Are Leg Extensions Bad For Your Knees?

are leg extensions bad for knees

Leg extension is a form of physical activity designed to strengthen the upper legs (thighs). It is a type of strength training exercise that immensely received negative feedback.

Leg extensions are performed with the help of an extension machine, but it is not the best quad workout, why?

Several fitness professionals quote that leg extension is bad for your knees as your knees become more prone to knee injury that will leave you nonfunctional for months.

In this article, you’ll find out why leg extensions are bad for your knees.

Also, we’ll guide you about the other alternative compound exercises which give better results with fewer chances of injury than leg extensions.

Muscles Involved In Leg Extensions

Leg extensions involve the upper leg targeting quadriceps muscles. The quadriceps, as the name suggests, is a group of 4 muscles located on the front of your thigh consisting of:

  1. Vastus lateralis– it is present laterally on the upper leg. It is considered the most significant quadriceps muscle.
  2. Vastus intermedius– it is placed in between the other two vastus muscles. Rectus femoris covers vastus intermedius.
  3. Vastus medialis– located medially of your thigh. The shape of this muscle resembles a teardrop that is present right above your knee.
  4. Rectus femoris– it is a long muscle originating from the iliac bone, going over the hip joint and extending the quadriceps tendon and the other end inserting the tibia. Rectus femoris interacts with the hip and knee joints as a result involved in both knee extension and hip flexion.

The anatomy of quad muscles will give you a clear idea of the functioning of knee or thigh muscles during the leg extension exercise.

Drawbacks of Leg Extension

are leg extensions bad for your knees

Leg extension has become notorious due to the increased risks of a knee injury. There is so much more than just causing knee injury, which is mentioned below;

One most intriguing drawbacks is that leg extension puts too much stress on a strong connective tissue band called the anterior cruciate ligament (ACL).

This ligament attaches the thigh bone to the shin bone. When some amount of stress is applied to a muscle, resistance is developed. But leg extension doesn’t impact adaption capacity.

It is just causing stress and not strengthening your muscles that, in the long run, can result in tender muscles.

If you keep carrying out leg extensions in such a situation, it will result in ligament tear or damage to a greater degree.

The study claims that different participants experienced different results depending on their collective capacity to withstand stress.

If leg extension is performed according to the range of motion of the knee joint, there are lesser chances of knee pain and injury.

Common Mistakes Were Observed While Performing Leg Extensions

It is not just to consider any activity hazardous; chances are that you are not performing it correctly.

You need to figure out with the help of your fitness trainer that you aren’t making the below mistake while doing leg extensions.

Going Too Fast

Leg extension doesn’t require speed to get the best results. Speed will develop momentum that is not required for this activity.

If you are focusing on attaining higher speed, it will cause the involvement of other muscles to complete the lift.

As a result, quad muscle engagement will be hampered, and no significant results will be seen.

Heavy Lifting

If you aim to lift on a heavier load during leg extension, be aware of the possible risks.

The leg extension is not the proper exercise for low-rep and high-load strength conditioning.

It can cause knee ligament strain or, in knee twists resulting in a severe, knee injury.

Locking The Knees

Putting your knees at full extension may lock your knees, and if you bring your leg back instantly to the starting position, it will cause a jerk.

Knee locking can put you at risk of knee strain. Just to be on the safer side, always keep a slight bend in your knee joint.

How To Perform Leg Extension Exercises

You might have seen an extension machine in your gym, and it is similar to some other workout machines. There’s a padded seat and a movable lever.

Here’s how you can use the machine to perform leg or knee extensions:

  • The first step is to adjust the backrest according to your knee location. The knees should rest against the pad of the seat.
  • Then make sure that the shin pad sits right above your ankles.
  • Hold the sidebars present adjacent padded seat.
  • Straighten your legs at the hip-width distance with your toes pointing upwards.
  • Raise the padded bar with the help of your flexed legs for a few seconds.
  • Repeat in sets.

Alternative To Leg Extension Exercises

Alternative To Leg Extension Exercises

The drawbacks of leg extensions make it an unfavorable exercise. So, it is better to look out for alternatives to get better results without causing knee injury or cramping.

Here is a list of some safe result-oriented alternatives which involve your quadriceps muscles;

Bodyweight Leg Extensions

It is performed at home without any equipment or weight. Sit on a regular chair, straighten your back, leave your feet free on the floor, and then one by one, lift your leg and extend it knee height.

Repeat again and again. Bodyweight leg extension doesn’t cause extra stress on your knees, as no added weight is involved.


A squat is considered a preferable exercise targeting quadriceps muscles.

It involves not only thigh muscles but also the butt, lower legs, and hips too. Proper posture is essential to achieve the perfect squat.

Reverse Lunges

Reverse lunges are similar to leg extensions. It tones and strengthens your upper leg muscles.

Reverse lunges are performed in a standing position, then you lower your knee at the angle of 90 degrees and then return to starting position. Alternatively, perform reverse lunges for both legs.


It may seem quite simple and easy, but when done repeatedly, it improves hip mobility by strengthening the quadriceps, glutes, and hip flexors.

For step-ups, you’ll require a box or bench of knee height.

Bulgarian Split Squats With Dumbbells

Bulgarian split squats target muscles of 4 regions of your lower body that are; hips, glutes, quads, and hamstrings.

It is a perfect alternative for leg extension. The movement and positioning are similar to reverse lunges.

But in Bulgarian split, a bench of knee height or slightly lower is required with some weight. You can perform it with or without the dumbbells.

Why Are Alternative Exercises More Suitable?

By now, you have an idea that leg extensions can cause knee injuries. So, it is good to rely on other practical exercises to tone your thigh muscles that have lesser chances of injury.

Above mentioned alternatives put lesser stress on your knee and hip joints but give the desired results when performed consistently.

It is wise to opt for exercises not involving machines as they help your body attain perfect balance and posture.


The leg extension is not considered the most suitable quad workout for everyone due to the risks of a knee injury.

It is a strength training exercise that involves your 4 upper thigh muscles. This machine-dependent exercise doesn’t give notable results on the target areas but improves your posture.

Similarly, other alternative exercises like reverse lunges, squats, and step-ups proved much more reliable and safe for strengthening and toning of your thigh muscles.

These alternative exercises are relatively safe and, when performed with consistency, give the best results.

No exercise is completely good or bad. Other underlying factors can impact the performance and overall results.

The risk of knee injury is directly proportional to the strength of your knee joint to withhold stress and the range of motion of the knee joint.

With the help of a fitness trainer and precautions, you can perform leg extensions effectively.

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