06/17/2026
The same hip-flexor stretch reaches a different muscle depending on how much the knee is bent.
The re**us femoris is part of the quadriceps. It crosses both the hip and the knee, and it also extends the knee. The iliopsoas - iliacus and psoas major - acts at the hip only.
Because those two reach their length-tension limit at different knee angles, the knee position during a stretch or assessment decides which one is targeted. High knee flexion with hip extension loads the re**us femoris. A straighter leg shifts the emphasis to the iliopsoas.
For a working coach: "tight hip flexors" is two muscles with two stretch positions. If every hip-flexor stretch in the program uses the same knee angle, one of them is being left out.