When a sprinter covers a hundred metres in under ten seconds, almost every visible feature of the performance - the explosive start, the smooth transition to full speed, the driving finish - is shaped by biological factors whose details became clear only in the last thirty years. Coaches had long known that sprinters were born with advantages that training could refine but not create. Modern sports science has been able to specify some of those advantages and, in some cases, to separate the effects of genetics from those of preparation.

The first and most widely discussed factor is the composition of the muscles themselves. Human skeletal muscle contains a mixture of fibres, broadly classified as slow-twitch and fast-twitch. Slow-twitch fibres contract relatively slowly but can sustain activity for long periods without fatigue, making them important for endurance events. Fast-twitch fibres contract rapidly and powerfully but tire quickly. Most adults have a roughly even mix of the two, while distance runners are biased towards slow-twitch. Elite sprinters, by contrast, have been found on biopsy to carry an unusually high proportion of fast-twitch fibres, sometimes more than seventy percent. This proportion changes only slightly with training.

A second factor concerns the stretch-shortening cycle, the mechanism by which tendons and muscles behave like springs. When a sprinter's foot strikes the ground, the Achilles tendon is briefly stretched and then rapidly shortens, releasing the stored energy into forward motion. Elite sprinters appear to have longer and stiffer Achilles tendons than recreational runners of the same height, allowing them to store more energy and release it more quickly. Sports physiologist Dr. Tendai Moyo has shown, through high-speed imaging at the University of Cape Town's movement laboratory, that the measurable difference in foot-contact time between world-class and club-level sprinters is only a few thousandths of a second, but that this difference accumulates into the gap observed over a full race.

Nervous system efficiency is a third factor. Producing maximum force in a short time requires the nervous system to recruit a large number of muscle fibres almost simultaneously. Laboratory measurements of reflex speed, grip strength and reaction time suggest that elite sprinters are very good at this coordinated recruitment, but that training plays a larger role here than in muscle composition. Over months and years, the connections between motor neurons and muscle fibres become more reliable, and athletes learn, at a level below conscious thought, to produce near-maximum force on demand. This aspect of sprinting responds measurably to practice.

A fourth factor is stride pattern. Casual observers often assume that faster sprinters take longer strides, but video analysis shows that the fastest athletes generally take more strides per second at similar stride lengths. High stride frequency requires rapid coordination of hip and knee flexion, and it places considerable demand on the lower back and pelvis. Coaches emphasise drills that strengthen these regions, because weakness there tends to show up as small inefficiencies in the stride that add up over a race.

There are limits to how much these factors can be changed. A child born with a high proportion of slow-twitch fibres and a short Achilles tendon is unlikely to reach the elite level in sprinting, regardless of training; a child born with the opposite profile may or may not, depending on coaching, nutrition and a great many other variables. Dr. Moyo cautions against treating any of the measurable differences as destiny. Her recent work suggests that late developers - athletes who begin serious training in adolescence rather than childhood - can still achieve world-class performances, but only if their underlying physiology is favourable.

The public appeal of sprinting lies partly in its simplicity. Two people run a fixed distance, and the first to cross the line wins. Behind that simplicity, however, is an unusual amount of measurable detail, and advances in biomechanical imaging, genetic testing and training science have made the detail visible. What remains stubbornly hidden is the moment in which an athlete's preparation meets their day. On a particular afternoon, in a particular stadium, the physiology that years of study have described explains only so much; the rest, as athletes themselves tend to say, is simply running.