Serena Williams is famous for both her fashion and serious upper-body strength on the court. But you can get arms like hers without touching a racquet.
Try Plank Rows, says Nike master trainer Marie Purvis, who helped Williams create her workout in the Nike+ Training Club app (free; iTunes and Google Play). "You're sculpting your arms as well as your shoulders, abs and legs," she says. Do it 3 times a week after cardio for sexier guns in a month.
Start in plank, with your toes tucked under, and a 5- to 20-pound dumbbell in each hand (A). Keeping your hips and chest face down, engage your abs and lift left dumbbell toward your chest, bringing your elbow behind you (B). Lower to start and switch arms. That's 1 rep. Do 20 reps.