I started doing Guerilla cardio about 3 months ago. I haven't lifted wieghts at all and my arms are getting bigger! Could this be from the guerilla cardio? I've heard that doing too much cardio over about 30-50min a day and your body will start taking away muscle. That is why marathon runners have the body types that they do vs. Sprinters. Is it possible that guerilla cardio has the INVERSE effect and you actually BUILD muscle? It seems like it makes some sense because during a sprint you have maximum intensity on your muscles and maybe its not weight per se, it still works your muscles enough to cause hypertrophy. Just a thought.