This week we have been breaking down the energy systems, how they work and how to train them. If you want to learn about the energy systems in general, the ATP-PCr system or the Anaerobic system follow the links.
Today we are talking about the Aerobic system. This is the most complex of the 3 systems and is how the body uses Oxygen to generate energy via a process called 'cellular respiration', which occurs in an organelle called the mitochondria. The mitochondria is commonly referred to as the "power house" of the cell because it is within the mitochondria that most ATP is produced.
During long-duration activity, the muscles need a constant supply of energy, much more than is stored in the muscle off hand. Unlike the other two energy systems, the aerobic system has somewhat of a delayed start and only turns on after exercise has been going for a while. This makes sense intuitively as well. You don't start breathing hard at the very moment of exercise. Instead, it takes several seconds or even a minute before the heavy breathing starts.
The aerobic system provides our muscles with energy during both prolonged exercise and day to day activities. We don't see an increase in respiration, or at least notice an increase, because the amount of oxygen we breathe in is more than adequate to meet the needs of our musculature. As soon as we start engaging in moderate to high-intensity exercise, though, the demands are increased, and our respiration rate and depth both follow suit.
Aerobic metabolism involves taking the byproducts of anaerobic metabolism (one of the acids created during anaerobic metabolism and ultimately convert it to ATP via a series of chemical reactions that require Oxygen. The aerobic system is much more efficient than the other two systems.
The aerobic system is active at all times. It is only after prolonged bouts of effort though that it kicks into high gear.
How to Train the Aerobic Energy System
It may make sense to train the aerobic using it as the primary source of energy generation. Wouldn't it make sense to exercise at an intensity that relies almost solely on aerobic metabolism, to improve the efficiency of aerobic metabolism?
Going for a run at low-moderate intensity isn't going to improve your aerobic metabolism. There may be justification to performing Long-Slow Duration (LSD) training, but it isn't for improving your fitness or even your health (see my article "Stop Doing LSD" for more on this topic).
The only reason I ever prescribe running at an intensity lower than 70% or maximum intensity is either for pacing or for focusing on technique.
In order to improve your Aerobic energy system, much like the other two systems, the intensity still needs to be high. At least 70% and as high at 90% of your HRmax. If you are training at an intensity that you can still have a conversation with your running partner, you aren't improving your aerobic system, despite the fact that your aerobic system is the primary energy supplier at this intensity.
The reason this is the case is because of how the aerobic system adapts. With the other two systems, just using them (i.e. training at the right intensity) is enough to cause a training stimulus and improve their efficiency. However, in order to train the Aerobic system, we need to overload it. This means training ABOVE what is known as the 'anaerobic threshold' (i.e. the intensity that the anaerobic system switches on).
Training at this intensity causes what is called "mitochondrial biogenesis", which is an increase in the number of mitochondria in the muscle cells. More mitochondria = more energy, and no type of training provides more of this stimulus than high-intensity interval training. A factor in exercise efficiency is mitochondrial density, which is how much mitochondria is present in a given muscle cell. Often, muscle bound jocks are thought to be slow and have terrible cardio. This is usually because they put on muscle without increasing mitochondrial density, making their muscles less efficient for aerobic exercise. In contrast, a long-distance runner has very low muscle mass but a high mitochondria-to-muscle ratio, making their muscle more efficient.
I argue that having adequate muscle mass for your sport, paired with the proper mitochondrial density (stimulated by the proper training methods) is more efficient and more desirable. How else can a heavyweight MMA fighter last 25 minutes of all out fighting? You don't see it often, but every now and then there is a fighter who has plenty of muscle and yet unreal cardio. Take Forrest Griffin as an example, a former light-heavyweight champion from the UFC. Weighing in at 205lb, conventional wisdom would tell you there is no way he should have lasted as long as he did at the intensity that he fought at, and yet he consistently lasted all 5 rounds without slowing down significantly. This can be attributed to his mitochondrial density and the efficiency of his musculature.
In sum... intensity matters. Train at high intensity to improve your aerobic system, don't slog away for 45 minutes at a low intensity on the stationary bike.
Aerobic System Summary
- Primary energy system at low intensities or long-duration bouts
- Efficiency: most efficient of the three energy systems, uses Oxygen to produce ATP
- Limits to performance: fuel (carbohydrates, fats) and oxygen available to muscle cells
- Fatigue: extreme breathlessness, exhaustion (similar to anaerobic fatigue)
- Training Frequency: train with anaerobic training (i.e. 2-3x/week)
Bringing it all together
While each energy system is very different, none function independently. For high-intensity movements such as 1RM snatch or Power Clean, we rely on the ATP-PCr system. For longer duration efforts (up to 2 minutes) we rely on the Anaerobic system. While for long duration, low-moderate intensity exercise we rely on the aerobic energy system.
To train the ATP-PCr system we need heavy, short duration exercise (ex sprint, back squat; 90-98% max intensity).
To train the Anaerobic we need to perform High-Intensity Interval Training (ex Tabata style training; 80-95% max intensity).
To train the Aerobic system we need to over load its' ability to produce energy (ex: 800m intervals with 15s rest; 70% and above HRmax).
-Mark Murdoch, B.Kin