The very essence of meditation is the inner process within your consciousnesses.

The physical posture is important only in its support for making this inner process optimal. That's it.

In certain types of meditations, when, for example, a flow of energy within your body is involved, the posture may be of importance.

Otherwise, especially in the pure sense of meditations (i.e. mental process), the physical circumstances are of less importance.

Listen to your body and choose the posture that you feel most comfortable with. This listening is actually a meditation by itself.

2000 years ago, when people were not using chairs but were used to sit on the ground, the lotus posture may have been natural for those people. It had the benefit of automatically keeping the back straight.

Nowadays, sitting in a lotus posture is a torture for an average mediator. Consequently, instead of being attentive to the meditation work, the meditator finds herself fighting with the pains and discomfort.

It is funny (and sometimes a bit sad) to see people in group meditations forcing themselves into the lotus posture and other external rigid formalities. Doing so sometimes indicate a misunderstanding of what meditation actually is.

Get loose and choose your posture with an open free mind.