The instability of the rings will engage stabilizing muscles. You will need 1-½" straps to fix them to whatever you have available (at the gym, playground, tree, in your basement, etc). The flat surface is to be able to print them flat on the bed without support. Note that the overhang angle at the interface is 60°. Print them as heavy or as light as you need them. Be careful when using them as the load rating will depends on the material and the printing parameters used. The one on the picture was printed with ESun ABS+ using 1.6 mm wall, 1.2 mm top/bottom and 50% cubic infill. The result is very sturdy.