A horse always has stiff hind legs when I pull her out to ride her. What can I do to prevent this and are there any exercises that do not include being mounted?

Turnout as much as possible, if older horse a lot of walking before trotting.
You could mention to your trainer having her see a chiropractor if y’all have on in your area. my pony was just like that and worse for a while, but we saw a chiropractor, and ever since then her tenseness has been slowly decreasing. I do stretches with my mare before every ride and as often as I can, but the chiropractor gave me the stretches to do.
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