HAVE YOU EVER looked at a guy with skinny arms and thought, “He must just be too busy to work out?” Of course not. That’s because no matter how short on time you are, you can train hard and build muscle—even if you’re the guy with skinny arms we’re referring to. The following no-excuses routine alternates biceps and triceps exercises, and manages rest periods so you can load your guns in only 15 minutes.