‘To give zazen instruction, we often say, “straighten your back,” “keep your eyes half-open, half-closed” to regulate the body, “make your out- breath long,” “do abdominal breathing” to regulate breath, and “do not think anything,” “focus your attention on your breath” to regulate the mind. I think there is a big problem here. Zazen should not be something forcefully built up by imposing a ready-made mold onto our body-mind from outside. It should be what is naturally and freely generated from inside as a result of non-fabrication. There is a danger that a rote way of giving instruction is leading us to change zazen into shuzen.
In zazen, the spine should elongate by itself instead of our lengthening it by effort. I would like to briefly touch upon the topic of “outer” and “inner” muscles. When we try to lengthen our spine consciously, we use the “outer muscles” – the volitional muscles. These are designed for purposeful movement. When the spine elongates by itself, the body is using the autonomously-controlled “inner muscles.” These are the muscles of “being” – the non-volitional muscles – designed as a system of supportive movement.’

I love how Issho frames zazen as very different to other kinds of meditation, as I have posted before – he name-checks the Alexander technique in this passage.