To prepare for his film role as a boxer, Jake Gyllenhaal gained almost seven kilos of muscle.

In a recent Southpaw trailer, the Hollywood actor shows off shredded abs and bulked-up biceps.

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His transformation was slow, taking six months, his trainer Claybon has revealed.

“Gradually we built up, day-by-day,” the trainer told Yahoo News.

But after losing 13 kilograms for his role as a scrawny journalist in Nightcrawler, the actor’s change was stark.

Here’s how Gyllenhaal, 34, got huge.

The gruelling Southpaw workout

Morning

One thousand sit ups

Three hours of boxing

Night

Another 1,000 sit ups

A further three hours of strengthening, conditioning and cardio