By Shasha Dai

A survey shows that U.S. midmarket companies are holding up well financially, but the confidence these companies have in the economy has begun to wane.

Private equity-backed companies have expanded their revenue and workforce faster than their nonsponsored peers, although executives from both camps expressed less optimistic views about the economy.

The most recent quarterly survey from the National Center for the Middle Market polled 1,000 senior executives at businesses with annual revenue of $10 million to $1 billion.

Surveyed companies reported an average 7.2% revenue growth over the trailing 12 months, up from 6.6% a year earlier.

As with surveys from the past few quarters, private equity-backed companies have expanded their revenue at a faster clip than their nonsponsored peers. For the most recent survey, private equity-backed companies expanded their revenue at 9.2% for the trailing 12-month period, compared with 6.2% for nonsponsored businesses.

Companies backed by financial sponsors also reported adding employees at a faster pace than nonsponsored companies. Private equity-backed respondents reported an expansion of 5.6% in their workforce during the trailing 12 months, compared with 3.8% for nonsponsored companies and 4.4% for the group as a whole, according to the survey.

Thomas Stewart, executive director at the center, said the revenue and employment growth rates among midmarket companies are noteworthy, given that the U.S. economy has expanded for more than seven years and is widely seen as nearing a downturn.

“This expansion has not run out of steam,” said Mr. Stewart. “The middle market has not run out of steam.”

How long the momentum will last, however, is an open question.

In the latest survey, 50% of the respondents said they are confident or somewhat confident about the global economy, down from 58% a year earlier; 69% said they are confident or somewhat confident about the U.S. economy, down from 73%.

Both private equity-backed and nonsponsor-backed companies were less confident about the economy going forward. The percentage of private equity-backed companies confident or somewhat confident about the global economy fell to 54% from 63%; the percentage for nonsponsored companies declined to 47% from 57%.

As for the U.S. economy, the percentage of private equity-backed companies who said they are confident declined to 68% from 71%; the percentage for nonsponsored companies went down to 69% from 74%.

Mr. Stewart said those confidence numbers were “very striking,” particularly because the survey was done before the Brexit vote. One could assume, said Mr. Stewart, that confidence in the economy could be further dampened by the U.K.’s decision to leave the European Union.

Still, Mr. Stewart said, concerns over the U.S. economy are somewhat baffling, given that the U.S. is deemed to be relatively insulated from the Brexit’s impact.

Mr. Stewart said that one possible reason for the changing outlook is the political uncertainty typical for an election year. General anxiety about the likelihood of another downturn may also have played a role, he said.

So far, the economic expansion has lifted all boats, with private equity-backed companies sailing a bit faster than their nonsponsored brethren. But perhaps these reduced levels of confidence may be an early sign that the cycle is about to turn.