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...
What is the point of Ch’an (or Buddhist) enlightenment in the modern age? Many, if not all of the world’s great scientific breakthroughs have been made by human minds that have not undergone the Buddhist training, and which have not uprooted greed, hatred or delusion, transcended duality or perceived the empty mind ground. My personal opinion is that Buddhist developmental methodology is not a religion, despite the fact that many manifestations of Buddhism have assumed the garb of religiosity. Buddhism is not anti-science as the theology of other religions is often presented, and yet the Buddha and his disciples (although many of them ‘learned’), could not read or write. Many are surprised by this, but at no point in any of the 5000 plus Buddhist texts does the Buddha mention the modern notion of literacy, despite the Buddha’s thought processes appearing to be very modern despite manifesting at sometime between 2,500-3000 years ago in ancient India. As the concept of modern science has now mainstreamed in the world, together with literacy being the preferred norm, the Buddha’s path no longer seems that special or important. An effective scientist does not need to meditate or gain enlightenment to be an effective servant of humanity, and profoundly assist in its development and welfare. On the other hand, I have read Professors at Oxford University state that in their opinion, the Buddha was the first ‘modern’ thinker at a time when logical thinking was thin on the ground, with Carl Jung opining that the Buddha appeared, through a sheer act of will, to think ‘outside’ the era within which he existed. This on its own is an extraordinary feat, if it is accepted that he was the world’s first modern thinker in the true sense. In today’s world, being intellectually astute is inherently linked to simultaneously possessing a high degree of literacy and coming from an economically rich background, and yet the Buddha had none of these things as a spiritual-seeker. Indeed, today he would be considered one of the homeless community and what he had to say would be deliberately excluded from what is considered the general (and valid) discourse of mainstream existence. It is perhaps ironic that most that refer to themselves as ‘Buddhist’ in the contemporary West are of the privileged economic class that the Buddha rejected. The enlightened Buddha combines poverty, homelessness and unemployment with selflessness, non-attachment and sublime wisdom. What is interesting is that if a person were to live in a peaceful forest or on top of a hill far from the cares of the ordinary world, then the Dharma certainly does prove to be a ‘way out’ of ordinary suffering (by following the Vinaya Discipline). However, within the Ch’an School (and the Vimalakirti Nirdesa Sutra), the Buddha explains the path of the enlightened lay-person. Such a person lives amongst the pleasures and pains of the world and remains non-attached to arising and falling thoughts (and emotions), and is unmoved by words of praise or blame. The empty nature of material reality is always perceived as underlying the continuous play of phenomena. Although a busy street or a quiet mountain top may differ in outward appearances, they both share exactly the same empty mind ground, and other than the practicalities of different manifestations, no real difference can be discerned. Understanding this is the further training required after enlightenment. From my own perspective, training as a young man directed, strengthened and freed the full intellectual and wisdom capacities inherent in my mind, whilst allowing me a completely different way of relating to and controlling my physical body. This led to tremendous academic success and the mastery of our family martial arts system.