The hills subdomain captures both natural and artificial terrain where changes in elevation are gradual (hills) rather than incremental (stairs). This distinction is meaningful for several reasons. First, gradual changes in elevation produce foot placement that is not level rather than level, which provides unique opportunities to develop proprioception and balance. Second, gradual changes in elevation allow for activities that cannot be completed safely on stairs, such as chasing a ball up and down, riding down in a wagon, or sliding down on a slip and slide.
The hills subdomain is composed of the following two categories: vertical rise and slope.
The vertical rise category is composed of the following four subcategories: small, medium, large, and extra large.
The slope category is composed of the following three subcategories: not steep, steep, and very steep.
