For strong, defined arms you'll want to show off, try doing tricep dips. This exercise will tone the muscles on the back of your arms, from shoulder to elbow. Watch the video to see Health contributing editor Cynthia Sass demonstrate how to do this move.
Here's how to do it: Sit down your mat with your legs straight in front of you, and heels on the floor. Place your hands behind you, right behind your butt. Engage your abs and bend your arms so your butt just taps the mat, then straighten them. That's one rep. For an extra challenge, lift one leg as you dip. Do 15 reps.
Try this move: Tricep Dips