A recent study by Larsen et al assessed how the position of the hip, whether being in a more flexed or extended position, influences hypertrophy. The researchers found “that rectus femoris muscle hypertrophy was greater with 40° [of] hip flexion compared to 90° [of] hip flexion with no differences between legs for vastus lateralis hypertrophy.”
They hypothesized that the greater hypertrophy seen in the rectus femoris at 40 degrees of hip flexion was due to training it in a more lengthened position.