When I first started out as a certified trainer, experienced trainers would oftentimes warn me against incorporating leg extensions.
They would say: “Bad for your knees,” or “It puts your knee joint at risk.”
Being inexperienced, I followed their advice and dared not question them. Since then, until now, leg extensions have been public enemy No. 1 for my knees. But you may have heard:
“Don’t do it—they’ll shred your patella.”
“That machine’s dangerous, just squat instead.”
The belief that leg extensions are dangerous has become a stubborn gym myth that won’t go away. For those who believe it means avoiding an effective tool for quad development, strength, and even post-rehab recovery. After Nick Tumminello, a strength coach with 27 years in the trenches, tackled this issue in a recent social media post, here’s the real question that should be asked: Are leg extensions a knee-saver or a knee-destroyer?
Here, we’ll provide the answers to this legday dilemma by breaking down the origins of the myth, biomechanics, and how to use leg extensions to build teardrop quads without issues.
Why Do People Say Leg Extensions Are Bad for Your Knees?
This myth of leg extensions didn’t come from lifters; it came from the rehab world. Cautionary tales of leg training trauma were misinterpreted and passed down like a bad game of telephone date back as far the 1960s, according to Tumminello.
“Clinicians observed that open-chain knee extension could strain a healing ACL graft, particularly early after surgery,” he says.
Over time, that caution escaped the clinic and morphed into the repeated myth that leg extensions are bad for everyone, all the time.
In the 1990s and early 2000s, physical therapists began limiting leg extensions in early-stage rehab for ACL injuries. The science suggested that when the anterior shear force on the tibia increases, a biomechanical force that pushes the shinbone forward relative to the thigh bone can stress healing tissues in a post-surgical knee.
While this recommendation might be valid for some, it became a blanket warning. Trainers were telling anyone within earshot that leg extensions were bad for your knees, and the warning spread fast and straight into my newbie ears. Combine that with gym bros shouting, “Just squat instead!” and people tossed leg extensions aside.
Are Leg Extensions Safe for Healthy Knees?
Machine leg extensions are an open-chain knee extension exercise that isolates the quadriceps while increasing shear force on the anterior knee and the cruciate ligament. These factors, combined with the gym grapevine, are why this exercise has its share of detractors.
Yes, leg extensions do place tensile forces on the ACL, but how much is too much? Research shows that ACL forces during leg extensions range from 158 to 396 newtons, even at near-maximal effort. The estimated failure strength of the ACL is around 2,000 newtons. That means leg extensions expose the ligament to less than one-fifth of its failure strength.
So, don’t worry, your knee isn’t going to blow out.
Another criticism is that they produce a forward pull on the tibia without co-contraction of the hamstrings and calves. But multiple biomechanical analyses have shown that:
- The hamstrings do activate during leg extensions, but at low to moderate levels. That’s what you’d expect during an open-chain knee extension.
- Knee joint reaction forces vary depending on knee angle and exercise parameters. So, the blanket statement that leg extensions are bad for your knees lacks context.
- Daily activities such as walking, stair climbing, or running often produce joint reaction forces that are equal to or greater than those from leg extensions.
- “Squats and leg presses show greater peak patellofemoral stress at deeper knee flexion angles at 60 to 90 degrees,” says Tumminello. “Using the same logic, squats would have to be labeled “bad,” too—which is obviously nonsense.” Although concerns about leg extensions arise in rehab settings, a review found that rehabilitation programs use open-kinetic-chain knee extensions safely to restore quadriceps strength and knee function after injury.
While leg extensions are great for developing knee and quad strength, they’re not suitable for every circumstance.
When Leg Extensions Can Actually Hurt Your Knees
Let’s not pretend leg extensions are bulletproof. Like any exercise, they can cause problems when misapplied, misused, or performed poorly. Here’s when this quad-blasting machine can go sideways:
Pre-Existing Knee Issues
If you’re dealing with patellar tendonitis, chondromalacia, or chronic anterior knee pain, leg extensions may aggravate your symptoms—especially at the top of the movement, where tension is highest. That’s because open-chain knee extension isolates force on the patellar tendon without full support from the other lower-body muscles.
