Jake Gyllenhaal muscles up for Southpaw

Jake Gyllenhaal in Nightcrawler (left) and Southpaw (right).

He shrunk himself down by 10 kilos to play an ambulance chaser in Nightcrawler.

But Jake Gyllenhaal has muscled up for his next role - a ripped boxer in Southpaw.

The first image from the Antoine Fuqua-directed film has been released to Deadline, with Gyllenhaal showing off an impressive physique as he tenses up in a boxing ring.

For Nightcrawler, which is screening in theatres now, Gyllenhaal told People magazine he lost weight because his character was "literally and figuratively hungry".

"It's not different than getting into character for anything. It's more about believing where you are and being present where you are. Who's to say what the process is? I have a strange one ... but I love what I do."

Jake Gyllenhaal in the movie Southpaw.

When asked to share his weight loss tips, Gyllenhaal told People: "I just love this movie. That's how I did it. There really was no technique."

Deadline reports Gyllenhaal put on 7 kilos of "pure muscle" for his role in Southpaw, which sees him play Junior Middleweight Champion Billy "The Great" Hope, a boxer on the comeback trail.

It's due for release in 2015.

- nzherald.co.nz