Leg extensions have long been a controversial exercise, often feared and misunderstood. You’ve probably heard that they’re dangerous for your knees, especially if you have knee issues. Some even say they should be avoided, particularly after knee surgery or during rehabilitation.

But should you really be avoiding leg extensions altogether? Let’s take a closer look at why this exercise has gotten a bad reputation, and whether it deserves the hype.

Why Do Leg Extensions Have a Bad Reputation?

It all started with a few key studies. In one study, researchers concluded that leg extensions put more force through the kneecap joint (patellofemoral joint or PFJ) than exercises like squats or the leg press. Another study suggested that leg extensions place more stress on the anterior cruciate ligament (ACL) than squats and should be avoided, especially during rehab after ACL reconstruction. Additionally, leg extensions have been criticized as a "non-functional" exercise — mainly because you’re seated with your foot off the ground, which doesn’t seem to mimic real-life movements.

With all this evidence, it’s easy to see why leg extensions have earned their bad rep. But, here’s the twist — more research has been done, and it's painting a very different picture. So, should we be scared of leg extensions? Not necessarily. In fact, they might be one of the best exercises for strengthening the quadriceps and improving knee function.

Why Is This an Important Topic?

When it comes to knee health, the quadriceps (the large muscles in the front of your thigh) are crucial. Stronger quads mean less knee pain and more protection for the knee joint. Research has shown that leg extensions can be a fantastic tool for strengthening these muscles, especially if you’re dealing with knee pain or recovering from an injury. 

Debunking the Dangers of Leg Extensions

1. More Force on the Patellofemoral Joint (PFJ)

In all fairness,  leg extensions do put more force on the PFJ than other exercises like squats, but this is only the case from a fully extended leg position to 45 degrees of knee bend. In fact, squats beyond 50 degrees of knee bend actually put more stress on the PFJ than leg extensions ever do (yes, squats can be worse in this regard!).

So, to avoid irritating the PFJ, try performing leg extensions from 45 to 90 degrees of knee bend. You can also consider doing mini-squats. A stronger quadriceps will help reduce stress on the PFJ over time. By building strength in the quads while avoiding excessive stress on the knee joint, you can gradually reduce PFJ pain and build up your quads to the point where you can perform full-leg extensions safely.

2. More Stress on the ACL

There’s a big fear that leg extensions increase the stress on the ACL and stretch the graft after knee surgery. However, research now shows that leg extensions do not significantly increase ACL laxity (the amount of give in the ligament), when added into the program at 4 weeks in a limited range of motion.

The stress on the ACL is quite low when leg extensions are performed between 60-90 degrees of knee bend. The strain on the ACL in this range  is actually lower than what you’d experience when walking. The highest strain occurs in the first 30 degrees of knee extension (a position that’s easy to avoid in your workout).

So, if you’re recovering from ACL surgery, wait at least 12 weeks before incorporating leg extensions into your routine without fear. However, with help from your physio, you may be able to add leg extensions after 4 weeks in a limited range (60-90 degrees). Working in the limited range helps you regain quadriceps strength more quickly. Quadriceps strengthening is essential for regaining normal function and returning to activities like walking, running, and sports.

3. Not a “Functional” Exercise

Another argument against leg extensions is that they are “non-functional” because you're seated with your foot off the ground, which doesn’t replicate typical real-world movements. But here’s the thing: strengthening the quadriceps is functional for a wide variety of everyday activities. If you’ve ever torn your quad, you’ll know how difficult it is to straighten your leg and walk properly. This highlights just how important it is to maintain healthy, strong quads.

Leg extensions isolate the quadriceps more effectively than squats or lunges, helping you build strength in that muscle group. They’re a great way to target the quads directly without compensating with other muscles — especially when recovering from knee injury or surgery. So while leg extensions might not replicate real-life movements perfectly, they’re necessary to rebuild strength and function in the knee.

Should We Be Scared of Leg Extensions?

In short: No! Leg extensions are not inherently dangerous. In fact, they can be a crucial part of your strengthening routine, especially if your goal is to strengthen the quadriceps.

For optimal quad growth, a combination of squats and leg extensions is ideal. But there are times when leg extensions are the best choice, especially when you need to isolate the quads or avoid overloading other areas of the knee.

If you’re still unsure whether to incorporate leg extensions into your workout routine, start slow. Try isometric leg extensions (holding the position without moving) in a limited range of motion (60-90 degrees). This will help build strength without putting too much stress on the knee.

Try These Leg Extension Exercises

Here are a couple of leg extension exercises to get you started:

• Isometric Leg Extension Using a Ball

  • 2 sets x 4 reps – hold each for 20 seconds

References:

1. Biomechanical considerations in patellofemoral joint rehabilitation. Am J Sports Med. 1993 May-Jun;21(3):438-44

2. Comparison of closed and open kinetic chain exercise in the anterior cruciate ligament-deficient knee. Am J Sports Med 1993 Jan-Feb;21(1):49-54

3. The relationship between quadriceps muscle weakness and worsening of knee pain in the MOST cohort: a 5-year longitudinal study. Osteoarthritis Cartilage. 2013 Sep; 21(9): 1154–1159

4. Quadriceps strength is negatively associated with knee joint structural abnormalities—data from osteoarthritis initiative. BMC Musculoskelet Disord. 2022; 23: 784

5. The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial. J Sports Sci. 2021 Oct;39(20):2298-2304

6. Patellofemoral joint stress during weight-bearing and non—weight-bearing quadriceps exercises. J Orthop Sports Phys Ther. 2014 May;44(5):320-7

7. The Effect of Open- Versus Closed-Kinetic-Chain Exercises on Anterior Tibial Laxity, Strength, and Function Following Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis. Orthop Sports Phys Ther. 2018 Jul;48(7):552-566