Track and field athletes have made nowhere near such consistent progress — with the exception of a few relatively obscure events like racewalking. In short-distance running events, for instance, medal-winning times have fallen by only about 2 percent over the 40-year period. Long-distance runners have made slightly more progress — about a 6 percent improvement since 1968 — but still much less than the swimmers.

The field events that make up the other half of the athletics completion have been a mixed bag. Although there has been a lot of progress in the high jump and the pole vault, the trend has actually been negative in some other competitions. The woman who won the shot-put competition in Beijing, Valerie Vili of New Zealand, would not, with her tosses, have won even a bronze medal at the 1976 Olympics in Montreal.

What accounts for those differences?

Part of the answer is simply that swimmers have benefited more from technology, in the form of everything from sleeker, computer-designed Speedos to deeper (and, for the swimmer, less turbulent) pools. But there is little a short-distance runner can wear to help improve her performance much, although the Nike and Reebok commercials might suggest otherwise.

Another factor: an athlete with the perfect swimmer’s build and a world-class work ethic would still stand little chance of competing in this year’s games if he happened to be born in a poor nation like Cameroon or Panama — he might never have gotten into a pool, let alone an Olympic-size one. But running, especially over short distances, can be practiced virtually anywhere and anytime.

Which leads to this: As Stephen Jay Gould noted, the more open to competition a sport is, the harder it may be to break records or to post extraordinary statistics. The .400 hitter disappeared in baseball once the color barrier was broken, and black Americans and players from Latin America were allowed to compete in the major leagues. This raised the average level of performance — but also made it harder for any one athlete to stand out quite as much relative to his peers.

In the track and field events, it is more likely that an athlete has already come close to what Gould called the “right wall” of human performance, simply because the human being who possessed the ideal build and work ethic is more likely actually to have competed in the Olympic Games.

This is not to diminish the accomplishments of Phelps or Lochte — and I’ll be rooting for them. But whatever records they set may well be broken in Rio in 2016, if not sooner. I wouldn’t put money on anyone out-jumping Beamon or outrunning Griffith-Joyner any time soon, however.