When you think about sexy, strong arms, your mind probably goes straight to your biceps and triceps, right? Well, those muscles are awesome — no shade at all over here — but the truth is that exercises that isolate just the biceps or triceps are simply not the best way to go when it comes to strengthening your arms. Might we suggest shifting your focus to the shoulders, as you'll do in this 30-day challenge?Here's why: For one thing, shoulder exercises tend to be more dynamic and involve several muscle groups (unlike, say, bicep curls, which primarily work, um, the biceps). In other words, when you work your shoulders, you're probably going to be challenging the rest of your arms at the same time. So efficient.For another thing, strong shoulders help provide an amazing foundation for beautiful posture, and for less pain in your neck and back. They're also a crucial way to prevent injury, and shoulder injuries are way more common than you might think. In fact, about 7.5 million people visit the doctor for a shoulder problem each year, according to the American Academy of Orthopaedic Surgeons.Which brings us to one word of caution: As with any exercise program, it's important to take things slowly and make sure you fully understand the proper form for each move before challenging yourself with heavier weight (you can even try the moves sans weight for a round or two, for practice!) — and since shoulder injuries are so common, just be careful, y'all.To give this challenge a shot, you'll need at least two sets of dumbbells: one lighter (think 2-3 pounds) and one heavier (think 8-12 pounds). And that's pretty much it! Get ready to give weak arms the cold shoulder.