Under Armour is looking for people who take the initiative in their personal and professional life. A person who believes in themselves doesn't wait around for something to happen that will motivate them. For example, you could say that you decided to try Crossfit. Talk about what you have learned and how it has trained you to focus and challenged your physical strength. Maybe you enrolled in training to help advance to the next step in your career. Get specific! Think about your goals and how you want to grow in your professional life. Even though they are asking about personal growth, you can still direct your answer towards ways it will help you at work.