I have an ankle holster for my Ruger LCP .380, that I occasionally carry as a backup. I have two different holsters, because the first one doesn't hold it as close to my leg as I'd like; I want it secure, well hidden, and staying in the same place as I walk, run, or whatever. More commonly though I carry two extra .40 Glock magazines for my main gun in an ankle carrier. Of course I do have two ankles, and the two together make me less off balance. Years ago I used to wear ankle weights when I walked and ran for exercise, and this is a more practical version of that.
For any holster I wear I do some fast running, falling, and intense martial art workouts wearing it (with an empty gun), because some gunfights are preceded by hand to hand combat, and the last thing I want is to find out that all this time I've been carrying a gun I was carrying it for the bad guy to use against me, which would mean that not only would I die, but I would die as a fool.
Years ago I was working at a nuclear power plant on a renovation job. They had security guards all over, and occasionally one would get a call on his radio and have to take off running to check something out. One guard (who looked like he was about 12 years old) took off running one time and his revolver fell out of the holster and crashed into the pavement. It happened in front of a bunch of construction hands, who all got a good laugh out of it. I thought it was a cheap lesson for him. And I don't want to learn a lesson that might not be so cheap.