Inside and outside is a set of yin and yang. This principle refers to the movements in this set of.
The outside is considered a physical structure made up of physical materials such as bones, ligaments, muscles, etc. The inside is empty. The outside is yang and the inside is yin.
In terms of movements, the outside cannot move as in. The inside moves. If movement is defined as an activity, there must be more activity inside, less activity outside.