The first step to reaching your fitness goals is creating a base of aerobic fitness, a process known as aerobic base building. Aerobic refers to the energy created in the presence of oxygen that you can later advance, and base building refers to building a base that you can later advance from. To build an aerobic base, you must exercise in the aerobic range, gradually adding more duration to absorb to the training effectively. A strong aerobic base will enable your body to better adapt to and benefit from anaerobic training when it is introduced.
You should commit to an aerobic base building period for 8–12 weeks when you are just beginning an exercise program, returning after a break from training, or recovering from an injury or overtraining. The longer the lapse in exercise, the longer the base building period should be. During this time your workouts should be completely aerobic, without any anaerobic exercise.
Aerobic base building workouts should be done at approximately 65%–75% of maximum heart rate so that the intensity does not cross over into the anaerobic range. Using a heart rate monitor is critical because it provides immediate, continuous, accurate feedback. You can shop for heart rate monitors