However complex meditation positions are unnecessary. Your posture should be such that while being comfortable it should also allow you to stay alert. Therefore lying down is generally discouraged as it is very easy to fall asleep. Slouching and otherwise sitting in positions where the back is bent or the feet are not resting firmly on the ground will cause physical discomfort after a while.
If you are suffering from any pain related ailments or are inflexible, do not force yourself into sitting positions that are torturous. Support your lower back or other parts of your back as necessary to ease as much discomfort as possible. If you are comfortable standing up, do so by all means.
Remember that your aim in the beginning is simply to focus on your breath or any object so as to calm your mind.
It is considered good practise to first do stretching exercise such as yoga before sitting down to meditate. This helps to strengthen muscles, releases aches and pains and loosens the body allowing it to naturally relax when sitting down for an extended period. Yoga can also be practised after meditation to remove any stiffness.
In the beginning do not worry too much about posture. Your aim at first is to learn to simply relax. and be aware of your own body, your own breath and even of your thoughts. Do not feel that you need to achieve anything or that there is any time limit.
If you feel you are unable to sit and breathe peacefully get up, take a break. Look out the window, go for a short walk, read a book. When you feel adequately calm and actually feel the desire to meditate then come back to your regular meditating spot and allow yourself to breathe relaxedly, to enjoy breathing and focus on the breath. As the time you spend in meditation increases, you will find your body will get attuned to the posture best for you.
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