Rate of Perceived Exertion (or RPE) is how you can gauge your training intensity when doing any kind of exercise. Although many are familiar with the Borg Scale of RPE, which ranges from 6-20, this scale covers the same range but from 1-10. 1 is very light activity (just above rest) contrasted with 10 which is the absolute highest intensity your body can achieve. This scale can help you determine what percentage of your VO2 Max you are utilizing.
Aerobic base is built by increasing the efficiency of the function of the mitochondria in your muscle cells. When training to increase your aerobic base you should strive to keep your intensity somewhere from 5-7 on this scale and maintain this for as close to 45 minutes as you can. This corresponds to Heart Rate Zone 2. If you are new to training you may need to begin with sessions of 15-20 minutes and increase over time. Try to perform at least 2-3 sessions per week in Zone 2. Work up to the point that you accumulate 90-150 min per week.
When training to increase your VO2 Max (or peak aerobic output) you need to push your intensities beyond 7 to 9 or even as high as 10 for intervals (HR Zones 3-5). Then recover with intervals that bring you back to your maintenance pace in Zone 2. While the high intensity intervals can range from 1-3 minutes, the recovery intervals can be 2-5 minutes depending on your current aerobic fitness level and how much time you are devoting to aerobic fitness. These sessions can be as short as 20-30 minutes and will leave you feeling very fatigued. The intense demand on your aerobic machinery will stimulate your body to generate more mitochondria, increasing mitochondrial density. Only 1-2 interval sessions per week are needed to improve VO2 Max.
When using RPE in can be difficult to distinguish between moderate to vigorous. The HR Zones are the key, but determining them accurately is best done through VO2 Max testing. From there we identify the heart rates that correspond to your key thresholds (aerobic, anaerobic and peak) and lay out your zones. We recommend that you spend 80% of your aerobic training time in Zone 2 and 20% training in Zones 3-5. It only requires small doses of higher intensity to drive mitochondrial density but it takes more time to build the broad base of mitochondrial efficiency.