I can’t find the damn link, but this is my favorite hip flexor stretching exercise/APT combatant.
Get one of those reebok steps and set it up against a wall. Put your front foot on the step and externally rotate your toes about 10 degrees. Step back as far as possible with that back leg, then bring it back towards you about 6 inches, so that you have quite a big stride. Make sure the toes on your back foot are facing forward.
Start with no added weight initally, but I really want you to keep your upper body tall (not hyper extended, as that shortens the hip flexors - imagine you have a string on the top of your head and you are ‘pulling’ yourself upwards.) Now, I want you to really force that top knee forward over the toes of that front foot. Really feel the stretch in that rear hip flexor.
I don’t want you to hold it, just do 12-15 reps on each side. Get right into that end range, as painful as it may be, because that’s the only way to get some benefit.
I also suggest this being your first exercise after your warm up, as stretching a semi-cold muscle as opposed to a warm one will lead to more long term gains.
Now for a stretch, I stole this one from Mike Boyle and use it with a lot of my clients… It’s painful.
This one is better than just the regular half kneel hip flexor stretch as it really gets the psoas involved, because of where that muscle originates, and the placement of that back leg.
Now, if you can’t tell, that bottom leg is situated at a 45 degree angle from the box, while the top leg is swung around to be directly in front. Prepare for some lack of balance, and a really good stretch. Hold this for at least 45 seconds, and do it at least twice a day.
Let me know how things go.