Back Squat vs Leg Extension: Which Exercise Builds Better Legs?
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When it comes to leg training, few debates are as common as the one between the back squat and the leg extension. Both are popular resistance exercises, but they target the muscles in different ways. A new study published in The Journal of Strength and Conditioning Research (2025) compared these two exercises to find out which one leads to greater muscle growth and strength gains. The results give us useful insights into how to choose the right exercise for your goals.
What the Study Looked At
Researchers from Londrina State University in Brazil studied 63 young untrained women. Half of them trained the back squat using a Smith machine, and the other half trained the leg extension. Both groups worked out twice a week for eight weeks, doing three sets of 8–12 repetitions until muscle failure.
Before and after the training period, the researchers measured:
- Muscle thickness of the quadriceps (rectus femoris and vastus lateralis) using ultrasound.
- Strength, measured through 3-repetition maximum (3RM) tests for both the squat and the leg extension.
What They Found
Both exercises improved muscle size and strength, but the details were interesting:
- Rectus femoris (front thigh muscle): The leg extension led to much greater growth. The increase was around 11–17%, compared to just 2–8% with the squat.
- Vastus lateralis (outer thigh muscle): The squat caused slightly more growth at the lower (distal) part of the muscle, while both exercises produced similar gains in the upper parts.
- Strength: The back squat group improved their squat strength by about 47%, far more than the leg extension group. However, both groups showed similar improvement in leg extension strength (around 20–23%).
What This Means for You
Both exercises are effective, but they do different things well.
- If your goal is to build the front part of your thigh (rectus femoris), the leg extension is the better choice. It isolates the quadriceps and allows that specific muscle to work hard without help from others.
- If your goal is to build the outer thigh (vastus lateralis) and improve overall lower-body strength, the back squat is superior. It works several muscles at once—quadriceps, glutes, and hamstrings—and improves coordination and balance.
Why These Differences Happen
The key lies in how each exercise moves your joints:
- The back squat is a multi-joint exercise involving both the hip and the knee. Because the rectus femoris crosses both joints, it doesn’t fully contract during squats. That’s why it doesn’t grow as much.
- The leg extension is a single-joint exercise focusing only on the knee. This allows the rectus femoris to fully engage, leading to greater growth.
Also, squats create more overall muscle tension when the leg is bent deeply, which may explain why they boost growth in the lower part of the vastus lateralis.
Practical Advice for Your Workouts
The best choice depends on your goal:
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For overall leg strength and athletic performance:
Prioritize the back squat. It trains multiple muscles, improves coordination, and boosts your ability to lift heavier weights.
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For muscle isolation or shape improvement:
Include the leg extension to specifically target the quadriceps, especially if you want more definition in the front of your thighs.
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For maximum muscle growth:
Combine both. Using both exercises together ensures that all parts of your quadriceps are trained evenly. For example, do squats first to build strength, then finish with leg extensions to fully fatigue the muscle.
Final Thoughts
The back squat and leg extension are not competitors—they are complements. The squat builds overall lower-body power and strength, while the leg extension fine-tunes and shapes the thighs. For beginners, the leg extension is a safe way to start strengthening the quadriceps. For intermediate and advanced lifters, squats remain a must for building real-world strength.
In short:
- Leg extension = better for front-thigh growth.
- Back squat = better for overall strength and outer-thigh development.
- Both together = best for total leg development.
Whichever you choose, focus on proper form, controlled movement, and progressive overload. That’s the real secret to stronger, bigger, and more balanced legs.
