Does doing leg extensions make your knees sore? It could be that you are using too much weight. While leg extensions are not inherently “bad,” performing them with heavy weights and low reps significantly increases the risk of knee discomfort or injury due to the high mechanical stress they place on the joint. For isolation exercises like the single-leg extension, high reps with moderate weight are generally considered “better” than low reps with heavy weight for most lifters, primarily due to safety and joint health. It is a great exercise for your quads but being mindful of the weight and rep range is important.

High Reps (12–20+ repetitions):

Joint Safety: Lighter loads are significantly gentler on the knee joint and connective tissues. High-load leg extensions can be “needlessly stressful” to the knees, making higher reps a safer way to reach muscular fatigue.

Metabolic Stress: This range induces greater metabolic stress and “the pump,” which are key drivers for muscle hypertrophy.

Muscular Endurance: It specifically builds local muscular endurance, helping muscles withstand fatigue for longer periods.

Low Reps (1–6 repetitions):

Strength Focus: This range is superior for building maximal strength and improving neural recruitment of muscle fibers.

Mechanical Tension: Heavy weights maximize mechanical tension, but in a single-joint isolation move like the leg extension, high tension can lead to form breakdown or increased injury risk. 

Why High Reps Win for Extensions

Scientific research indicates that as long as you train near failure, both high-rep and low-rep sets build a similar amount of muscle. Because leg extensions are an isolation movement—meaning all the force is concentrated on one joint—using extremely heavy weights can cause joint irritation without providing a unique muscle-building benefit that you couldn’t get with more reps. 

Recommended Rep Ranges

• Hypertrophy (Muscle Growth): 10–15 reps per set.
• Endurance/Safety: 15–25 reps per set.
• Advanced Trainees: 8–12 reps with heavy weight only if form is perfect and joints feel healthy. 

For more specific guidance, check the ACSM resistance training guidelines or use an app like Alpha Progression to track your progressive overload across different rep ranges.