Endurance athletes rely on slow twitch fibres to keep them going for extended periods of time.
Slow twitch fibres are also known as Type 1 or slow oxidative fibres, and have a slow contraction speed. They are designed for steady, continuous activity and are highly resistant to fatigue.
Analysis of these structures shows them to have active motor neurones, with a low firing frequency. They also have thin cells with a high surface to volume ratio, which permits good capillary supply and efficient gas exchange.
The up-shot of this is that they are very efficient at bringing oxygen into the cells and disposing of carbon dioxide as waste. While they can use glucose for fuel, they prefer fat as a fuel supply, and will use fat in extended efforts.
The beauty of our muscle structure is that we have three different types of muscles, each with different functions. Utilising all three in different combinations allows us to perform a vast variety of movements, with different demands.
However, the balance of these is not set any individual can change the balance with appropriate training, allowing us to increase our ability to perform various tasks.