Yes, leg extensions can be bad for your knees, especially if performed incorrectly or with excessive weight. However, they can also be a safe and effective exercise with proper form, and in moderation.
Check how leg extensions can impact your knees and how to minimize risks:
Why Leg Extensions Can Be Bad for Knees
Shear Force on the Knee Joint:
Leg extensions place a significant amount of stress on the anterior cruciate ligament (ACL) and patellofemoral joint (where the kneecap meets the thigh bone).
This shear force is highest at the top of the movement (when the leg is fully extended), which can strain the knee structures.
Isolation of the Quadriceps
Unlike compound movements like squats, leg extensions isolate the quadriceps without engaging the hamstrings or glutes.
This imbalance can lead to patellar tracking issues or knee instability, especially if your hamstrings are weak.
Excessive Weight
Using too much weight can overload the knee joint, stressing the ligaments, tendons, and cartilage.
This increases the risk of knee pain, inflammation, or injury.
Existing Knee Issues
People with knee conditions like patellar tendonitis, arthritis, or ACL injuries may experience more discomfort or worsen their condition with leg extensions.
How to Perform Leg Extensions Safely
Use a Moderate Weight
Start with a light to moderate weight that allows you to perform the exercise with control and proper form.
Tip: Reduce weight if you cannot control the movement or feel pain.
Maintain Proper Form
- Sit upright with your back against the pad.
- Keep your knees in line with the pivot point of the machine.
- Avoid locking out your knees at the top of the movement; stop just short of full extension to reduce shear force.
- Lower the weight slowly to prevent momentum from stressing the knee.
- Adjust the Machine Properly
- Ensure the machine is adjusted to your leg length, with the pad resting just above your ankles.
Focus on Controlled Reps:
Perform slow, controlled repetitions rather than fast, jerky movements. Aim for a 2-3 second lift and a 2-3 second lower.
Limit Range of Motion:
Avoid moving through the full range if it causes discomfort. A partial range of motion may be safer for your knees.
When to Avoid Leg Extensions
- If you experience sharp or persistent knee pain during or after the exercise.
- If you have a history of knee injuries or joint issues.
- If your physical therapist or trainer advises against it based on your knee condition.
Leg extensions are not inherently bad but require careful execution to avoid knee problems. Focus on proper form, moderate weight, and listen to your body. If you’re unsure, consult a fitness professional or physical therapist to ensure you’re protecting your knees
