Celebrity Workout With Kellan Lutz

To stay in shape after The Twilight Saga: Eclipse, Lutz starts every workout with 5 minutes of cardio and follows it up with core work. "Activating the core maximizes muscle utilization in every other exercise," McDonald says.

Core work makes your body ready to rock. After a few sets of powerful full-body movements, Lutz and McDonald do simpler exercises that target specific muscles, letting their bodies recover a bit before another blast of energy and muscle-fiber recruitment.

Here's a game Lutz plays: Spread out a deck of cards, pick a card, and guess its color. If you're right, your partner must do the number of pushups displayed on the card. If you're wrong, do double that number in crunches. Go through the deck and you'll both do about a combined 300 crunches and push-ups.

When Lutz first began exercising in high school, he'd do crunches for hours every day. Wrong move: Those muscles need time to recover. "I saw much better results when I hit my abs harder and less frequently," he says.

Grow Proportionally Lutz used to focus on his chest and arms, and ignored his skinny legs--but that made him awkward and out of balance. "You can't just build a huge chest and arms. You need to work out your body evenly," he says. "Otherwise, A, you'll look weird, and B, it's not the best for your body."