Personal trainer Mark Jarvis has been putting the 'Best Song Ever' heartthrobs through their paces in the gym and has seen some impressive results in the lads since working with them.

Jarvis's manager, Ramzy Khachnik, told Yahoo OMG! that singer Liam Payne has the best body in terms of sculpting and muscle definition although Harry Styles has made some serious improvements to his body:

"Harry hasn’t got a Brad Pitt body yet, but Liam has. Liam’s got the best body, a washboard stomach now. He’s the one responding the best and enjoys it the most."

Referring specifically to Styles and his lean physique, he added: "Mark says Harry’s grown into a man. He’s shed his puppy fat, his body has really changed... but he’s not ripped. Mark says his shape has changed completely, he’s harder and more muscular."

"But Harry’s never going to be completely ripped, its harder for his body type than others, he’s more fleshy than Liam. Mark says he’s lost body fat and gained muscle and his shoulders and chest have filled out."

Watch the boyband's 'This Is Us - Sky Movies Special' feature below: