It is possible to meditate in a chair, as long as it is a straight-backed chair, and sometimes, when you are traveling for a long period in a train, plane or bus, it is the only option available. However, there are drawbacks to meditating in a chair and at the same time there are many advantages to learning how to meditate sitting on the floor.
One of the drawbacks to meditating in a chair is that your back is touching the chair’s back. This makes it more difficult to withdraw your attachment from the physical world. During the first part of meditation you should imagine that you are alone and that there is nothing around you. If the whole length of your back is touching the chair, then this effort to withdraw your attention away from the world takes a longer time. On the other hand, if you are sitting on the floor the surface area of your body touching the earth is much less and withdrawal is much easier.
The whole issue of withdrawal from the physical surroundings is also the reason why the classic yoga postures for meditation all involve sitting on the floor and not in a chair. When the legs are folded and crossed, as they are in the lotus posture, half lotus or adept’s pose (siddhasana), the lower part of the body rests. The folding of the hands in the lap also contributes to this process of rest and withdrawal. If you then sit quietly and keep your back erect, you can perform the meditation exercise nicely.
The problem for most people is that they are not in the habit of sitting on the floor and the position quickly becomes uncomfortable. It may be difficult to keep your back straight. Your knees may hurt, your foot may even fall asleep. It may become so painful that instead of thinking about the mantra, you end up mentally cursing the yogi who told you to sit for 25 minutes in such an uncomfortable position. So, what should you do in this case, retire to the comfort of a chair? I would suggest that you persist in learning how to sit on the floor. While it may be more uncomfortable in the beginning, later it will help you to reach deeper states of meditation. In fact, if, in the first few months of meditation, the only thing you accomplish is to learn how to sit quietly on the floor for 30 minutes, then you have made a good step forward in your spiritual life.
One more thing I can add is that there are some limbering exercises that can help you to learn how to sit more comfortably. Visit your nearest Ananda Marga Center and ask an instructor to show them to you.