I have a pair of the regular Herm Sprenger flexible stirrups... In my trunk. My trainer made me get regular fillis irons and put rubber on them. Something about the flexibility and the thin foot surface was NOT working well for me. I kept trying to grip the irons with my toes and bringing my heels up.
If you have issues with ankle flexibility, they may not work well. Maybe try to borrow some to try first?
If your ankles/heel position are already good and you're looking for something to help your knees, then maybe these would help.
But if you need help keeping your heels down, I can't see these helping you bring your toes up (in order to help your ankles).
I have the regular Sprenger flexible irons, and I switched from them to regular Fillis irons. I did like the extra comfort to my knees and ankles when I first started riding in the Sprenger irons, but I get much more stability from the Fillis irons. My first ride in the Fillis irons after several years in the Sprengers, and I had terrible achy ankles, but since then no problems at all. And the Fillis are just so much more attractive too.
I stick with the regular fillis irons. I have chronic tendonitis in one knee and I found the Sprenger 4-ways to be a bit unstable and made my knee ache more. If you can borrow a pair for a ride or two, that would be safest.
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I have them and quite like them a lot. I don't always use them, I started using them when a knee injury made riding incredibly uncomfortable and then when I got my new horse I found the bow balance irons to allow me to jam my heels down and it made my ankles really sore so I went back to fillis irons.
When I sold my new horse and was just riding my original guy, I went back to the bow balance irons after a year because my knees and back were acting up quite a lot and they've seemed to help again.
I've owned regular Sprengers for a long time and have really liked them, but I bought a pair of Bow Balances to see whether I could benefit from them. Big mistake -- my ankles and outer lower legs were killing me, and so I ended up giving them to a friend. I've been thinking of trying fillis stirrups again, but I just rode in a friend's saddle equipped with $25 composite stirrups from Dover and loved them.
For comfort and/or fatigue purposes, the original HS flex stirrups, hands down. For a truly rock-solid lower leg/prettier picture, old school plain fillis. That said, I get sore so quickly in plain irons that the trade off is not worth it for me. I do find I need to ride at least hole shorter on the flex stirrups. I also find the weight of the HS makes them easy to pick up if you drop them.