Premier League: Why has it become so hard to defend the title?

Manchester City smashed record after record in winning the Premier League title last season.

But will they be able to retain it? When the English top flight returns on Friday, Pep Guardiola's side will attempt to do what no team has managed since 2009 and record back-to-back title triumphs.

Prior to 2009 the Premier League was defended on seven occasions but in the subsequent nine seasons no team has successfully achieved the feat.

In that time Bayern Munich won the Bundesliga six times in a row, Barcelona claimed back-to-back Spanish titles on two occasions, Juventus won Serie A for seven straight seasons and Paris St-Germain lifted the French title four consecutive times between 2012 and 2016.

So why is it different in the Premier League? BBC Sport has identified some key areas which might be holding teams back.