Endurance events such as road cycling, long-distance swimming, marathon, triathlon, and 10K run all require a high level of endurance and place a relatively low premium on anaerobic power. These events force competitors to perform at the margin of their maximal aerobic capabilities over long distances. As training, nutrition, and selection of athletes in endurance sports improve, records continue to fall. This suggests that doing the right things can and will result in moving the known envelope of speed in endurance events. The winner of the marathon at the Atlanta Olympic Games in 1996 won with an average running pace of slightly under a 5-minute mile, a feat that is not commonly achieved. Despite this incredible speed, the athlete had to maintain this pace at a level that allowed a sufficient oxygen uptake to sustain, primarily, aerobic muscular metabolism. That is, the majority of all muscular work took place with fuel being burned in the presence of oxygen. This is an efficient means of obtaining energy, allowing the athlete to sustain muscular work for long periods of time.
Aerobic training does some wonderful things to the athlete’s ability to use oxygen. The intermediary (type IIa) fibers, which tend to behave more like fast-twitch (power) fibers than slow-twitch (endurance) fibers, dramatically increase in mitochondrial content and the in enzymes involved in oxidative metabolism. The training impact on oxygen usage is well known. In studies looking at blood lactate concentration, trained athletes are far more capable of tolerating high levels of blood lactate than are untrained subjects doing the same intensity of work. The conversion of the behavior of the intermediary fibers results in an improvement in the athlete’s aerobic endurance. The increased ability to use oxygen results in an improvement in the ability to burn fat as a primary fuel, reducing the reliance on carbohydrates.
Since even the leanest athletes have a great deal of energy stored as fat, this increased ability to burn fat dramatically improves endurance. However, since carbohydrate is needed for the complete combustion of fat, carbohydrate is still the limiting energy source for endurance work because athletes have relatively low carbohydrate stores. This is clearly demonstrated by findings that athletes consuming a high-fat diet have a maximal endurance time of 57 minutes; on a normal mixed diet their endurance rises to 114 minutes; and on a high-carbohydrate diet, their maximal endurance rises to 167 minutes.