WASHINGTON — The U.S. economy flexed its old muscles in 2014.More than five years removed from the Great Recession, worries had taken hold that perhaps the world’s largest economy had slid into a semi-permanent funk.Consumers, businesses and investors, after enduring a brutal winter, showed renewed vigor as the year wore on and set the United States apart from much of the world. Stocks repeatedly set record highs. Employers were on pace to add nearly 3 million jobs, the most in 15 years. Sinking oil prices cut gasoline costs to their lowest levels since May 2009. Auto sales accelerated. Inflation was a historically low sub-2 percent.The U.S. economy proved it could thrive even as the Federal Reserve ended its bond buying program, which had been intended to aid growth by holding down long-term loan rates.All told, the United States remained insulated from the financial struggles surfacing everywhere from Europe and Latin America to China, Japan and Russia.So what explained the U.S. economy’s resilience this year?Economists say it largely reflected the delayed benefits of finally mending the damage from the worst downturn in nearly 80 years. Unlike past recoveries that enjoyed comparatively swift rebounds, this one proved agonizingly slow. It took 6½ years to regain all the jobs lost the recession — 8.7 million — far longer than during previous recoveries.