response in generalInvoluntary body shaking in sitting meditation can be perfectly normal. The Buddha talks about it in this sutta SN 54.7:
Shaking and vibration can range from mild, like nose or mouth twitching, to strong and even violent shaking, but with persistent correct meditation done over a long period of time, the energy will smooth out and eventually shaking will disappear. Doing yoga, accupressure, other healing health modalities can accelerate the smoothing of the energy flow.
Naturally, the more violent the shaking, the more scary it looks to outside observers, and the more worried they are. Probably most assistant teachers don't have personal experience with shaking and vibration, and are not taught how to deal with the situation.
But with that said, meditation can trigger serious mental health issues, with or without body shaking, and even for people who've been meditating a long time and getting the expected positive results. So it's reasonable for assistant teachers, and even expert meditation teachers with tons of experience, to take precautions to err on the side of safety. If in any doubt, check with qualified professionals. Safety first.
why i don't feel the strong vibrations anymore?
when i first started meditating i felt strong vibrations in my body but after 20 days later i don't feel the strong vibrations anymore? now it's so week. anyone feels like me?
sorry for my bad English.
The body condition is always changing. If you meditate long enough (years), you'll likely experience all kinds of strange bodily phenomena. It's normal. In general, body shaking and vibrations are going to get smoother and eventually disappear, the body will get softer, more pliable, feeling lighter. The feeling of mass, tightness, and vibrations generally are the subtle energy channels being blocked, and the energy is bouncing back. The more you meditate, the channels are going to open up wider, vibrations will dampen and gradually disappear. Another way vibrations might be decreasing is your overall energy declined, from doing energy draining activities. Sex, undersleeping, partying, gambling, etc. But if someone is celibate, meditates frequently, then the body is going to get softer, lighter, mind gets sharper, clearer, vibrations will smooth out and energy channels widen.