You measure your fitness rating by measuring the maximum amount of oxygen that your muscles are able to absorb within one minute. That amount of oxygen is then divided by your body weight (in kilos), showing you the number of millilitres of oxygen that may be absorbed per kilo of bodyweight.  

 

 

 

 

 

If those extra kilos on your body are mainly made up of fat, you will notice that once you loose weight, you will automatically be able to walk or run further. This is because the oxygen will be distributed amongst fewer kilos. This is why some athletes decide to loose weight prior to important competitions in endurance sports such as cycling or running.   If you do effective fitness training on a regular basis, the amount of oxygen absorbed by each muscle will increase. Once the level of oxygen absorption increases, you will be able to train harder during the training time period, or you will be able to train for longer periods of time at the same level of intensity (thereby becoming even fitter!). This also makes many everyday strains easier to cope with.