U.S. workers were more productive in the April-June period than previously estimated, while labor costs were unchanged from the first three months of the year, the government reported Thursday. The Labor Department said second-quarter productivity grew at an annual rate of 2.3 percent, up from an initial estimate of only 0.9 percent growth. Unit labor costs were flat in the second quarter, less than the 1.4 percent increase the department had initially estimated. The combination of stronger productivity and less of an increase in wages should provide assurance to the Federal Reserve (Fed) that inflation is not a threat. The revised figure for productivity—the amount of output per hour worked—was a reflection that economic output had been revised higher for the second quarter to a growth rate of 2.5 percent. Despite the increase, productivity growth has been weaker than during the recession and early stages of the recovery. It rose only 1.5 percent last year and 0.5 percent in 2011. In 2010 and 2009, productivity increased at annual rates above 3 percent, reflecting the fact that millions of Americans lost jobs as companies struggled to deal with a deep downturn. While output was down as well, the number of workers fell more, thus increasing productivity.