Your aerobic fitness is your ability to take air from your surroundings and produce energy for your muscle cells. Many factors influence aerobic fitness, including your lung efficiency, cardiac function, gender, age, training status and genetic makeup.
Aerobic base training is like driving a car in cruise. You should be able to coast along and an average pace and still be able to talk complete sentences. This is an effort level that you can sustain for a longer period of time. The more we train at this level, the more body fat we burn, as fat sores are a slower release energy source.
Aerobic respiration needs oxygen. It is the release of a relatively large amount of energy in cells by the breakdown of food substances in the presence of oxygen.
Unlike aerobic respiration, anaerobic respiration does not need oxygen. It is the release of a relatively small amount of energy in cells by the breakdown of food substances in the absence of oxygen. Anaerobic respiration happens in muscles during hard exercise: glucose → lactic acid. Glucose is not completely broken down, so much less energy is released than during aerobic respiration.
If we had just low intensity slow effort exercise, we would build up our aerobic base well, meaning you could continue at a lower effort level for longer. However, without the HIIT work, you might struggle to sprint for the bus, or to the end of the street, for example.
Someone with a good aerobic base will be able to complete low intensity exercise for a long time, but will struggle with higher intensity. This type of client is one that I might give heavy weights and high intensity reps to, for example.
This was a mistake I made when I first started training for triathlons. Too much high intensity high effort training alone will mean that your body will become ineffective at burning fat stores, and much more effective at burning carb stores (i.e what you ate today). You will be very good at sprinting for that bus, but would struggle to jog around the block.
This is the key to all around fitness: a mixture of both.