When I did a consult with Dr. Gillis, PhD in tendons, she never mentioned supplements. JB is right, you want your nutrition to be top notch and tight and then it’s about your horsekeeping.
Soft tissue is bothered by hills. Etc.
I just attended a presentation by a vet specializing in rehab and we do know now that things like bodywork and laser/red light therapy really are legitimate and do good.
That’s what I’d pursue. I’ll be curious to hear what others say.
Oh, and your hoofcare MUST be impeccable. Your trim. No long toe. Shoot xrays and confirm you have a perfectly balanced foot That was the #1 recommendation by this vet.