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.

Money talks - rivals outspend reigning champions

The Premier League's £5.14bn TV deal gives clubs unrivalled spending power compared with most of their European counterparts.
That means after failing to win the title, leading teams are able to strengthen significantly in the following summer to improve their chances in the next campaign.
In the nine seasons since the last successful title defence, by Manchester United in 2009, the defending champions have been outspent in the summer transfer window seven times by the team that went on to win the title.
The only exceptions were United in 2010-11 and Leicester in 2015-16, when the clubs spent just £21.87m and £34.38m respectively before winning the Premier League.
The overall trend suggests clubs have been guilty of standing still - in spending terms at least - and allowing others to overtake.
However, former Manchester United assistant manager Mike Phelan, who was at United when Sir Alex Ferguson's side won three consecutive titles between 2007 and 2009, said even during that period the club would never try to spend big on a significant number of players after a title win.
"When you get to that stage with a winning mentality and being champions, you have done something right - you have got a good group of players," Phelan told BBC Sport.
"What you need to do is back it up with one or two signings that would take you to the next level and change the dynamic of the team.
"It is about tweaking certain things. We would bring in the likes of a Juan Sebastian Veron, introduce one mega player."
Veron might not have worked out but that is far from an isolated case. Even finding that one difference maker has proved elusive in recent seasons...
In the last six campaigns, Pedro and Marouane Fellaini are the only players signed by the defending champions who have made a lasting impression on their club's first team.
As for Maicon, Jack Rodwell, Baba Rahman and Bartosz Kapustka, the less said the better.
A trend has emerged of Premier League champions recruiting the wrong players between winning the title and beginning their defence.
In the summer of 2017, reigning champions Chelsea spent £186.03m on seven first-team players, many of whom had a difficult season, including Tiemoue Bakayoko, Davide Zappacosta, Danny Drinkwater and Alvaro Morata, and manager Antonio Conte grew frustrated at being unable to sign his first-choice options.
They made a similar mistake in 2015 when Rahman, Kenedy, Nathan, Matt Miazga, Asmir Begovic, Falcao, Papy Djilobodji and Michael Hector joined alongside Pedro.