I tend to have a muscular build for a woman - so when I was doing my workout DVDs people often assumed I lifted weights due to the definition in my arms. It came from doing planks, pushups etc as part of my aerobic workout. Anyway, I always assumed there are genetic differences, but this is a write up of an interesting rat study.

http://well.blogs.nytimes.com/2015/04/29/why-some-people-get-fitter-than-others/

"The most interesting rat in the world" sorry, had to make the joke!!