The United States economy grew at its fastest pace in more than six years at the end of 2009, even as businesses resisted hiring and continued to do more with less.

The broadest measure of economic activity, gross domestic product, expanded at an annual rate of 5.7 percent in the fourth quarter, after a 2.2 percent increase the previous quarter.

“It was an excellent report, but it’s not clear how sustainable this pace of growth is,” said John Ryding, chief economist of RDQ Economics.

The growth rate was the fastest since the third quarter of 2003, when the economy grew at a rate of 6.9 percent. But even 2009’s fourth-quarter surge was not enough to overcome a terrible start to the year. The economy finished 2009 with its biggest contraction since 1946, when the country was still cooling off from World War II.