The numbers signify two distinct energy systems in the body. On the aerobic efficiency index, we represent the development of your aerobic efficiency. Meaning that the higher it is, the more you can do while your body remains producing energy with the aerobic energy system. This obviously happens in the low ranges of your hear rate zones.
Once you start pushing your body more, the anaerobic energy system kicks into gear. There is always a balance between the energy systems, but depending on which zone you exercise, you use one system more than the other. The idea to place them side by side is to give you a solid and clear representation on how your energy systems are co-functioning, and what system was used the most last time you exercised, and which one is improving better in terms of efficiency.
We have built the scales based on our own science and they should only be seen in dynamics. Having said that, there will be times where your anaerobic energy efficiency might go below your aerobic efficiency. This is when you have depleted your body’s ability to produce anaerobic energy efficiently. Those days you should not push your body too much. Light recovery runs, walks or cycling rides should do it!