The Great Debate: Squats vs. Leg Extensions
For decades, lifters have argued over the "best" leg exercise. While the back squat is often hailed as the king of movements, the leg extension is frequently dismissed as a mere isolation accessory. A new study published in the Journal of Strength and Conditioning Research (2026) provides a nuanced look at how these two movements influence muscle growth and strength differently.
The Study Design
Researchers (Kassiano et al., 2026) recruited 63 untrained young women and split them into two groups: one performing Smith machine back squats (SQ) and the other performing leg extensions (LE). Both groups trained 2 days per week for 8 weeks, performing 3 sets of 8-12 repetitions to failure.
It is important to note that this article is based on the study abstract, which provides the primary findings regarding muscle thickness and strength gains.
Key Findings: Regional Hypertrophy
The most fascinating aspect of this study is the regional specificity of muscle growth. The quadriceps are not a monolithic block; they respond differently based on the mechanical demands of the exercise.
- Rectus Femoris (RF): The leg extension group saw significantly greater growth across all measured sites (proximal, middle, and distal) compared to the squat group.
- Vastus Lateralis (VL): The back squat group showed superior growth at the distal site compared to the leg extension group.
Strength Adaptations
When it came to strength, the specificity principle held true. The back squat group saw a massive 46.7% increase in their 3RM squat strength, significantly outperforming the leg extension group. However, when testing 3RM leg extension strength, both groups improved similarly, suggesting that the squat is highly effective for building strength in the extension movement, even if it doesn't build the rectus femoris as effectively as the machine.
Practical Takeaways for Your Training
- Don't skip the isolation: If your goal is maximum rectus femoris development, the leg extension is a superior tool.
- Squats for the sweep: The back squat remains a powerhouse for overall leg development, particularly for the vastus lateralis.
- Specificity matters: If you want to get better at squatting, you must squat. If you want balanced quad development, a combination of compound movements and isolation exercises is likely the most evidence-based approach.
For more details, you can view the full study here: PubMed ID 41379528.