Why are some muscle fibres fast and others slow?

Because different jobs need different muscle. Your skeletal muscles are built from two broad kinds of fibre, slow ones called Type I and fast ones called Type II, and each muscle holds a mixture suited to what it does. Fast fibres contract fast but tire more quickly. Slow fibres contract more slowly but are better at keeping going.

The difference is in how they make energy. Slow fibres use aerobic respiration, meaning they burn fuel with oxygen to produce ATP. They look red because they hold a lot of myoglobin, an oxygen-storing pigment, and they tend to have more mitochondria, the cell's power plants, and more capillaries, the tiny blood vessels that feed them. That makes them slow to fatigue and well suited to endurance.

Fast fibres lean on a short-term, anaerobic system called glycolysis, which does not need oxygen. They can contract and develop tension at two to three times the rate of slow-twitch fibres, but they fatigue more quickly than the others.

Most human muscles contain all of these types, in different proportions depending on the muscle's job. In humans, the quadriceps in your thigh are about 52 percent slow Type I fibres. The soleus, a calf muscle, is about 80 percent. The small muscle around the eye is only about 15 percent.

Sedentary adults and young children have about 55 percent type I and 45 percent type II fibres. Athletes' mixes tend to tilt towards their sport: endurance athletes show more Type I, while sprinters require large numbers of the fastest fibres, type IIX.