“The human body is equipped with two systems to produce energy for fast swimming, aerobic (requires oxygen) and anaerobic(no oxygen required). Both are used in tandem to produce ATP, the fuel for our muscles, but there are significant differences between the two energy systems that you should consider when choosing a breathing pattern for a specific race.
While the purpose of this article is not to describe these different energy systems in detail and how they work, it is to try to come up with the best possible breathing solution for the 100 freestyle. The reason that the breathing pattern in the 100 freestyle (and butterfly) is more controversial than for other distances is that for events shorter than 100 meters, the energy is predominantly derived from the anaerobic system. In events longer than 100 meters, the energy is predominantly derived from the aerobic system. In the 100 meters, the energy derivation is about half aerobic and half anaerobic.”