There are indeed people who gain muscle slowly, but it's heavily related to diet. If someone simply can't eat much food to begin with, they won't force themselves. Detraining will definitely cause muscle loss, but it's mainly the sarcoplasmic hypertrophy from high-rep, lighter-weight training that diminishes, while actual muscle tissue is hard to lose. Look at this brother I just came across—check out his muscle separation when he was skinny at the beginning. I mentioned his situation isn't just about fitness, it's also related to my own mindset. Whether it's gym training or street workout, there's always a bit of competitive spirit involved, wanting to get stronger and be able to perform tricks and that sort of thing. He definitely shows good muscle separation, and if the upper part's SFP (separation/fat partition), then...