Answer by Suminda Sirinath S. Dharmasena for Shaking REM and movement

In short this can be due to:

• past fabrications surfacing rapidly and creating uncommon experiences
• perhaps less likely but a possibility is Piti which sometimes create vibrating and / or pulsating experiences

Also see: eye movement during meditation, I get rapid eye movement as soon as meditation starts

These experiences are neither good nor bad. Also do not get attached or averse to them. In additions concentrate on the task at hand than giving importance and distracting your self when there experiences do arise in your meditation session.

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