Have an older horse who I seem to be smacking more than ever. Nothing else works, and even if he gets a few sharp snaps of the lead rope, or smacks on the chest he just keeps pushing... or will move off it for a while, the drag his feet (in the back up for example). Or sometimes he'll stop when I do, and sometimes he wont. For that I'm very consistent and give him a sharp smack on the chest every time he keeps walking after I saw whoa and stop - he doesn't get it all the time???? He's not stupid, and not being crazy, he's just invasive and doesn't have a "get off right away" attitude.
If I wanted him to listen 100% of the time I'd have to beat him up quite a bit in the beginning and not back off when I think I'm getting way to aggressive and he's getting worse.
How firm is too firm? And I mean I have to get pretty tough and he still wont listen, or will slow down.