Martinez is eighth Chelsea-Villa deal in four years

Chelsea have signed Argentina goalkeeper Emiliano Martinez from Aston Villa for £7.5m - the eighth deal between the two clubs in the past four years.

The 33-year-old, who won the World Cup in 2022, has moved to Stamford Bridge on a three-year deal.

Former Arsenal goalkeeper Martinez joined Villa in 2020 and made 256 appearances, helping the club win the Europa League last season.

"I'm delighted to be at this football club," Martinez said.

"Coming to Chelsea, for me and my family, was an easy decision to make and I'm just over the moon - I can't wait to get started.

'I want to be successful here. I'm someone who wants to win at everything I do and I want to bring that here to this football club because it's a football club that wins titles, so I think it's a good match."

Martinez's move to Chelsea comes after Nicolas Jackson's transfer the other way last week.

No two Premier League clubs have exchanged more players, on permanent or loan deals, in the past four years.

The relationship began after Chelsea's owners, BlueCo, took control of the club in 2022 and signed Carney Chukwuemeka from Villa for £20m that summer. Ian Maatsen (£37.5m, to Villa), Omari Kellyman (£19m, to Chelsea) and Axel Disasi (a £5m loan deal, to Villa) subsequently moved between the clubs.

This summer alone has brought four more deals. Morgan Rogers completed a £117m move Chelsea, while Alejandro Garnacho joined Villa on loan with a conditional obligation to buy in a deal worth around £43m.

Jackson then moved to Villa for £65m, before Martinez followed.

Since 2022, Chelsea have paid Aston Villa £163.5m for players. Villa have spent £102.5m on Chelsea players including Axel Disasi's loan fee. That figure could rise to £150.5m should certain criteria be met to make Garnacho's loan move permanent. Chelsea believe the terms are easily reachable.

Chelsea and Villa have done business before. Tammy Abraham, Ross Barkley, Danny Drinkwater, Ryan Bertrand, Steve Sidwell and Andy Townsend are among those to have moved between the clubs during the Premier League era, while John Terry joined Villa after leaving Chelsea.

But the recent volume of business between the clubs has not gone unnoticed elsewhere in the Premier League, at a time when both have had to navigate domestic and European financial regulations. One source told BBC Sport that a number of club executives have raised questions about the frequency of the dealings.

There is no suggestion, though, that either club have broken any rules.

BBC Sport has approached Chelsea and Aston Villa for comment.

BBC Sport has counted every permanent transfer and loan between Premier League clubs since the summer of 2022, restricting the analysis to the 15 sides who have been in the top flight throughout that period.

There is a tie at the top: Chelsea have done eight deals with Villa, and another eight with Brighton.

Newcastle United and Nottingham Forest are next on six, followed by Arsenal and Chelsea on five.

Chelsea feature in six of the 10 busiest club pairings, illustrating the scale of their player trading since the BlueCo consortium completed its takeover.

BlueCo's £4.25bn purchase of Chelsea ended Roman Abramovich's 19-year ownership and ushered in a period of significant squad turnover.

But Villa's close relationship with Chelsea also has a more specific explanation.

Chelsea co-owner Behdad Eghbali and Villa owner Nassef Sawiris have developed a close working relationship and have communicated directly over transfer business.

That relationship has spanned several transfer windows and included Rogers' move to Stamford Bridge this summer.

Chelsea did not have a clear run at Rogers, though - as Arsenal also wanted to sign him, and it was the player's decision to move to Stamford Bridge.

Uefa has the power to scrutinise transfers completed between the same clubs within a 45-day period. Garnacho's move to Villa - on an initial loan deal - came two days after Rogers signed for Chelsea. Jackson's permanent transfer went through 38 days after the Rogers one.

But a timeline on its own is not evidence that the deals were in any way related.

Villa, for instance, could point to their long-held interest in Jackson, who previously worked with head coach Unai Emery at Villarreal.

Chelsea, meanwhile, could argue their interest in Martinez was independent of previous dealings with Villa and would have been pursued regardless of the club he played for. The goalkeeper had attracted interest from Juventus earlier in the window, with a finger injury understood to have scuppered a move.

There are broader reasons why domestic transfers remain attractive, but money is often the biggest factor.

Premier League clubs generally have greater spending power than most teams elsewhere in Europe, meaning another English side may be among the few buyers capable of meeting the fee and wage demands of a selling club.

That can make domestic deals particularly attractive when clubs are trying to move on high-value players. Signing from within the league also reduces some of the uncertainty, with recruitment teams already familiar with a player's qualities and performances.

Yet even in a league where domestic trading is commonplace, the frequency of deals between Villa and Chelsea stands out.