My girl Timber has the tremble in her hind legs. Sometimes when she's relaxed or, sometimes when she gets really excited. I asked my Vet and they were not concerned at all because it is only occasionally and some of their large breed dogs do it too.
For a long while I was not satisfied with this explanation and I asked everyone I could: massage therapist, chiro's, acupuncturist, holistic Vets, nutritionalist, my breeder, and some different trainers. Afer many months I am sorry to report no one really knows why. It was put to me that giant breeds may be more prone to having their limbs "fall asleep", a circulation thing, maybe the way they sleep on their leg or arm. I doubt that's it.
I did read that the Revolution heartworm treatment I "had" Timber on may have side effects that include "tremors", so I've taken her off that. It still happens once in a while.
To be honest, I am not worried about it anymore. We've lived with it for so long and anytime she's doing it all I have to do is startle her, call her name, or I rub her legs and it stops immediately. I massage her weekly so I know its not a circulation problem and her muscle tone is super. In 3 years it has not slowed her down at all. All of her wellness testing has come back great.
I will always be interested in any new information anyone may have. Sorry I have nothing better to report for all of my research thus far.
Marina