The aerobic and anaerobic thresholds
What is the aerobic threshold?
In Chapter 7 we discussed the different ways that the body can convert stored energy into motion. The aerobic energy system uses oxygen, and burns carbohydrate, fats and proteins. It is relatively slow at producing energy, and therefore cannot be used when a lot of energy is needed quickly. It is, however, the main energy system used for long distance running. For more intense activity, the body uses anaerobic energy systems which don’t require oxygen, but which produce waste products (notably lactic acid and protons) which build up in the muscles, and use up limited stores of glycogen.
The aerobic threshold is the level of effort at which anaerobic energy pathways start to be a significant part of energy production. Runners want to increase their aerobic threshold because this will enable them to run faster for longer before they tip into anaerobic metabolism which cannot be sustained for as long.
The pace which corresponds to your aerobic threshold is generally around your marathon pace. As we shall see later, this is about 70% of VO2 max, or about 75% of your working heart rate.
What is the anaerobic threshold?
The anaerobic threshold is defined as the level of exercise intensity at which lactic acid builds up in the body faster than it can be cleared away. For this reason, it is also sometimes called the lactate threshold or lactate turnpoint (LT).
There is some controversy about whether there really is a level of exercise intensity at which lactic acid in the bloodstream begins to increase. There is evidence that lactic acid levels increase smoothly as exercise becomes more intense, and there does not appear to be an identifiable discontinuity or kink in the curve. On this view, the anaerobic threshold is simply point at which the lactic acid accumulation becomes noticeable to the runner.
When you run at close to your anaerobic threshold, running should be hard but not uncomfortable. Some runners observe a change in their breathing rhythm at the anaerobic threshold, from one breath for every four steps to one breath for every two steps. At this level of exercise you can continue to speak, but not in complete sentences.
For experienced runners, your anaerobic threshold is somewhere between your half marathon pace and your ten mile race pace. It should be about 10–30 seconds a mile slower than your 10km race pace.