I can only speak from personal experience but I am suspect of the idea that one can only meditate effectively in the sitting (ideally, lotus or half-lotus) position and that not doing so will impede one's development on the spiritual path. In the end, all the arguments and pronouncements for one or the other postures, are somebody else's opinion and must be taken on faith. Meditation, and all that emanates from it, are private experiences; perhaps the ultimate of what is meant by 'personal experience'. The only way you could really determine the superiority of one posture over an another is to divide oneself in two and spend a lifetime mediating in the sitting position and another mediating walking, lying down or whatever. The fractal nature of the mind which is illuminated and rarefied during the meditation experience is simply too subtle to measure by ordinary standards. I presently practice Shamatha and I meditate lying down. I've been doing this for just over a year now. When I first began the practice I tried sitting but lower back and knee problems interfered. The pain I experienced was not the discomfort suffered by the neophyte who sits in the lotus position. Here the instruction is to ignore it and come back to the object of reference and it will eventually subside. My pain was based on physical malady and I discovered through well-intentioned trial and error, that it would not subside in a sitting position, only deepen. When this happens you are not meditating, you are focused on pain. For a period it was discouraging: one person even going so far as to say I should give up the practice altogether. I took naturally to lying down and though you are warned that this posture too often leads to drowsiness or sleep--I have never experienced either, once, in the whole time I've been doing it. I correct my position, sometimes before, sometimes during practice, so that the line running from my feet to my neck is straight. If I have a bit of congestion I will slightly elevate my torso until it clears. I try not to get too hung up in the do's and don'ts. I don't scratch an itch but I do get up and go to the bathroom if I have to and this has never interfered with--in the sense of ruining-- my sessions. So my advice, if I were asked, is to find what best works for you. Shamatha is a challenging form of meditation but if one can simply stay focused as best as able on the object of reference, progress will be made. And remember, while you're interested in making progress, it is more important to forget the whole idea of progress: in other words, you're only doing it when you're not doing it.

I can only speak from personal experience but I am suspect of the idea that one can only meditate effectively in the sitting (ideally, lotus or half-lotus) position and that not doing so will impede one's development on the spiritual path. In the end, all the arguments and pronouncements for one or the other postures, are somebody else's opinion and must be taken on faith. Meditation, and all that emanates from it, are private experiences; perhaps the ultimate of what is meant by 'personal experience'. 

The only way you could really determine the superiority of one posture over an another is to divide oneself in two and spend a lifetime mediating in the sitting position and another mediating walking, lying down or whatever. The fractal nature of the mind which is illuminated and rarefied during the meditation experience is simply too subtle to measure by ordinary standards. I presently practice Shamatha and I meditate lying down. I've been doing this for just over a year now. When I first began the practice I tried sitting but lower back and knee problems interfered. The pain I experienced was not the discomfort suffered by the neophyte who sits in the lotus position. Here the instruction is to ignore it and come back to the object of reference and it will eventually subside. 

My pain was based on physical malady and I discovered through well-intentioned trial and error, that it would not subside in a sitting position, only deepen. When this happens you are not meditating, you are focused on pain. For a period it was discouraging: one person even going so far as to say I should give up the practice altogether. I took naturally to lying down and though you are warned that this posture too often leads to drowsiness or sleep--I have never experienced either, once, in the whole time I've been doing it. I correct my position, sometimes before, sometimes during practice, so that the line running from my feet to my neck is straight. If I have a bit of congestion I will slightly elevate my torso until it clears. I try not to get too hung up in the do's and don'ts. I don't scratch an itch but I do get up and go to the bathroom if I have to and this has never interfered with--in the sense of ruining-- my sessions. 

So my advice, if I were asked, is to find what best works for you. Shamatha is a challenging form of meditation but if one can simply stay focused as best as able on the object of reference, progress will be made. And remember, while you're interested in making progress, it is more important to forget the whole idea of progress: in other words, you're only doing it when you're not doing it.