Back 100 million years or so when people mainly spent their time chasing down food, running away from hungry carnivores, attacking (or being attacked by) neighbors, digging in the earth and, one assumes, generally keeping active, their life spans may have been nothing to brag about, but their cardiovascular systems were surely in great shape and, oh, the strong backs those cavemen must have had.

Now, of course, a lot of the things that tended to keep Alley Oop and friends from reaching a comfortable old age -- man-eating tigers, deadly microbes, poisonous snakes, exposure, starvation and other prehistoric exigencies -- have been more or less eliminated as daily threats to life.

What we have now, sighs Dr. Richard O. Keelor of the President's Council for Physical Fitness, is "the desk and the swivel chair," which Keelor says may be an even greater occupational hazard than coal dust to a miner, simply because of the numbers.

"The human body is designed for and requires regular exercise to maintain itself, and if it is not subjected to daily doses of physical exercise then it tends to deteriorate," said Keelor. "It's all summarized by a handy adage: You either use it or lose it."

Even as recently as 1890, he notes, Approximately 85 percent of all productivity in the farms and factories of our country were supplied by muscle power. Today it's less than 1/2 of 1 percent."

The result is what Dr. Keelor calls "the plague of our age" -- degenerative diseases. These include heart attacks, high blood pressure, strokes and other vascular problems, lower back problems, obesity, and chronic fatigue.

It isn't only that this new plague is shortening our lives. It's causing the deterioration of our way of life, cutting ominously, dangerously, into the bottom lines of a lot of business and government agencies, having an adverse affect on the country's productivity and sending healthcare costs out somewhere past Voyager-2.

Here's are some numbers:

In 1979 General Motors spent more ( $825 million) on its employe health plan than it spent to buy steel from U.S. Steel.

The executive branch of our government lost 25 million workdays and $1.34 billion from absenteeism in 1976.

Why, you ask, doesn't somebody do something about it?

Keelor says we've gotten to the point where, "given the nature of disease today it is far more important what you do for yourself than what your doctor does for you."

But we've let ourselves (as a group) get so deconditioned, that we're long beyond being able to do it safely by ourselves, even if we would if we could. So the answer is that the proprietors of the work place, be it public or private, recognizing that good fitness is good business, are beginning to join with health community "to reintroduce people to their bodies."

Enter the American Association of Physical Fitness Directors in Business and Industry.

This outrageously named organization was formed about five years ago when a group of 25 fitness executives met under the auspices of the President's Council and "thought we'd have a nice little club to get together and meet once a year," recalls Keith Fogle, current president of AAPFDBI and fitness director of Prudential Life Insurance Co.

Today, Fogle reports in awe, the group has 900 members. Between 250 and 300 are businesses or agencies which have introduced some kind of fitness programs for their employes.

Members also include forerunners of a new American industry: The fitness-by-contract firm -- like Cardio-Fitness Center in New York City, whose clients (on behalf of their employes) include Time, Inc., Simon & Schuster, Dun & Bradstreet, Sperry Rand and a number of other midtown Manhattan firms. It and its ilk programs with elaborate and sophisticated equipment.

Another member is a midwestern YMCA which has 19 industrial clients for which it has tailormade fitness, and, in some cases, cardiac-rehabilitation programs. Other Ys in other places, including the Washington area, are rapidly following suit.

Industrial fitness projects range from Kimberly-Clark's $5-million medical-fitness recreation facility for employes and families and friends at its Wisconsin headquarters, to small classes on fitness on education or smoke-ender programs or cardiac-risk identification programs. In addition to AAPFDBI, Dr. Keelor thinks there must be "thousands more" budding programs.

Another statistic:

The cost in industry for recruiting replacements for executives who've had heart attacks is estimated at about $700 million a year.

And another:

A billion dollars a year is lost in output because of backaches, excluding Workmen's Compensation payments. Back problems are the single largest cause of absenteeism in this country.

Probably the most sophisticated federal government fitness program is at the Department of Transportation where volunteer employes are screened for cardiovascular risk factors, given stress tests under medical supervision at the in-house stress laboratory and work out at the Department gym or run around its own track -- on the roof. Its directors are positively gleeful about its acceptance and some individual success stories. It is likely to be joined soon by other agencies. NASA, the Justice Department and HEW also have programs.

The Metropolitan Washington YMCA, the Board of Trade and the President's Fitness Council will meet in mid-October to launch a major attack on the desk-bound of the Washington area. The YMCA'S Herman Gohn said this week that the Y is working with cardiologists from George Washington and Georgetown University medical schools to come up with fitness blueprints for area agencies and businesses. They will have a dozen or so pilot programs "to establish credibility," Gohn said, and are about to set up contract- fitness programs geared especially to this area.

"One of the problems about D.C." Gohn said, "is that there really aren't very many corporate headquarters here, so we have a unique opportunity in the field."

So far there is little statistical proof of the cost-effectiveness of these programs. Much too early to tell. But studies are underway and early signs point not only to physical well-being, but psychological benefits as well. Says Dr. Keelor: "When that shell we live in is lean and fit and muscular and strong it has to affect our outlook on life. . ."