The crowning of Affirmed as Horse of the Year marks the end of an extraordinary decade. Probably no 10-year period in the sport's history produced so many authentic thoroughbred superstars as did the 1970s.

Not only Affirmed, but Secretariat, Seattle Slew, Forego, Spectacular Bid, Hoist The Flag and Ruffian dominated their own equine generations. Today we may take for granted the development of such great horses, but 10 years ago such a succession of champion would have been almost inconceivable.

In the 1960s, not a single horse had captured the 2-year-old championship and gone on to win the next year's Kentucky Derby. From 1949 to 1972, not a single horse had swept the Triple Crown. And there was a logical reason for the rarity of such dominant thoroughbreds.

Man O'War had been one of fewer than 2,000 thoroughbred foals in 1917. When Citation won the 1948 Triple Crown, he proved himself superior to the 8,400 other 3-year-olds in the country.

But now, in the 1970s, the size of each year's thoroughbred crop wass approaching 30,000. There was so much more competition that the emergence of dominant racehorses was becoming less and less likely.

That reasoning seemed valid in 1970; it is just as valid today. What happened in the decade that is about to end was probably a fluke of nature. We should savor it, not expect to see anything like it in the 1980s. (Given proper odds by a bettor with a sufficient life expectancy, I would be willing to wager that no horse will win the Triple Crown in the next 10 years).

Yet as rich as the 1970s have been, I have no difficulty making my choice of its greatest horse, and its greatest moment.

Secretariat was the Horse of the Decade.

Having become a student of speed handicapping in 1970, I have measured the running times of all the great horses, and I know for sure that Secretariat was the fastest horse of the era. On an average day (for him), he could have whipped all the other champions of the decade at their very best. h

But a racing fan in 1972 and 1973 did not have to understand the nuances of speed handicapping to realize that Secretariat was doing things that no other horse could. As a 2-year-old at Saratoga, he circled a field and went from last place to first place around a turn, running a quarter-mile in less than 22 seconds. In the Kentucky Derby he ran every quarter-mile faster than the one before it.

And his 31-length victory in the Belmont Stakes was the great performance of the decade, maybe the greatest race any thoroughbred has ever run. His time of 2:24 for 1 1/2 miles is the most unassailable mark in the sport.

No horse can measure up to the standards of greatness that Secretariat set, but the one who came closest in the 1970s was Spectacular Bid. If he had not hurt himself on the morning of the Belmont Stakes, and if he had not challenged Affirmed at an inopportune time, he might be getting the recognition he deserves.

Ruffian, too, might have proved she was in the class of Secretariat if a fatal injury hadn't cut short her career.

With the memory of these horses, plus Forego and Seattle Slew, so fresh, I find it difficult to join in the chorus of acclaim for Affirmed. He was a very good horse, of course, but also a very fortunate one.In almost all the major races of his career, he had the advantage of being the only front-runner in the field. On the two occasions last year when he did face another tough speed horse, Seattle Slew blew him off the track.

If he had reached maturity around 1970, when such animals as Arts And Letters, and Fort Marcy were being voted Horse of the Year, we would probably be hailing Affirmed as a true champion of champions. It is a measure of the exceptional nature of this decade that a thoroughbred can win 22 of 29 lifetime starts, earn championships in three straight years, win more money than any horse in history and still not seem to be particularly special.