One of the reasons for US economic dynamism compared to the UK and Europe has been its openness to immigration. Current immigration into the US is 1.3 million a year. This is projected to rise to two million a year by the middle of the century. http://www.nytimes.com/2008/08/14/washington/14census.html?_r=1&hp&oref=slogin
Immigration into the US has been particularly strong from Latin America and Asia - two of the most dynamic sectors of the world economy which therefore strengthens links between the US and these key economic regions as well as boosting the US population and labour force itself.
The current trend of immigration into the US is beginning to surpass the great wave at the beginning of the 20th century, when the proportion of the US population born abroad reached its peak at 15 per cent in 1910. The proportion of the population of the US born abroad is expected to rise from 12 per cent today, to 15 per cent in 2025, to 20 per cent in 2050.