Not to disagree with the callus analogy, I know that, at very least, you do toughen or even thicken fibers through resistance training, but I wonder if you interpret the following study the way I do. If 'anabolic stimulus' is synonymous with protein synthesis, it seems to cast down on the connection between it and muscle growth:
I'm leaning more and more to the theory that muscle growth has more to do with mechanical stress (and less to do with nutrition) than is commonly believed.
My son experienced the glories of juvenile muscle growth the last year-and-a-half. He actually reduced calories, lost five pant sizes ... and his weight essentially didn't change, leading to the assumption that he lost a lot of fat and gained a lot of muscle pretty much simultaneously (probably more circuitous than that). Point is that he sure didn't eat for gaining twenty pounds of muscle. My surmise is that he had it in him and the strength training brought it out.
He's not quite the ectomorph his dad is. But I remember doing Nautilus, in college, and probably gained all the muscle due me in a few short months. And I don't remember eating for that either. Certainly didn't take a protein supplement.