All Shook Up
Beginning yogis often shake a lot. As muscles become stronger from regular practice, the fibers learn to trade off between firing and resting with smoother coordination. Eventually, quivering subsides (though there will always be teachers who turn even the strongest students into yoga jelly). To calm the body, 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 that the body is overworked. So tune into the brain, the eyes, the root of the tongue, and, most of all, the breath. If any of these areas feel hard or constricted, take it a little easier.
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