Robert Downey Jr. was asked to add 20 pounds of muscle in 5 months for his role in Iron Man. His trainer had him use a machine called the Vortex, which has several cables, pull up bar, dip bar, etc. This made it easy for Downey to quickly switch exercising one muscle group to another.
Another machine he used is called the Jacob's Ladder . It's a cardio machine that is a revolving ladder as you climb. Also, he used a machine called the Pineapple which is a vibration platform used to exhaust the muscles when exercising. I'm also assuming Downey's trainer had him working out with general weights as in another interview he remarks that he changed Downey's workout from the traditional heavy weight training.
He was down to 151 pounds when he did Sherlock Holmes so he need to bulk back up to 175 lbs for Iron Man II. Feeling kind of bored with the same old routine Downey's trainer put him on a less traditional workout. He had Downey using bamboo bar instead of a regular bench press. They would hang plates on rubber bands off the bar. His trainer said it was like bench pressing a snake. It bounced all over the place.
Downey also pushed a custom-built wheelbarrow modified with 600 - 700 lb weight stacks around an obstacle course. The workout targets the chest, shoulders, and back. Downey also would beat tires with sledge hammers and drag fire hoses around that were filled with sand and water to build up his shoulder muscles and pecs. He also juggled weighted Indian clubs for shoulders, hulled around kettle bells, and used a war machine. No, he didn't carry around the War Machine suit of armor from the movie. His trainer said, it is a rope with handles on it with a pulley you suspend yourself from.
Robert Downey Jr. at 151 lbs for Sherlock Holmes
His trainer filled a sprinter's sled with 50-pound weights and tied it to the end of a heavy, 50-foot rope. "Robert would stand stationary and then pull it to him, drop it, then sprint 50 feet away again. He would pull, run, and pull again," his trainer Brad Bose said. The full-body exercise especially targeted the glutes, lats, rhomboids, biceps and triceps, as well as the torso, abs, and core.
To round it off Downey also trained in kung fu two to three times a week. Hey what's a superhero who doesn't know kung fu.
The biggest lesson to take here is the enormous benefit to your muscles to keep them guessing. Changing a workout around on a regular basis keeps your muscles growing. So if you get stuck without any muscle gains or weight loss. Change things around. Also another important principle Downey presented by his change in workout routines is that it pays to never get bored with a workout. By changing things around it made it interesting, and that's extremely important to keep yourself committed to your workout and putting forth your best effort.