Amid what is likely to become the longest period of sustained economic growth on record, a new report shows that millions of middle-class and low-income Americans still aren't on solid enough ground to weather a sustained downturn.

Since the Federal Reserve's annual report on household well-being began in 2013, the survey (most recently of more than 11,000 Americans) has become a key measure of whether the benefits of the recovery have reached beyond the upper end of the socioeconomic spectrum.

Although this year's report painted a positive picture overall, officials said, it identified underlying fragility and exposed pockets of distress. In line after line, the report lays out the everyday concerns that plague U.S. households.

Almost four in 10 people (39 percent) said they wouldn't be able to scrape together the cash to meet a $400 emergency expense. Even without any sudden expense, about 17 percent of adults said they would miss a payment on at least one bill during the month surveyed.

More than 6 in 10 said losing their job would mean they couldn’t cover three months of expenses, even if they took out loans, sold assets, or borrowed from friends and relatives.

Only 36 percent said their retirement savings are on track.

Almost a quarter of Americans skipped some form of medical care in the past year because they couldn't afford it. Separately, 1 in 5 faced major, unexpected medical bills. About 4 in 10 of those folks were still carrying debt related to those bills.

The survey covers 2018, when the unemployment rate averaged 3.9 percent, the lowest since 1969, and the economy grew 2.9 percent, matching its post-Great Recession high. Average hourly earnings grew 3 percent, easily the fastest rate since the recession's end. But those figures are broad national averages — if gains are going disproportionately to the wealthy few, trends among the majority of U.S. workers could be missed.

When David Moore, owner of D&D Automotive in Las Cruces, N.M., heard he will soon be living through the longest economic expansion on record, he sounded indignant.

"It's not expanding for me," he said. "It's getting worse."

Moore has been a mechanic for 40 years. He said these days, customers often have to leave their cars with him until payday rolls around, or until they can scrape together the money.

"I had three jobs this week that I lost because it's too much money," Moore said. "They hauled the cars off. They're not going to fix them."

"There's not any extra money laying around for a lot of people," Moore said. "I get sticker shock adding up tickets sometimes," he added later. "Everything's gone up."

In fact, when his son had to go to the emergency room last year, Moore couldn’t cover the $2,000 bill up front. He’s still sending the hospital $100 a month to pay off the bill, he said.

In the report, the authors wrote: “Another year of economic expansion and the low national unemployment rates did little to narrow the persistent economic disparities by race, education, and geography.”

In particular, measures of economic distress continue to spotlight black workers and, to a lesser extent, Hispanics. Only 47 percent of black adults rated their local economy as good or excellent in 2018, compared with 68 percent of whites.

Black Americans are less likely to be working and less likely to be satisfied with how many hours they're getting on the job.

The disparities are sharp even among Americans who attended college. About 28 percent of black respondents were behind on their student loans, as were 15 percent of Hispanics.

The number for white people was just 7 percent. The gap may be related to access to education — black Americans were more than five times as likely than whites to have attended a for-profit university.