Interval training uses short periods of intense, anaerobic work, interspersed with periods of recovery. A larger, stronger heart has a larger stroke volume, resulting in higher cardiac output and more oxygen to the working muscles. The muscles themselves can also be improved with training. The most effective type of training is that which places a demand on the muscle fibres to utilise oxygen. The best type of training for this is endurance or aerobic training.
Endurance training utilises long periods of work at medium intensities with few or no rests.
This type of training increases the number of functional capillaries around the muscle fibres and increases the activity and mechanisms in the muscle cells to utilise oxygen. As discussed above, there is a correlation between exercise intensity and the type of energy system used. Cardiac output is the major determinant of oxygen supply to the muscles. When the intensity of the exercise reaches a certain point, the cardiac output is no longer able to supply enough oxygen to the muscles, and anaerobic systems take over to supply the extra energy. Thus, you can see that if we know the exercise intensity level at which the cardiac output begins to fail in oxygen supply, we can work out which energy systems we are using at given exercise intensities.