As always, Tim Cartmell’s explanation is clear and effective:

Zhan Zhuang initially is the most efficient method of correcting poor alignment, developing correct breathing, strengthening the postural muscles evenly, creating the correct balance between tension and relaxation, developing the kinesthetic sense, developing basic endurance and raising the threshhold for pain tolerance.

The method is so efficient because all possible variables are reduced to a minimum (since you are standing still). The only concern is your relationship to gravity, since the complications of momentum and other external forces are not a concern. It is, therefore, easier to consciously inhibit any bad habits of body use you have acquired.

Shili is used to test the attributes cultivated through Zhan Zhuang in motion, with the added complications of momentum and the ever changing forces acting on the balance.

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