12-y-o Schnauzer's leg trembles - why?

by Herbert Gene
(Fayetteville NC)

We have a Mini-Schnauzer, female: 12 years old and appears to be in good health except for one issue. Her rear legs tremble a lot.
The only thing that appears abnormal is the muscle (upper part of thigh)inside the right rear leg seems to be larger than normal. It is not sensitive to touch or pressure. The vet has checked for cause but sees nothing to cause the trembling. Sometimes it is worse than other times.
She does not seem to have any serious arthritis/joint issues. She has on occasion pulled tendons chasing creatures out of the yard: currently the vet is treating that with AllereG3 and Flex 2500.
Is this prevalent in the breed?
Gene