The relation between moving speed (running speed, swimming speed etc.) and the energy demand – expressed as oxygen demand (see also economy) is expressed as a mathematical equation. The terms A,B, and D are the three terms of this equation. A is a static value, B is a linear value and D is a cubic value. Example: in a sport like swimming the terms could look like: A=5 (resting VO2); B=0,1 (expressing a small linear increase of energy demand), D=15 (expressing a large energy increase with speed). In Swimming, the cubic term D is larger compared to a sport like running. This is because of the influence of the hydrodynamic drag of the swimmer and the higher density of water compared to air.