Are leg extensions bad for your knees? The Truth About Quad Growth, ACL Stress, and Knee Health – Muscle & Fitness

When I first started as a certified trainer, experienced trainers often warned me about using leg extensions.

They’d say, “Bad for your knees,” or “It puts your knee joint at risk.”

Being inexperienced, I followed their advice and did not dare to question them. Since then, until now, leg extensions have been public enemy No. 1 for my knees. But maybe you’ve heard:

“Don’t do it, they’ll tear your kneecap.”

“That machine is dangerous, crouch down.”

The belief that leg extensions are dangerous has become a persistent gym myth that won’t go away. For those who believe this means avoiding an effective tool for quad development, strength and even post-rehab recovery. After Nick Tumminelloa strength coach with 27 years of experience in the trenches, tackled this issue in a recent social media post, here’s the real question to ask: Are leg extensions a knee saver or a knee destroyer?

Here, we provide the answers to this leg day dilemma by unraveling the origins of the myth, its biomechanics, and how you can use leg extensions to build teardrop quads without any hassle.

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 rehabilitation world. According to Tumminello, cautionary tales about leg training trauma were misinterpreted and passed off as a bad game of telephone dating back to the 1960s.

“Clinicians noted that open-chain knee extension can stress a healing ACL graft, especially early postoperatively,” he says.

Over time, that caution escaped the clinic and turned into the repeated myth that leg extensions are always bad for everyone.

In the 1990s and early 2000s, physical therapists began limiting leg extensions during rehabilitation in the early stages of ACL injuries. Science suggests that as anterior shear force on the tibia increases, a biomechanical force that pushes the tibia forward relative to the femur can stress healing tissues in a postoperative knee.

While this recommendation might be valid for some, it became a general warning. Trainers told everyone within earshot that leg extensions were bad for your knees, and the warning spread quickly and directly to my new ears. Combine that with gym buddies shouting, “Squat!” and people were throwing 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 ACL. These factors, combined with the gym grape, are why this exercise has its detractors.

Yes, leg extensions exert pulling forces on the ACL, but how much is too much? Research shows that ACL forces during leg extension range from 158 to 396 Newtonseven at near maximum effort. The estimated failure strength of the VKB is approximately 2,000 Newtons. That means leg extensions expose the ligament to less than one-fifth of its breaking strength.

So don’t worry, your knee won’t explode.

Another criticism is that they cause a forward pull on the tibia without simultaneous contraction of the hamstrings and calves. But multiple biomechanical analyzes have shown that:

• The hamstrings are activated during leg stretching, but at a low to moderate level. That’s what you would expect during an open chain knee extension.
• The reaction forces of the knee joint vary depending on the knee angle and training parameters. So the blanket claim that leg extensions are bad for your knees lacks context.
• Daily activities such as walking, climbing stairs or running often produce joint reaction forces equal to or greater than those of leg extensions.
• “Squats and leg presses show a greater peak of patellofemoral tension at deeper knee flexion angles of 60 to 90 degrees,” says Tumminello. “By the same logic, squats should also be labeled as ‘bad’ – which is obviously nonsense.” Although concerns about leg extensions arise in rehabilitation settings, a review found that rehabilitation programs safely use open kinetic chain knee extensions to restore quadriceps strength and knee function after injury.

While leg extensions are great for developing knee and quad strength, they are not suitable for every circumstance.

When leg extensions can actually damage your knees

Let’s not pretend that leg extensions are bulletproof. Like any exercise, they can cause problems if misapplied, misused, or performed poorly. Here’s when this quad beam machine can go sideways:

Pre-existing knee problems

If you’re dealing with patellar tendinitis, chondromalacia, or chronic anterior knee pain, leg extensions can worsen your symptoms, especially at the top of the movement where tension is highest. That’s because the open chain knee extension isolates the force on the patellar tendon without full support from the other muscles of the lower body.

Fix it: Use shorter ranges (for example, 90 to 45 degrees), lighten the load and slow the pace. Or opt for closed-chain options like step-ups or high-heel squats.

Postoperative or early phase of ACL rehabilitation

In the early phases of ACL repair, open-chain movements between 40 degrees and full extension cause increased anterior shear on the tibia, which can stress a healing graft. That’s where this myth comes from, and it is valid in that specific context.

Fix it: For post-ACL clients, consult your PT. Many protocols now include leg extensions after early stages, but not right away.

Bad technique

Swing the weight stack, bounce the pad, lock hard at the top or hyperextend the knees? Yes, that’s how you tap your knees. Some pain when performing leg extensions comes from poor reps, not the machine itself.

Fix it: Use a controlled pace, avoid momentum and stop before locking out.

Why leg extensions are one of the best quad builders

They’re not squats, they’re not lunges, and they’re not going viral on Instagram unless someone uses the whole stack while someone else screams, “It’s all you, man.” But if you’ve been foregoing them because you believe in this myth, here’s what you’ll be missing.

Laser-targeted quad development

Leg extensions are among the best isolation tools available for your quads. Machine leg extensions train the quads without hip involvement, perfect for increasing lagging leg size or cutting definition in a cutting phase, says Tumminello. “If you want to maximize quad size, leg extensions provide a boost to the rectus femoris that lower-body compound exercises alone don’t provide,” he says.

Compound lifts distribute the load across the glutes, hamstrings, adductors and lower back, while leg extensions do not.

Collectively friendly volume

Barbell squats and Bulgarian split squats are great for building quad strength and size, but they tax your nervous system. Leg extensions allow you to add muscle-building volume without spinal compression, balance demands, or high CNS costs.

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 solve your quad sensation problem. On the other hand, it’s great as a finisher. Finish your leg day with 3 sets of 20 reps and a 3-second squeeze at the top, then try walking down the stairs at the gym.

Superstar after rehabilitation

People often give leg extensions a bad rap in rehabilitation, but many return-to-play protocols rely on them once coaches dial in timing, loading and knee readiness.

“Quad strength loss is a defining problem after a knee injury,” explains Tumminello. “Weak quads not only limit performance, they change the way you move, disrupt your gait, and increase your risk of re-injury, especially after ACL reconstruction. If you want resilient knees, restoring quad strength is not optional, it’s mandatory.”

The best way to program leg extensions for muscle and joint health

Here’s how to turn this misunderstood machine into 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 squeezed.
• Or as a pre-exhaust tool, especially if you have trouble “feeling” your quads during squats. Energize and don’t destroy before heavy barbell work.
• For hypertrophy: 3-4 sets of 10-15 reps, moderate weight, controlled pace.
• For endurance: 2-3 sets of 15-20 reps with short rest between sets.
• For strength: 3 sets of 8-10 reps, heavy weight, targeting peak contraction and full ROM.

Are leg extensions bad for your knees? Final verdict

The leg extension machine won’t ruin your knees; it simply remains a misunderstood exercise.

Yes, it creates shearing 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 rehabilitation.

• Leg extensions are safe for healthy knees when done properly.
• They do not destroy ACLs.
• They are not a replacement for compound lifts, but are a complement.
• The problem is not the machine, but the misuse.

No, leg extensions are not bad for your knees. Bad form, ego lifting and bad programming are. Give them the respect they deserve – and your quads and knees will thank you later.