Joining Damon on set is his personal trainer Jason Walsh.

"Matt is absolutely the best client anybody could ever hope for," Walsh told E! News last month at the opening of his new L.A. fitness studio Rise Nation. "He is hard working and he is funny as hell. So we keep it light, but we work really, really hard."

Damon also shows of his body in The Martian, his new film in which he plays an astronaut stranded on Mars when his fellow crew members believe he was killed during an unexpected storm on the red planet.

The Martian, which premieres at the Toronto International Film Festival on Friday, will be in theaters Oct. 2.

—Additional reporting by Holly Passalaqua