My arms have always been skinny but surprisingly not weak. I would like to work out on my arms more often but I'm a cyclist and that's what I choose to do. I ride and ride and then ride some more. Is it unhealthy? I don't think so. Maybe an imbalanced workout but my heart rate is low and I feel rather darn good. Isn't that important. If I was to work out my arms, they get more tone, but I really don't really think my arms would gain serious mass.