Richard Rosen’s reply:
Your muscles are made up of many fibers. When you use a muscle, not all the fibers contract at the same time. Some rest while the others work, and then they trade places. When you really challenge your muscles, the changeovers can get a little ragged.
You could think of it as one runner passing the baton to another in a relay race. When you quiver, someone fumbled the baton.
Beginning yogis often shake quite a lot. As your muscles get stronger from regular practice, the fibers learn to trade off between firing and resting with smoother coordination. Your quivering will probably subside (though there will always be teachers who turn you into yoga jelly, no matter how strong you get).
For the time being, to calm things down, try to hug the quivering (contracting) muscle against its underlying bone and press the bone into the muscle being stretched.
Quivering is not necessarily bad, but it may be a sign you’re overworking. Several years ago, when slugger Mark McGwire was mired in a terrible slump, a sportscaster asked Mac’s hitting coach what the problem was. The coach opined that McGwire was trying too hard, and needed to “try easier.”
Tune into your brain, your eyes, the root of your tongue, and, most of all, your breath: If any of these areas feel hard or constricted, take the coach’s advice: Try easier.