The only form of energy which can be used by muscles is the chemical energy from ATP (adenosine triphosphate) which is provided by sugar and fats in our diet.
When a muscle contracts, the ATP stored there is broken down and some of the available energy is used to perform work. However, since the ATP store is small, it must be replenished rapidly if we are to continue running. With sub-maximal running, ATP stores can normally be restored aerobically (ie. with oxygen). But when high intensity effort is called for, ATP is more likely to be restored anaerobically...without oxygen.
The contribution of the aerobic and anaerobic systems to energy production in the following events is as follows:
|Distance||Aerobic %||Anaerobic %
However, all runners can benefit from training of the anaerobic system. Even at marathon distance, the ability to produce a high rate of energy is useful. Anaerobic training makes it considerably easier to run up hills and keep going in the teeth of a strong wind. Interval training improves your anaerobic capacity while a thorough pre-exercise warm up has the effect of increasing the temperature of your muscles, speeding up the delivery of oxygen to the muscles and enhancing total energy output.
Distance runners primarily use the aerobic system where oxygen produces energy from carbohydrate and fat stores. Training will increase the muscles' ability to use oxygen to burn fat and after some weeks those who could barely run one mile can run for 2 hours!Strength training also improves anaerobic capacity as the size of the muscles is increased, so enhancing the capacity of the anaerobic system and enabling you to produce more work.