[paulrudy]

When I think of isometric exercise, I think of co-contracting opposing muscle groups so that the body is exerting more effort than would be minimally necessary to maintain a posture. That kind of exertion would be antithetical to the practice of taichi, but perhaps that wasn't what you meant.

If you meant simply that a posture or position is held against the resistance of gravity or some other resistance (like a partner), then that's isometric contraction by definition, since there is muscle activity but the joints are not moving.

Still, by that definition, describing taichi as a form of isometric exercise doesn't really cut it for me. A fundamental part of practice is to continue discovering how to muscularly engage less, in order to free up the sensitivity, availability, and responsiveness of the body. The phrase "isometric exercise" doesn't conjure up that important aspect in my mind, but that's entirely subjective.

Another aspect is that in practice, there is constant motion in the joints in taichi. Holding static postures is a common and useful aspect of training, but the actual use of taichi (a martial art, after all) is entirely dynamic. To an outside observer, a movement might appear as though a practitioner is holding their arm, spine, and head in fixed positions while turning the waist or stepping, but in actuality, every joint should be adapting and moving in concert with its neighbors. Nothing is held in a fixed position--one reason being that as soon as you're committed to holding something in a fixed position, your partner/opponent will exploit that as a fulcrum to destabilize you.