There are two main energy systems and two distinct muscle fibre types involved in all endurance exercise.
By far the most important energy system to train for endurance is the vast aerobic fat-burning system. The muscle fibre type most associated with fat-burning ability is the Type I slow-twitch muscle fibre. The Type I muscle fibre is made for continuous lower-power contractions in the presence of fuel and oxygen, and is redder than the fast twitch muscle fibres because it has far more myoglobin, which is a stationary muscle-based variant of the same red, iron-rich protein haemoglobin in the red blood cells.
Haemoglobin is the oxygen-binding protein in the red blood cells. The slow twitch muscles have less contractile protein (actin and myosin) than the powerful fast twitch fibres, but far more mitochondria. Mitochondria are the bacterium-sized energy-furnace organelles in organs, glands, and muscle which generate energy from the aerobic metabolism of fat or glucose.
There are two main types of fast twitch fibre we need to be concerned with in athletics, and only one of them is strongly associated with endurance potential.
The most involved fast twitch fibre for endurance is the Type IIA fast twitch muscle fibre. This is the second-most powerful fast twitch muscle fibre. It’s most associated with powerful contraction for 20 seconds to 2 minutes, and for doing this by rapidly generating power from the breakdown of glucose either with oxygen (aerobic glycolysis) or without (anaerobic glycolysis).
With aerobic endurance training, genes are switched on that enable mitochondria to proliferate in the IIA muscle fibre, as well as a vast increase in aerobic enzymes, so that the IIA muscle fibre acquires fatigue resistant aerobic qualities. Genes are also activated that allow for angiogenesis, or the growth of new blood vessels, throughout the IIA muscles, so that far more oxygen and fuel can be delivered into the muscle, and far more carbon dioxide and acidic waste products can be rapidly removed.