If you look at an Xray of a healthy spine you'll notice how closely it resembles that of an archers bow, when sitting properly this is the optimum posture (or asana) for meditation, especially when you want to raise Shakti to her mate Shiva; an advanced state of meditation.
Arjuna and his bow, as mentioned in the Bhagavad Gita, is symbolic of the spine. When Arjuna "took up his bow" it meant to imply he assumed the lotus posture of meditation, where the spine is kept straight, and began to meditate. Slouching during meditation cuts off the vital flow of prana and renders meditation less effective and/or leads to sleep. SBS meditated in the Lotus posture.
