Johnny D. wrote:
The one exception I think of to this is standing Qigong postures like holding the tree, where the spine is almost stacked straight, even then, there is a kind of muscular exertion involved to hold oneself up.
Differences exist between sitting meditation and static standing meditation. In the first, the spine is held erect over a mostly unyielding base (one's butt!). In the second, although the spinal shape may be the same it is now resting on a base (pelvic region) that is further supported by the legs (which are bent and not locked). This adds an extra dimension of shock-absorption and an increase in postural sway that is less marked when sitting meditating.
In performing standing meditation (zhan zhuang) it should be noted that emphasis is placed on two additional practices - Sung, the mindful relaxation (softening, not slackness) of the joints and muscles, and correct abdominal breathing. Part of the reasoning behind this is to slightly elevate the oxygen intake and encourage the use of postural muscles (stabilisers) to hold the position rather than our phasic muscles (mobilisers). Stabilisers tend to derive energy from oxygen, whereas mobilisers get theirs from glucose. Which is why if you stand in meditation employing mainly the mobilsers you will tire very quickly due to finite reserves of glucose-based energy in comparison to the perpetual intake of oxygen.
Overall I believe the lower back is far less mobile in zuo chan than in zhan zhuang, and this is despite the fact that many/most beginners to either practice go to great lengths to attempt to remain utterly motionless. This is a) impossible unless you are dead, and b) uses more muscle energy and leads to fatigue more quickly.
That's my opinion. If you don't like it let me know and I will go and form some others!