Are Leg Extensions Really Bad for Your Knees?

The leg extension is a popular strength exercise found in most gyms. It uses a seated machine that allows you to straighten your knees against resistance. Many people use it to build quadriceps strength, improve muscle tone, or support knee rehabilitation. Because it isolates the front thigh muscles, it remains a common choice for beginners and experienced gym users.

Muscle Targeting in Leg Extensions

Leg extensions work the quadriceps group at the front of the thigh. This machine targets these muscles without much involvement from the hips or surrounding muscles.

Muscles involved:

• Vastus medialis

• Vastus lateralis

• Rectus femoris

• Vastus intermedius

Anatomy of the Knee and Biomechanics During Leg Extensions

The knee is a hinge joint made up of bones, cartilage, ligaments, and joints (including the patellofemoral joint). During a leg extension, force travels through the patella and along the quadriceps tendon. Because the movement is open chain, the load on the knee changes as you move through the range. The greatest force usually occurs near full extension.

Are Leg Extensions Bad for Your Knees? What Does the Evidence Say?

Research shows that leg extensions are not harmful when performed with suitable technique and appropriate weight. Problems usually arise when the exercise is done too quickly, with excessive load, or with poor control. Leg extensions can support strength gains for many people, including those in rehabilitation programs. For some individuals with knee irritation, especially at the front of the knee, the exercise may need modification. A physiotherapist can help adjust the movement, so it suits your needs. Some evidence also suggests that knee extensions should be introduced later in rehab, particularly in those who have had ACL reconstruction to reduce strain on the graft site early on.

Common Risks and Injury Concerns

• Excessive weight that places high compressive force through the patellofemoral joint

• Fast movements that reduce control

• Locking the knees at the end of the movement

• Poor seat positioning on the machine

• Preexisting knee pain that increases during the exercise

• Moving through a painful or uncomfortable range

Safe Techniques to Perform Leg Extensions

• Start with light resistance to learn correct control

• Keep movements slow and steady

• Avoid locking your knees at the top of the lift

• Keep your back supported against the seat

• Adjust the machine so your knees line up with the pivot point

• Work within a comfortable range

• Increase resistance gradually

• Stop if you feel sharp or increasing pain

Alternatives to Leg Extensions for Safe Quadriceps Strengthening

Closed-chain exercises are movements where your feet remain in contact with the ground or a stable surface. These exercises involve multiple joints and muscles, which spreads load more evenly through the leg and often feels safer for the knee. They also mimic everyday activities such as standing, climbing stairs, and squatting.

Examples include:

• Squats

• Leg press

• Lunges

• Step ups

• Sit to stand practice

• Wall sits

Pros: improve joint stability, use several muscle groups, transfer well to daily life.

Cons: may be difficult during early stages of knee pain, some movements require balance and hip strength.

A physiotherapist from Body Fit Physiotherapy can guide you on which closed-chain exercises suit your knee health and goals.

Role of Physiotherapy in Knee Health and Exercise Rehabilitation

Physiotherapy supports safe strengthening and can help you understand the right exercises for your goals. Treatment may include movement assessment, load management advice, manual therapy, and an individualised exercise plan. At Body Fit Physiotherapy, we help you build confidence in your knee, reduce pain, and return to the activities you enjoy.