4 yr Old QH legs shake when he lies down

I found out his back is fine and he does have some soreness there - but nothing serious. He hasn't gotten better - as I thought he would with age.

Has anyone had this happen- what do you recommend?

Shakes--how? Do you have a video? Has a vet seen him?

Yes vet saw him and we did exrays no problems. She thinks he's just growing into his legs yet. Someone also mentioned he may need more muscle in his back end.

He rolls the minute I get him into the arena - if the legs shaking bothered him would he do it EVERY time he gets in thhe arena. He walks in about 10 ft and then loves to lay down and roll.