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Dr. Layne Norton Breaks Down the Science Behind Leg Extensions


Dr. Layne Norton, Ph.D., has previously linked up with fitness influencer Jeff Nippard to examine science-based leg training. Continuing that exploration, Norton further explained the current science regarding quad training.

Norton examined a study conducted in June 2023 that dissected the benefits and modality of leg extensionsOn Aug. 2, 2023, Norton shared his findings in a video published on his YouTube channel. Check it out below:

[Related: Mixed Martial Artist Josh Barnett and Mike O’Hearn’s 4 Training Tips for Young Lifters]

Leg Extension Biomechanics

Leg extensions are a common machine exercise that targets the quadriceps. However, Norton isn’t a big fan of most of them for two reasons:

Limited range of motion: Layne Norton prefers a machine that allows the legs to come beneath the seat so the quadriceps can be trained in the fully-lengthened position, as doing so is key for growth.
An upright seat is awkward for hip angle: When a person is bent forward, achieving a good stretch in the quadriceps is challenging. A straight-backed leg extension seat doesn’t allow its user to recline for more quad activation.

Norton would prefer a leg extension machine that lays the upper body flat on a bench so the quads can achieve their range of motion.

What the Science Says

The 2023 study in the European Journal of Applied Physiology that Dr. Norton reviewed tested the leg extension machine at three different hip angles: upright (zero degrees reclined), slightly reclined (40 degrees), and fully reclined (80 degrees). Researchers measured quadriceps activation levels at each angle. They asked subjects to rate how they felt their activity was and compared it to the data. (1) 

To Dr. Norton’s surprise, researchers found that a slight or full incline led to significantly better activation of the rectus femoris — the deepest muscle in the quadriceps. Subjects also felt more activation in these reclined positions. Norton suggests additional padding between the shins and the pad when using a gym’s leg extension machine. This will allow the legs to move farther underneath the seat, allowing a deeper stretch.

Ideally, most gyms have leg extension machines that recline, but that isn’t always the case. This study implores people to maximize hip angle and leg movement for the best leg extension training results.


Mitsuya, H., Nakazato, K., Hakkaku, T., & Okada, T. (2023). Hip flexion angle affects longitudinal muscle activity of the rectus femoris in leg extension exercise. European journal of applied physiology123(6), 1299–1309.

Featured image: @biolayne on Instagram

The post Dr. Layne Norton Breaks Down the Science Behind Leg Extensions appeared first on BarBend.

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