Contra-indications
Those without sufficient flexibility to place both knees on the ground when in full lotus position should not use it, as it strains the knees and lower back of such people. Other meditation asanas are indicated until sufficient flexibility has been developed to sit comfortably in the Lotus. Sciatica, sacral infections and weak or injured knees are contra-indications to attempting the posture.
Read more about this topic: Lotus Position