Fist, thank you for answering!
Useandpets: "being cold legged" is a french term
But you had understand what I mean!!
In fact, he isn't fine in indoor arena too. When I go in forest, he walk very well, he is very powerfull, but when I want to work in an arena he turn off... I work with him twice in a week with outdoor riding, arena, and work on foot horse. When I use my spur, he walk well just two second, I am required to bump all the time and I don't like that.
Saddlebag: I agree with you but he needs to work on a ring to muscle his back...