Martin, 37, has taken up a boxing-based workout regime with an expert recommended by his rapper friend Jay Z, reported Daily Mirror.
"Chris is a big fan of boxing and loves nothing better than working out with a punch bag. Chris has been doing it for a little over a month now to get in shape for his forthcoming tour," a source said.
"He doesn't want ridiculous biceps, but he wants to build some definition in his skinny arms and improve his stamina," the source added.
The singer recently separated from his wife of 11 years, Gwyneth Paltrow.