Doing leg extensions with your hip bent at 40 degrees instead of 90 degrees probably makes the front thigh muscle (rectus femoris) grow more in guys who haven’t lifted weights before.

Scientific Claim

Training the leg extension exercise with 40° hip flexion likely produces greater hypertrophy in the distal and proximal rectus femoris muscle compared to 90° hip flexion in untrained men, based on ultrasound measurements over a 10-week period.

Original Statement

“Results indicated a greater hypertrophic response in the rectus femoris for the 40° condition, with 'extreme' evidence supporting a hypertrophic response favouring the 40° hip angle for the rectus femoris (BF > 100; p(Distal/ATE & Proximal/ATE >0) > 0.999)”

From study:The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise

Evidence Quality Assessment

Claim Status

appropriately stated

Study Design Support

Design supports claim

Appropriate Language Strength

probability

Can suggest probability/likelihood

Assessment Explanation

The abstract explicitly states 'extreme' Bayesian evidence and uses probabilistic language ('likely' implied by BF > 100). The study design (RCT, within-participant) supports causal inference, but small sample size (n=22) and lack of blinding justify probabilistic verb strength.

Evidence from Studies

Supporting (1)

The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise

Randomized Controlled Trial

Human

2025 Jan

This study found that doing leg extensions with your hips bent at 40 degrees made the front thigh muscle (rectus femoris) grow more than doing it with hips bent at 90 degrees, and this was true for both the top and bottom parts of the muscle.

Contradicting (0)

No contradicting evidence found

Source Study

The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise

Score: 54

DOI: 10.1080/02640414.2024.2444713

Similar Assertions

If you're a guy new to lifting and want your front thigh muscle to grow as much as possible, doing leg extensions with your hip more bent (40°) is probably better than keeping it straighter (90°).

88%

For the outer thigh muscle (vastus lateralis), bending your hip to 40 or 90 degrees during leg extensions doesn’t seem to make a difference in how much the muscle grows in guys who haven’t lifted before.

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Doing leg extensions makes the front thigh muscle grow more evenly along its length than doing squats, which don’t stimulate that muscle as much.

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Doing leg extensions (kicking your leg out while sitting) makes the front thigh muscle right above your knee grow more than doing leg presses (pushing a heavy platform with both legs).

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Which leg exercise you choose matters: leg extensions grow the front thigh muscle more, squats grow the lower outer thigh more and make you stronger at squats—but both help you get stronger at leg extensions.

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