The principle of interval training is to work at alternating intervals of higher and lower intensity.
The objective is to use the body’s recovery during the intervals at lower intensity to make it possible to increase the total amount of activity at higher intensity in the workout (adding up all the intervals at higher intensity), compared to if you were to work continuously at the same intensity.
A classic series of experiments with running showed that an individual who could sustain a total of 4 minutes covering 0.8 miles at a certain fast continuous pace before exhaustion, could sustain a total of 20 minutes at that same pace (covering 4 miles) when the running was done intermittently in intervals of 10 seconds of exercise and 5 seconds of rest.¹”
The body has three different energy transfer systems, that it uses according to how fast it needs energy and how long it needs to sustain it for. The fastest energy transfer system(Alactic) can generate energy almost immediately, but it is exhausted within 30 seconds.
The intermediate energy transfer system (Glycolysis) takes about ten seconds to get going. After after about 30 seconds of activity this will be the predominant energy provider. This is very convenient because it can then take over as the fast energy system is depleted and while the long term energy system still is powering up its engines. It takes about ten minutes for energy to be transfered in (aerobic), however it can keep going for a long time if you stay at an intensity that doesn’t demand a sudden increase in energy.
You can vary the intensity and duration of the intervals and in this way effectively train the capacity of a specific energy transfer system.
In sports specific training these principles are used to prepare the body for work in specific energy transfer systems relevant to the particular sport.If for example you work 1 minute of moderate intensity followed by 1 minute low intensity, you will mainly be training the aerobic energy system. If in stead, you increase the intensity from moderate to high and you decrease the length of the interval to 20 seconds at this intensity, and keep the one-minute recovery interval, your body will have time to recover between the intervals at high intensity and you will mainly be working the anaerobic systems.
You can also play with the number of repetitions of the interval. The intensity and duration of the intervals will decide how many repetitions you should go for, together with your fitness level and your goal.