David and Mary Heimerdinger are not poor or out of work. He's a Clinton, Mich., farmer with 300 head of dairy cattle; she's a public school teacher. Together they earn about $40,000 a year.

Yet, like millions of American consumers made skittish by the recession, they just don't spend like they used to.

Last year the Heimerdingers decided not to build a garage, not to hang wallpaper and not to put up dry wall in their house. They stopped carrying $600 or $700 of credit-card debt from month to month.

Mary Heimerdinger stopped playing racketball at a private club and quit taking flying lessons. "I get more comfort from knowing I have money saved," she says. "When times are good I don't think, I just buy."

For more than a year economists have been waiting for the Heimerdingers of America to stop fretting and start buying. What keeps them from spending money? What might induce them to use their credit cards more? How great is their pent-up need for a new car or desire for a microwave oven or a Caribbean vacation?

These are some of the imponderables of the worst recession since World War II.

It is hard to overstate the importance of these questions to the American economy. Consumer buying in this country accounts for about two-thirds of all economic activity. When consumer buying jumps sharply and remains high, the recession is over.

It's been nearly a decade, however, since consumer spending was so consistently strong that it could be counted on to smooth out wrinkles in the national economy. The University of Michigan's national survey of consumer attitudes shows a long-term downward trend in consumer optimism since 1973.

The usual suspects for economic malaise--inflation and unemployment--have in the past decade soured consumers on the future, according to Richard T. Curtin, director of Michigan's Survey Research Center.

"In the 1950s and 1960s consumers more frequently reported improvement in their personal financial situation than deterioration. From 1973 to 1982, however, consumers more frequently reported that their financial situation had worsened rather than improved," Curtin says.

All this, Curtin says, means that "rather than a source of satisfaction and confidence, the economy since the early 1970s has become a source of discontent and uncertainty."

While American consumers may be uneasy, they nevertheless spent more last year than the year before. Often overlooked in this recession--amid post-Great Depression records for joblessness, bankruptcies and unused factory capacity--are statistics showing that consumer spending, while hardly robust, has not nose-dived. In the past year and a half, with the economy bedeviled by high interest rates, inflation-adjusted consumer buying increased in all but one three-month period.

Compared to the consumption-crazy '60s and early '70s, the spending growth has been slow. But that weak growth, economists say, has helped ward off economic collapse.

In 1983, with the Federal Reserve Board having triggered lower interest rates, there are strong indications that growth in consumer spending will increase.

Sales of new houses increased dramatically last fall. November's 12 percent monthly increase was a stunning 47 percent ahead of sales in the same month of 1981. New housing starts in November were up 26 1/2 percent--a 22-month high. On the strength of pent-up demand and mortgage rates in the 14 percent range, home builders and real estate agents from the Sun Belt to the Frost Belt reported in October and November that sales were up 20 to 50 percent.

Automobile sales, which fell last year into their worst slump in 20 years, rebounded sharply in the final months of 1982. With cut-rate financing luring buyers, sales were up 10.6 percent in November and 24.3 percent in December, compared with similar periods in 1981. Pent-up demand is unprecedented, experts say, because America's auto fleet is older than it has been in 30 years.

Finally, the crucial Christmas spending season, which each year accounts for more than one-third of consumer buying, showed modest improvement over recent sluggish sales trends. Sears, Roebuck & Co., the nation's leading retailer, announced at the end of the year that "it is clear to us that the recovery in durables such as washers, driers and refrigerators and home furnishings has started. Our sales of appliances were strong throughout the fourth quarter of 1982."

Part of the incipient recovery, undoubtedly, is due to lower interest rates and the inexorable swing of the business cycle. When consumers hold off buying for a long time, some of their possessions wear out, and they have to replace them.

Credit for the growth of spending last year and the recent upturn should also be given to the surprisingly optimistic attitudes of consumers not personally hurt by the recession, according to Jay Schmiederskamp, director of the Gallup Organization's economic survey.

"Given that we are in the worst soup since the Depression, consumers have held up their end of this thing remarkably well," says Schmiederskamp, an economist who has been polling consumer sentiments for 25 years.

About 70 percent of the work force is not unemployed, has not been out of work recently and does not fear imminent job loss, Schmiederskamp says. "For these people, the reduction in inflation is good news, and they are not worried about unemployment. We have had so much bad news for so long that people have developed a tin ear," he says.

A Gallup poll in October, during a time when politicians and the press were making a great deal of noise about double-digit unemployment, found that 31 percent of those questioned were optimistic about their own personal financial future while 28 percent were pessimistic. The rest either had no opinion or saw no change. Three years earlier, a similar poll found 28 percent optimistic, 34 percent pessimistic.

"This is not what you would expect. In spite of this terrible recession, the American people have tended to remain optimistic about their personal situation," Schmeiderskamp says.

One sharp change, however, for both working and jobless consumers has been a refusal to shoulder increased debt.

Like the Heimerdingers in Michigan, who've stopped carrying debt from month to month on their credit cards, consumers across the country--including those with high incomes--are in no mood to add to their indebtedness.

"I know I can survive some catastrophes if I have money saved," says Mary Heimerdinger. "It is easier for me to budget without the credit cards hanging over our heads. If I pay with cash, I know the money I have. It is more secure."

Last year, according to preliminary estimates, consumer debt did not increase. The about-face in spending habits is seen by looking at 1978, when consumers ran up their installment and mortgage debt by $135 billion.

Instead of making down payments on big-ticket items, some Americans put more into savings. Michigan consumer surveys in the third quarter of last year showed that 72 percent of all families were reluctant to use savings to make large purchases. That's up from 54 percent just two years earlier. The Commerce Department says the savings rate last year was about 6.6 percent of after-tax income, compared with 6.4 percent in 1981.

This increase was not as great as some economists expected, considering the severity of the recession. The savings rate, in fact, declined at the end of the year. This decline, combined with increased sales of houses, cars and some expensive merchandise, may signify diminished consumer wariness about the economy.

Yet, even if consumers are getting less uneasy, recent history indicates they will not increase their spending or indebtedness until their earnings increase, according to economic consultant Peter L. Bernstein.

"When people perceive that their wages are down, they don't spend money," says Bernstein, a consultant to Morgan Guaranty Trust Co. and the Ford Foundation. He points to figures showing that the increase in average hourly earnings in November 1982 was 5.6 percent compared with 10 percent in the same month of 1980.

Bernstein recently studied the relationship between wages and consumer spending from 1953 to 1981 and says he discovered a strong correlation. "The burden of proof is on those who argue that consumer spending can jump upward spontaneously in advance of an improvement in the labor market," he commented.

It's possible to argue that when inflation slows down--as it did so dramatically in 1982--that wage increases can also slow without reducing consumer spending.

The Michigan survey of consumer attitudes, however, found that the victory over inflation may have harmful side effects on consumer spending.

In the late 1970s, with inflation raging, 30 to 40 percent of consumers said they were buying cars and houses to avoid paying higher prices later. In the third quarter of last year, that buy-in-advance rationale was mentioned by less than 10 percent of consumers.

Ironically, then, slowed-down inflation may well be lulling consumers into feeling they can afford to postpone the purchases that would rev up the economy.