So the heavy duty hip flexor is the iliopsoas muscle group, half of which I included as part of the core series because the psoas portion has its origin point on the lumbar spine and because the iliopsoas can have an impact on pelvic tilt and therefore a number of other core muscles.
To recap quickly, the psoas starts on the front of the lumbar spine and crosses over the front of the pelvis, sort of like this:
As it passes over the ilium (the wide, gently curved bones that make up either side of the pelvis), it meets up with the iliacus, and together they continue until they both insert onto the front of the femur:
Because a lot of common activities — walking, running, bicycling, sitting — involve hip flexion (and therefore the iliopsoas being shortened/contracted), it’s generally more common for people to have too-tight hip flexors instead of hip flexors that lack that sort of contracting strength. And because my hips are definitely in this group — I can feel them tighten as I sit and type — I will be devoting more time to stretches rather than strengthening exercises for the upcoming section of the series.