Inside the Wild West of football's private trainers: Bitter rows with clubs over freak injuries in behind the scenes sessions, the jaw-dropping tales of unqualified 'cowboys' who get it all wrong and why 90 per cent of stars still swear by them

‘I'm really angry about that,’ Preston boss Paul Heckingbottom fumed.

The season was less than a week old but Heckingbottom’s patience was already wearing thin. This wasn’t about a dodgy refereeing call or the sale of a key player though, but an increasingly contentious issue in football that one source described as a ‘minefield’: the rise of private trainers working with footballers.