If you stretch your hips back while doing leg extensions, your quad muscle gets longer and grows more—especially in the middle and upper parts—than if you do the exercise with your hips bent.
Evidence from Studies
Supporting (1)
Community contributions welcome
The study shows that when the hip is extended during leg exercises, the upper part of the rectus femoris muscle becomes stiffer, which means it’s working harder. This supports the idea that doing leg extensions with hips extended builds more muscle in the upper thigh.
Contradicting (2)
Community contributions welcome
The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise
The study found that doing leg extensions with hips more extended (less bent) led to more rectus femoris growth, which matches the idea that stretching the muscle during exercise helps it grow more.
Hip flexion angle affects longitudinal muscle activity of the rectus femoris in leg extension exercise
The study looked at how different hip positions affect muscle activity during leg extensions, but it didn’t measure actual muscle growth, so we can’t say for sure if one position builds more muscle over time.
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.