We have two mini schnauzers and our 2 year old does the same thing. Every so often his front leg starts shaking for no apparent reason. He doesn't seem to have any pain, doesn't bother him, and I don't even think he knows it's shaking. Lasts for just a little bit and then stops. This is really interesting and also good to know that other dogs do this too. But we always wonder what it is. Thanks for sharing, everyone!