Even though the lotus posture is one of the most stable postures for meditation, it is not essential for practising meditation. In fact, forcing yourself in an uncomfortable or painful position will only hamper your ability to meditate. You can sit in Lotus position, half-lotus position, kneel down, sit on a chair or stool, or even lie down.
Source: Art and Science of Meditation - by Ms. Chitra Jha and Dr. Newton Kondaveti.
