Can any yoga teachers or practitioners recommend some good sitting positions when sat in an armchair?
As a yoga therapist i would recommend either sukhasana or vajrasana provided your base of the arm chair is strong enough. If not then do not bother about any good sitting position but try to maintain a hasta mudra i.e. hand position yoga. If you are prone to anger, have difficulty in concentrating then do Gyan mudra, if you have joint pains then do vayu mudra, if you want to loose weight do surya mudra and if you want to gain weight do prithvi mudra. If you want to strengthen your eyes and at the same time feel energised then do prana mudra. Take your pick and Good luck and God bless. Cheers! :o) By the way mudras should not be held more than 45 minutes at a stretch.
