The turnover of chief executive officers in the United States has reached its highest level since the peak of the economic crisis in 2008.

According to executive recruiter Spencer Stuart, 43 companies in the Standard & Poor's 500 Index hired new chief executives during the third quarter of 2013.

Previously 2011 has seen a high turnover of top brass, but 2013 will record the highest rate since the financial crash.

Flagship companies such as Microsoft and Wal-Mart have changes their CEOs in recent months.

The increased changeability of chief executives is down to the prominence of technological advancements, the increased complexity of such roles, and competition from places like China, India and Brazil, reported Bloomberg.

In addition, pressure from activist investors also appears to have risen, with Proctor & Gamble, Chesapeake Energy Corp. J.C. Penny, and Sotheby's among those firms responding to demands for change.