Shaking leg

5 year old intact bullmastiff. A couple of months ago we noticed shaking in his back leg. Usually the right but sometimes the left. Only one at a time, only ever when standing and if he stepped or shifted weight it would stop. Maybe 1-2 times per day for less then a minute. It didn’t seem to correspond to exercise or anything.   Brought him to the vet for an exam. Nothing conclusive but vet thought possibly a partial ccl tear. Put him on a 3 day trial of anti inflammatories to see if it responded. There was no difference.   Journaled for a couple of days with his regular routine and the only notable connection was that it occurred most if he was excited. Exercise didn’t seem to be a factor. When he went to the vet, saw other dogs besides the ones he lives with, or friends came over he would shake more.   Decided to try rest. Spent 2 weeks with no stairs or jumping on/ off anything and only very short leash walks. No change. Worked in longer walks over the following week, no change.   Brought in for x rays. He was sedated and 2 vets tested for drawer sign which was negative. X rays showed no spine or hip issues but moderate inflammation in both knees. X rays were sent to radiologist who did not observe sign of ACL tear.   He seems perfectly happy. No limping, no hesitation in walking. Eating and drinking normal. Just intermittent shaking.   I think the most confusing part is how it is provoked by excitement.   The vet is going to refer him to an orthopaedic specialist for consult. But I thought it was also worth posting here to see if you have any other ideas.

Hello,  I have little to add to this, although I wonder whether the orthopeadic vet also knows about neurology, because for me this may well be a neuromuscular abnormality (connected to the effect of signals from the brain travelling along the nerves to the muscles).  If you can film it when it happens, a specialist will find this very useful to look at.  Best of luck with this interesting case.