Athletic events can be divided into those that rely on explosive bursts of power and those that are tests of endurance, races such as the 100 metre dash and the marathon. There must be upper limits to the levels of performance achievable but the records to date indicate that substantial further improvements will be made.
The organs and systems we depend on for athletics are our muscles, our heart and blood circulation system, and our lungs. Our muscles move the body and our lungs and cardiovascular system supply the muscles with fuel and remove waste products. The science of how the body works is described by G.C. Brown in The Energy of Life (The Free Press, 2000).
Just as a motor car burns fuel in order to power movement, our muscles must also generate energy by burning fuel. The main muscle fuel is carbohydrate, mostly glucose, but fat is also important.
Muscles convert the energy locked in food molecules into chemical energy in a form known as ATP and use ATP to power the work done by the muscle. Muscles are generally long cylindrical structures and work by actively shortening along the long axis, pulling on the bones to which they are connected and causing limbs to move.
Muscle generates ATP from glucose by breaking it down in a long sequence of steps, eventually producing carbon dioxide and water. The breakdown steps can be divided into two parts, an initial fast part called glycolysis that can take place in the absence of oxygen (anaerobic) and a subsequent slower part called the TCA cycle that requires oxygen (aerobic).
Ninety-five per cent of the ATP generated by the entire process comes from the TCA cycle. Within muscle cells the TCA cycle is localised to little bodies called mitochondria.
If your athletic event relies on a short explosive burst of movement, your muscles use glycolysis to quickly produce ATP. If your event is a long trial of endurance (e.g. 10,000 metres), your muscles rely on the TCA cycle to keep up a plentiful and extended supply of ATP.
Let us consider endurance events first. The lungs must get oxygen into the blood which is pumped by the heart to the muscles where it is used to burn glucose. Obviously a bottleneck might exist anywhere along this chain that could limit the overall rate of the process.
The lungs don't pose a bottleneck because, even in the most strenuous exercise, they work at only two-thirds of maximum capacity. What about the heart and vascular system? Heavy exercise enlarges the heart, allowing more blood to be pumped with each contraction (beat) and the rate at which blood circulates can increase as much as 40 per cent.
A fully conditioned heart works at about 90 per cent of its maximum rate in endurance events, i.e. close to its limit. Blood contains red blood cells which in turn are full of haemoglobin which carries oxygen to the muscles. If the heart could pump even more blood through the system, the athlete could run (swim, cycle) faster.
The fact that the cardiovascular system is a limiting factor on performance is shown by the improvements in performance that result from the variety of methods (many of them banned by sporting organisations) used to increase the oxygen carrying capacity of blood. The more red cells per unit volume of blood, the greater the oxygen-carrying capacity. The red cell count can be increased by blood doping. In this technique the athlete draws off a litre of his/her blood and stores it several weeks before a race. In the meantime the body makes up the loss by making more red cells. Shortly before the event the athlete reinfuses the stored blood which boosts haemoglobin levels above the normal. Another way to increase the red cell count is to inject the hormone erythropoietin (EPO) to stimulate the production of red cells.
The muscle cell mitochondria that produce ATP in the TCA cycle may also be limiting factors. Training increases the number of mictochondria in muscle cells but an optimally trained athlete has to make do with the limit imposed by his/her enhanced mitochondrial level.
The level of performance in power events is limited by the muscles themselves. Muscles are made up of cells called fibres. There are two basic types: fast and slow fibres.
Fast fibres contract and relax quickly, but they fatigue rapidly. Slow fibres act slowly but have high endurance. Fast fibres rely mostly on glycolysis to make ATP and don't need much oxygen. Slow fibres rely mostly on the TCA cycle to make ATP.
The muscles of a sprinter or a weightlifter are about 80 per cent fast and 20 per cent slow fibres. The proportions are reversed in a long distance runner. Training can change the relative proportions of fast and slow fibres. Training also helps the power athlete by enlarging muscle size, thereby increasing the power of the muscle.
The conventional way to increase muscle mass is weight training. Seductive shortcuts are available of which the most familiar example is the anabolic steroid. These drugs are similar to the male sex hormone testosterone and, when taken with a diet rich in protein and carbohydrate, will increase muscle mass. Use of these drugs is banned because they confer unfair advantage and also because of serious side effects, ranging from acne to impotence.
Advances in genetic engineering, physiology and biochemistry will undoubtedly create (if permitted by sporting committees) new interventions to produce super-normal athletes. These might include interventions to introduce faster muscle fibres, increase numbers of muscle mitochondria, etc.
I have no interest in "technologically-enhanced" athletes. When we eventually reach our natural biological athletic limits, so be it. World records for power and endurance events have been improving linearly with time since 1900. If we were approaching natural limits we would be into steadily diminishing rates of improvement.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.