The reason I say that the first timed footrace was a seminal event is that runners run differently when they are timed than they do when they are competing against others or running as hard as they can alone without timekeeping. So the advent of timed races created a whole new way of running. Specifically, it made runners run faster. How so? Well, before the advent of timekeeping, a runner could do no better than run faster than everyone else in a given race.
After the advent of timekeeping, runners could aim higher: They could try to cover a given course or distance faster than anyone had ever done before.
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And while only the fastest of the fast could aim so high, runners of all ability levels could now try to run faster than each individually had ever done before. Indeed, before timekeeping there was not much point in racing if a runner was not capable of winning (which is probably why most races in eighteenth-century England were one-on-one matches staged for betting purposes).
But with timekeeping, anyone interested in running had a motive to run hard. The reason that runners are generally able to run faster in timed than in untimed races has to do with how the brain regulates running performance. As we have seen, in events in which a runner wishes to complete a defined running task in the shortest time possible, the brain's job is to ensure that the runner completes the task in the shortest time possible without any serious harm.
Through communication with the rest of the body, the unconscious brain has a sense of the absolute physiological limits of the muscles and other organs and tissues. During hard running it monitors the proximity of the various physiological systems to their ultimate limits. As necessary, the brain acts to prevent these limits from being reached by reducing muscle activation and by making the runner feel miserable.
This mechanism is not robotically exact and consistent. While the brain never allows the body to reach its true performance limits, exactly how close it allows the body to get depends on the details of the situation. The presence of competition is one situational detail that typically enables runners to come closer to running themselves to death, hence to run faster.
Timing is another, and it works in basically the same way: Running against time is a competition against a device or against yourself or against nature or against other runners indirectlydepending on how you look at it.