While only half of an average person's muscle fibres are fast twitch, Olympic sprinters commonly have 80 per cent. The stars of the future are going to need even more to beat this year's champions. The oxygen-free, or anaerobic, metabolism of fast twitch muscles is rapid. It's also inefficient and quickly causes lactic acid build-up, which leads to muscle fatigue. But since male sprinters blast through 100 metres in less than 10 seconds, that isn't a concern.
Height also matters.
Longer legs produce longer strides, so the historic trend towards taller sprinters is bound to continue. To make the most of what they have, top sprinters overdevelop the upper leg muscles, especially the gluteus and hamstrings so they can produce short explosive bursts of acceleration. They also run differently to most longer-distance runners. A college level sprinter, for instance, pushes off the ground in about 0.12 seconds, while top Olympians do it in 0.08 seconds. Over the course of a race of 50 paces, maintaining that difference on each step shaves two seconds off their times.