Because I'm always right about these things.
....and I never said squats were just a hip flexor exercise.
You are likely as strong as an average middle age woman. Why on earth would you be doing split squats and lunges?
The average Australian middle age woman is a fair bigger than me, wouldn't surprise me if they were stronger LOL.
You told me that I was stupid for saying that squats were a leg exercise. You also told me that squats were the best hip flexor exercise. Anyway lets put your incredible statements aside for now and discuss your question.
My answer is that ever since I took up skimboarding a bit over a year ago I have been prone to getting sore hip flexors. Skimming alone doesn't seem to be a problem, it appears to be a combination of surfing and swimming done on the same and consecutive days that causes the DOMS. The reason I believe is because I use a lot of leg kick to get into waves - I remember when I was doing some triathlon squad training (a long time ago) I was always rather pathetic at swimming, but I beat everyone in a kickboard only race (which shocked me), so it makes sense for me to catch waves using a fair bit of hip flexor effort. The problem with skimboard appears to be the step on where I run briskly for a short distance and simultaneously push with my left leg and step on the tail with my right. This is a mongo run and drop which would be extremely dodgy on a skateboard, but is considered the norm for skimming. Anyway the left leg is pushing and extended behind me and quickly needs to swing forwards on to the front of the board and this seems to give my hip flexor grief when it has already had some work from surfing.
So I am working on the theory that some hip flexor strength training will help protect me against DOMS in that region and maybe keep overuse injuries away too. Reasonable or not?
Both bulgarian split squats or lunges done with a very light weight (or no weight) seem to work my hip flexors hard. The light weight means I can distribute a fair bit of body weight on to the leg behind me and work that flexor. A heavy weight does not seem to be as effective or at least not give me the same control, because the flexor is quite weak compared to the stronger quads and glutes and a heavy weight forces me to use my front leg more. With light or no weight I can do a controlled slow decline (for eccentric benefit) and also do an isometric pause at the bottom without killing myelf.