There are two key types of fibre that build muscle within most organsims, humans included. These are slow twitch fibres and fast twitch fibres; the differences between the two are outlined below.
Fast twitch muscle fibres are designed to create large amount of energy in a short space of time. They are used for very strenuous activity, such as running or weight lifting. Slow twitch fibres, however are designed to create more steady, sustained contractions; muscles in the leg for instance are adapted to sustain standing up for long periods of time.
Slow twitch muscle fibres are red, because of a high quantity of myoglobin in the cells. Meanwhile the fast twitch fibres are white because they contain a comparatively low amount of myoglobin. This is because slow twitch fibres are active for longer periods of time and thus require a larger amount of oxygen to fuel their metabolic reactions that provide energy for movement.
Because of the higher energy demands presented by the slow twitch muscle fibres they also have a larger amount of mitochondria (the organelle is the site of respiration) than fast twitch muscles.
The fast twitch muscles have a larger store of glycogen than the slow twitch muscles do. This is because slow twitch muscles are expected to be in action regularly, therefore it would be unnecessary to store glucose as it will be used immediately. Glycogen is stored in fast twitch muscles because they are used less frequently, but large amounts of glucose is required for when they are active.
Slow twitch muscle fibres have a high abundance of capillaries in comparison to fast twitch fibres. This is because the always need a means to access oxygen and fuel, as well as a quick means to get rid of waste products. Fast twitch muscles need fewer capillaries as they are in action less frequently.
Fast twitch muscles fatigue much more quickly than slow twitch muscles, largely because of their explosive use of fuel and lower density of capillaries (meaning they have a harder time getting rid of waste products too). Slow muscles however are required to be in use a large majority of the time and therefore they are required to fatigue very slowly, if ever.