situations require you to stop doing leg curls. They require you to modify the setup, load, or range of motion.
Alternatives That Train the Hamstrings Differently
If you can’t access a leg curl machine, don’t like it, or want to round out your hamstring training beyond knee flexion, several alternatives exist that challenge the hamstrings in their hip-extension role as well.
Romanian deadlifts and stiff-leg deadlifts load the hamstrings in a lengthened position under hip extension, training the muscle in the pattern where strains most commonly occur. Nordic hamstring curls, where you kneel on a pad and slowly lower your body forward while a partner or anchor holds your ankles, are considered one of the most effective exercises for building eccentric hamstring strength at long muscle lengths. However, Nordics require substantial baseline strength and can be intimidating for beginners.
Sliding leg curls offer a middle ground. By lying on your back with your heels on sliders or a towel on a smooth floor, you bridge your hips up and curl your feet toward your glutes. This exercise works both hip extension and knee flexion simultaneously. The sliding leg curl provides an alternative conditioning exercise that can develop strength at muscle lengths where hamstring injuries frequently occur, while its single-leg variations can address strength imbalances between legs.15Strength & Conditioning Journal. The Sliding Leg Curl For people with knee sensitivity, the sliding leg curl has the added benefit of keeping the foot in contact with the ground, which some find more comfortable than the free-hanging lower leg of a machine curl.
A well-rounded program for knee health would ideally include both a hip-dominant hamstring exercise (like a Romanian deadlift) and a knee-dominant one (like a leg curl or sliding curl). Each targets the hamstrings through a different function, and the knee benefits from both: the hip-extension pattern builds the muscle’s capacity to absorb force during deceleration, while the knee-flexion pattern directly trains the protective pulling action that limits anterior tibial translation.
Hamstring Training and Knee Osteoarthritis
People with knee osteoarthritis often avoid resistance exercises out of fear that loading the joint will accelerate cartilage wear. While this concern is understandable, the evidence runs in the opposite direction. Strengthening the muscles around the knee, including the hamstrings, is one of the most consistently recommended interventions for managing osteoarthritis symptoms. Stronger muscles absorb more force before it reaches the joint surfaces, effectively acting as shock absorbers.
For someone with osteoarthritis, the seated leg curl may be particularly practical because it allows you to work the hamstrings through a controlled range of motion without having to bear your full body weight. The machine constrains the movement so there’s no balance demand, no impact, and no requirement to load the hip and ankle at the same time. If deep knee flexion aggravates symptoms, you can simply limit how far the pad travels. This kind of fine-tuned control over range and resistance is harder to achieve with free-weight or bodyweight hamstring exercises.
The caveat is that an acutely inflamed knee with visible swelling needs to calm down before you add resistance. But for the typical chronic, low-grade osteoarthritis pain that millions of people live with, regular hamstring strengthening through leg curls is more likely to reduce symptoms than worsen them. The key is starting light, progressing gradually, and paying attention to how the joint responds in the 24 hours after training rather than expecting zero sensation during the exercise itself.