One of the phrases that is used often in endurance training literature is "lactate threshold." What does phrase mean and why is it important to endurance athletes?
Intensity
When I prescribe workouts to tell athletes how fast to swim, bike or run I use seven different intensity zones. These training zones can be correlated with heart rate, rating of perceived exertion, power and pace. To help athletes improve endurance, speed and performance, each workout within a training plan is assigned an intensity based on the strategy and purpose of that specific training plan.
Energy Production
In order to make more sense out of training zones, it is important to understand a bit about energy production within the body. Our bodies need to have a continuous supply of energy--even to sleep. Energy is supplied by complex chemical reactions. The end result of these chemical reactions is a rich compound called adenosine triphosphate or ATP. The potential energy within the ATP molecule is utilized for all energy-requiring processes of the cells of your body.
There are two basic methods your body uses to produce ATP. One method is aerobic, or with oxygen, and the second method is anaerobic or without the presence of oxygen. The method of energy production your body uses depends on the rate of demand for energy, or intensity, and the length of demand for energy, or duration. Short bursts of high speed utilize the anaerobic system of energy production to fuel the muscles. For longer efforts, fat and glycogen are burned in the presence of oxygen to create ATP.
A small amount of energy is readily available to be utilized "on demand." For example, when you sprint to make it through an intersection before the light changes, a small amount of energy is needed instantly. The majority of the energy necessary for this sprint is created anaerobically. After you have made it through the intersection and a slower speed is resumed, energy is created mostly by aerobic means.
For short sprints, energy is created primarily anaerobically and uses ATP stored in the muscle cells to complete the work. ATP is stored in the cells in limited quantities. It is readily available but is used quickly. Aerobically produced ATP, on the other hand, takes more time for the body to produce, but it is available in huge quantities. These large quantities of energy allow an athlete to exercise for several hours at easy to moderate speeds.
The energy production system within the body is quite complex. It is important to note that although an athlete may be swimming, riding or running along at a moderate pace, some of the energy it takes to do so is produced anaerobically. In other words, both systems are working at the same time. As the intensity or speed increases, energy production and utilization need to happen more rapidly. Remember, the aerobic system needs time to produce energy; it is not as quick as the anaerobic system. So the body relies more on anaerobic energy production as the pace increases.