Being or feeling helpless or vulnerable can represent: Actual feelings of helplessness somehow in your real life. Some aspect of your life where you feel vulnerable or like you could use some help. Losing, or a fear of losing, power or control somehow in your life. A focus on the unpredictable nature of life.
Because in your dream you are trying to run, but your actual body isn't moving. And your brain is interpreting the disconnect as you aren't able to run, scream or whatever it is you are trying to do in your dream. It's a conflict between your dream conscience, your subconscience, and your physical self.
Before you believe me in this one you should try it yourself! If you are sleeping in a position with your legs straight and happen to be dreaming that you are running, you will run fast in your dream too, but if you are sleeping in a position where your legs are bent then in your dream no matter how fast you try to run you will be in slow motion. So test out this theory and when you wake up from a running dream next time. also if you sleep with your hands on your chest or belly area you'll have nightmares, not sure if you'll believe that but it happens to me so I always sleep on my side with my hands tucked under my pillow ;D sweet dreams zZZZzzzZz