This is a question that regularly pops up in students that take my classes. Many people experience the vibrating, trembling or even shaking of their muscles while doing a yoga pose that requires effort, or a movement that is experienced as physically intense. The trembling may even happen when you are in a pose for a longer time, like a resting pose. The sensation of muscles trembling, seemingly out of nowhere and completely outside of your own control, may feel very strange and unusual. It’s like your body has a mind of its own!
There is more truth to that than you may have thought…
You can approach your “whole being” as consisting of several (interconnected) parts, two of them being: a conscious mind and an animal body. So for a moment, we will separate into these two parts and focus on the body. Your animal body only has a few “objectives” of which basic survival, eating and sleeping are the most important ones. Impulses that trigger your body’s survival mode, are situations, events and events people that your body perceives as dangerous, even when your mind does not! As your body is an animal body, it will react to whatever it perceives, just like a wild animal. Your body is very well equipped to deal with these situations; your metabolism alters instantly; stress hormones are activated, your muscle energy increases (you basically become stronger and faster) and your state of mind is geared towards survival too. Everyone has these amazing capabilities, they are encoded in your body and nervous system. But…. your body does not want to be in this survival state for very long. It is not very healthy because this state demands a lot of energy.
So, it is vital for your body’s health to go back into a resting state after being activated, simply for the sake of restoring energy levels and countering many negative health effects listed . When your body is in the state of relaxation it has the chance to repair damage done to your muscles and tissues, bring the stress hormone levels back down, so it can go back to producing enzymes and hormones for the digestion of foods (for energy), and get nourishing sleep to restore good health to your body and brain. And in the state of relaxation, your emotional and mental states will return to their normal balance. And, quite logically, this is where your animal body wants to be most of the time (who could argue with that?).
So taking this back to your yoga class; you are standing in Warrior II and your front leg starts to tremble 
like what feels like crazy. What is going on exactly? Whenever your body feels safe, either after but also be during a state of high activation, your animal body will look for opportunities to go back to the “baseline” of relaxation. In your yoga class, when you have the focus on your breath, it signals to your body that the situation you are in, is safe. When your body feels safe while you are activating your muscles, this combination can “trigger” an energy release. Your body will perceive this safe state as a chance in which to let go of either energy that you are generating in that moment or release the energy that was generated in a previous situation that is still stored in your body. Your animal body is doing exactly what it is made for and does so beautifully. If you are trying to control or stop this, you are in essence preventing your body from going back to its naturally preferred state of relaxation. At the same time, you may think this feels funny or weird or abnormal. Know that this is only your mind making a judgment about it. Your animal body doesn’t care; it only wants to go to back to its baseline! So the question is; will you allow your body to do this? In a yoga class with many people around, you may not feel so cool to shake freely. If that is the case, you can try to do this at home, by yourself. There are also many practices that help to encourage the body’s stress release ability, for example psoas work in yoga and the TRE® Method(Tension Release Exercises).
If you are ok with your body doing its beautiful and natural shaking, just find a comfortable spot to liedown and let it happen for a few minutes. Make sure that you lie in Savasana (stretched out legs and arms) for a few minutes too. And really focus on the sensations you experience in your body during Savasana and right afterwards. And be sure to appreciate and thank your body. It knows what is best for you! Happy Shaking!