As I gain more experience teaching Tai Chi I see a reasonable percentage of people with lower back pain and stiff lower backs. Causes vary of course (anything from arthritis, being sedentary, trauma, scoliosis, to disc problems) and so does the suitability of any chosen lower back exercise, but in general these people are looking for an exercise which gives them some relief from their lower back pain. Usually they try Yoga first but may find the postures, or the bending/twisting/turning of the spine unsuitable for their condition or age. They may try brisk walking but may find the constant jarring impact with the ground aggravating. They may try massage or other forms of manipulation but this may be expensive and generally only offers relief of the symptoms.
So to answer any questions regarding this topic, good Tai Chi differs significantly from the aforementioned exercises & methods in the way we exercise the spine. Tai Chi practitioners (in general) slowly lengthen & release the spine whilst the spine is aligned vertically and connected (rooted) to the ground via the hips, legs & feet. Using an illustration, it is like the pulling of a spring (spine) that has its top attached to the ceiling - we never "twist" nor “excessively bend” the spine, all the turning is done at the hip joints (the millstone turns). The result of this repeated slow lengthening and releasing (along with several other Tai Chi principles) is a gentle – but substantial - spinal workout that can bring relief to tension & discomfort in the lower back area.