when my husky was younger I would notice that when I took her for walks and she would stop at a stand still her hind legs would shake. Shortly after she started limping
and under went tplo surgery on her hind knee and than on the other knee. This was the result of a tear in her acl. I was told this is a common problem in dog that are very active.
I recomend that you take your dog to have checked right away. It may be nothing but its
better to be safe than sorry. This happened to my dog at a very young age.
Please have your dogs hips and knees X-Rayed. If your dog has hip dysplasia or a knee problem this would be the optimum time to have surgical correction, if necessary, because be will not have developed degenerative joint disease yet. If he does have muscle atrophy once his hips are corrected he should regain much of his lost musculature.