Sitting-up with neck and back support in a comfortable chair can be useful - but lying down is just as good. Posture can involve any relevant position that you need rather than conformity to a universal standard. The Buddha talks of standing, walking, sitting and lying-down (he died lying on his right-side). The point is that a posture should allow an individual to 'forget' about the body.
Pain is one of the three stages of sensation mentioned by the Buddha - together with 'pleasure' and 'neurality'. A convenient meditation posture should generate either a relaxed 'pleasure' or an indifferent 'neutrality' to the body. However, pain has the ability to 'breakthrough' any indifference being cultivated - until the 'indifference' becomes stronger than the pain.
If you focus on 'Who is hearing?' and attempt to return all that is heard back to its non-perceptual essence - then the empty mind ground will be revealed. If a single sense can be 'returned' in this manner, then ALL of the other five senses (smell, touch, sight and touch) will automatically be returned generating a permanent unity of inner vision. Gone will be the duality that separates mind, body and environment. When this is achieved, pain is transformed into a distant 'drop' of water falling into the ocean...
‘...After my death, if you carry out my instructions and practise them accordingly, my being away from you will make no difference. On the other hand, if you go against my teaching, no benefit would be obtained, even if I continued to stay here.’
Then he uttered another stanza:
“Imperturbable and serene the ideal man practises no virtue.
Self-possessed and dispassionate, he commits no sin.
Calm and silent, he gives up seeing and hearing.
Even and upright his mind abides nowhere.”
Having uttered this stanza, he sat reverently until the third watch of the night. Then he said abruptly to his disciples, “I am going now,” and in a sudden passed away. A peculiar fragrance pervaded his room, and a lunar rainbow appeared which seemed to join up earth and sky. The trees in the wood turned white, and birds and beasts cried mournfully.
...The Patriarch inherited the robe when he was 24, had his hair shaved (i.e. was ordained) at 39, and died (sat upright) at the age of 76. For thirty-seven years he preached for the benefit of all sentient beings. Forty-three of his disciples inherited the Dharma, and by his express consent, became his successors; while those who attained enlightenment and thereby got out of the rut of the ordinary man were too numerus to be calculated.
The Altar Sutra of Hui Neng (Chapter 10 – His Final Instructions)