For his starring role in Iron Man 2, Robert Downey Jr. put on 20 pounds of muscle in just one month using DIY gym equipment. (I'm surprised it was necessary for him to bulk up at all, given that his body's hidden under a big metal costume much of the time.)
Examples: "Instead of doing regular bench presses, we used a bamboo bar... and we would hang plates on rubber bands off the bar... so it was like trying to bench press a snake!" [trainer Brad] Bose explains. "It bounced all over the place."
Downey Jr. also pushed "a custom-built wheelbarrow that we modified with 600 to 700 lb. weight stacks" around an obstacle course.
"We went out and bought truck tires, and we used sledgehammers -- anywhere from, two 4-lb. sledgehammers in each hand, all the way up to a full 20-lb. sledgehammer -- and we were just beating tires with it. We used fire hoses that we filled with sand and water... and we'd drag and whip them to build the shoulder muscles and pecs back up."
They also juggled "weighted Indian clubs," hauled around kettle bells and even used a war machine, which Boyes explains is "like a rope with handles on it with a pulley -- you suspend yourself from it," says Bose.
"We tried to find the most unique, and yet funnily enough, inexpensive training stuff to use," he says. "You couldn't find more practical stuff to use."(Source: US Weekly)