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Strong Support

causal

History

In untrained young men, doing leg extensions with the hip bent at 40 degrees causes more growth in the upper and lower parts of the rectus femoris muscle than doing them with the hip bent at 90...

Pro

Against

Mechanism

Synthesis from 1 study

How it works

Bending the hip to 40 degrees during leg extensions stretches the front thigh muscle more, making it work harder and grow bigger at the top and bottom ends. The side thigh muscle doesn't stretch differently, so it grows the same either way.

Most probable mechanism

In Simple Terms

When the hip is bent to 40 degrees during leg extensions, the front thigh muscle that crosses both the hip and knee gets stretched more at the start of the movement. This stretch makes the muscle produce more force during contraction, which stresses the muscle fibers more than when the hip is bent to 90 degrees. The increased stress activates signals inside the muscle cells that build more muscle protein, especially in the parts of the muscle near the hip and knee. The outer thigh muscle does not stretch as much with hip position, so it grows the same no matter the angle.

Causal chain

Hip flexion at 40° elongates the rectus femoris muscle by placing its hip attachment in a more stretched position while the knee is extended, increasing passive tension across the entire muscle-tendon unit

Verified by multiple studies

which leads to

Increased muscle length during contraction enhances motor unit recruitment and mechanical tension, particularly in the proximal and distal regions of the rectus femoris, due to its bi-articular anatomy and length-dependent force production

Verified by multiple studies

which leads to

Elevated mechanical tension activates intracellular mechanotransduction pathways, including mTOR signaling and satellite cell proliferation, specifically within the stretched regions of the rectus femoris

Supported by evidence

which leads to

Increased protein synthesis and myofibrillar accretion occur preferentially in the proximal and distal segments of the rectus femoris, leading to measurable thickening of those regions

Verified by multiple studies

which leads to

The vastus lateralis, being a single-joint muscle unaffected by hip position, experiences identical mechanical loading regardless of hip flexion angle, resulting in equivalent hypertrophy across conditions

Verified by multiple studies

Evidence from Studies

Supporting (1)

Community contributions welcome

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

Randomized Controlled Trial

Human

2025 Jan

Contradicting (0)

Community contributions welcome

No contradicting evidence found

Gold Standard Evidence Needed

According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.

Source Study

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

Score: 60

DOI: 10.1080/02640414.2024.2444713

Science Topic

Do leg extensions with 40° hip flexion cause more quadriceps hypertrophy than 90° hip flexion in untrained young men?

Supported

Leg Extensions & Hypertrophy

We analyzed one assertion on this question and found that in untrained young men, performing leg extensions with the hip bent at 40 degrees led to greater growth in both the upper and lower parts of the rectus femoris muscle compared to 90 degrees of hip flexion over 10 weeks [1]. The vastus lateralis, another quadriceps muscle, showed no meaningful difference in growth between the two positions [1]. This suggests that hip angle may influence how certain parts of the quadriceps respond to the exercise, but not all muscles in the group are affected the same way. What we’ve found so far is limited to one study group and one muscle-specific outcome — we don’t yet know if this pattern holds for trained individuals, women, older adults, or longer training periods. The evidence we’ve reviewed leans toward 40° hip flexion being more effective for targeting the rectus femoris in this specific population, but we can’t say it’s better overall for leg growth or strength. The difference in the vastus lateralis shows that muscle response isn’t uniform, which means where you feel the burn doesn’t always match where growth happens. For someone looking to target the rectus femoris — the muscle that crosses both the hip and knee — adjusting hip angle during leg extensions might offer a subtle advantage. But if your goal is general quad development, changing hip position alone likely won’t make a big difference.

0 items of evidenceView full answer

Similar Assertions

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.

91%

In untrained young men, performing leg extensions with the hip bent to 40 degrees results in greater growth of the rectus femoris muscle than bending the hip to 90 degrees, while 90-degree hip flexion produces similar overall quadriceps growth without favoring any specific muscle.

88%

Changing the hip angle during leg extensions does not change how much the vastus lateralis muscle grows in untrained young men, meaning joint position does not selectively target one part of the quadriceps over others.

86%

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°).

85%

During leg extension exercises, the rectus femoris muscle grows more in response to changes in hip angle than the vastus lateralis, reflecting structural and mechanical differences between these muscles.

84%