Fix it: Use shorter ranges (e.g., 90 to 45 degrees), lighten the load, and slow the tempo. Or sub in closed-chain options like step-ups or heel-elevated goblet squats.
Post-Surgical or Early-Stage ACL Rehab
In the early phases of ACL recovery, open-chain movements between 40 degrees and full extension place more anterior shear on the tibia, which can stress a healing graft. That’s where this myth originated, and it’s valid in that specific context.
Fix it: For post-ACL clients, consult your PT. Many protocols now include leg extensions after the early stage, just not right away.
Poor Technique
Cranking the weight stack, bouncing the pad, locking out hard at the top, or hyperextending the knees? Yeah, that’s how you tick off your knees. Some pain from performing leg extensions stems from poor reps—not the machine itself.
Fix it: Use a controlled tempo, avoid momentum, and stop short of lockout.
Why Leg Extensions Are One of the Best Quad Builders
They’re not a squat, they’re not a lunge, and they don’t go viral on Instagram unless someone’s using the whole stack while another screams, “It’s all you, bro.” But if you’ve ditched them because you believe this myth, here’s what you’ll miss out on.
Laser-Focused Quad Development
Leg extensions are among the best isolation tools available for your quads. Machine leg extensions train the quads with zero hip involvement, perfect for bringing up lagging leg size—or carving definition in a cutting phase, according to Tumminello. “If you want to maximize quad size, leg extensions deliver a stimulus to the rectus femoris that compound lower-body exercises alone don’t provide,” he says.
Compound lifts share the load across the glutes, hamstrings, adductors, and lower back, while leg extensions do not.
Joint-Friendly Volume
Barbell squats and Bulgarian split squats are great for building quad strength and size—but they put a drain on your nervous system. Leg extensions allow you to add muscle-building volume without spinal compression, balance demands, or high CNS cost.
Strategic Pre-Exhaust or Finisher
Want to feel your quads during squats? Try 2–3 sets of leg extensions before your first working squat set, and it will fix your quad feeling problem. On the flip side, it’s great as a finisher. End your leg day with 3 sets of 20 reps and a 3-second squeeze at the top, then try walking down the gym stairs.
Post-Rehab Superstar
People often give leg extensions a bad rap in rehab, but many return-to-play protocols rely on them once coaches dial in timing, loading, and knee readiness.
“Quad strength loss is a defining issue after knee injury,” explains Tumminello. “ Weak quads don’t just limit performance—they change how you move, mess with your gait, and increase the risk of reinjury, especially after ACL reconstruction. If you want resilient knees, restoring quad strength isn’t optional—it’s mandatory.”
Best Way to Program Leg Extensions for Muscle and Joint Health
Here’s how to make this misunderstood machine your secret leg-day weapon without succumbing to the myth hype.
- Use them after compound lower body lifts when your joints are warm and your quads are juiced.
- Or as a pre-exhaust tool—especially if you struggle to “feel” your quads during squats. Stimulate and not annihilate before heavy barbell work.
- For hypertrophy: 3–4 sets of 10–15 reps, moderate weight, controlled tempo.
- For endurance: 2–3 sets of 15–20 reps with short rest between sets.
- For strength: 3 sets of 8–10 reps, heavy weight, focusing on peak contraction and full ROM.
Are Leg Extensions Bad for Your Knees? Final Verdict
The leg extension machine isn’t ruining your knees; it’s just remains a misunderstood exercise.
Yes, it creates shear forces, isolates the quads, and, if you slam the pad around, can aggravate cranky knees. But if you train smart and follow the advice here, the leg extension is a valuable tool for quad growth and knee rehab.
- Leg extensions are safe for healthy knees when done well.
- They don’t destroy ACLs.
- They’re not a replacement for compound lifts—but they’re a complement.
- The problem isn’t the machine, it’s the misuse.
No, leg extensions aren’t bad for your knees. Bad form, ego lifting, and poor programming are. Give them the respect they deserve—and your quads and knees will thank you later.