where BV is bone volume fraction (often corresponding to the BV/TV bone morphologic measurement), k f is the formation rate, and k r is the resorption rate. The variables OBL and OCL usually represent changes from the steady state population, sometimes called active cell populations. Additionally, bone volume may be replaced with bone mass or other relevant bone properties. Bone volume, total osteoblast population, and total osteoclast population cannot have negative values.