Sure that is part of it but that isn't the only thing that happens. For example: when I cycled 4 days about 60 km/day at age ~10 I was completely amazed how muc

Sure that is part of it but that isn't the only thing that happens. For example: when I cycled 4 days about 60 km/day at age ~10 I was completely amazed how much easier & faster it was when I cycled to school after this vacation. This is not due to better coordination, as I had cycled every day for many years. I'm not saying that you get significantly bigger muscles quickly, but you can get better muscles quickly (or perhaps it's due to the lungs/heart?). BTW 10 lbs per year is a lot. It's more than 12 grams per day which is about 12 cm^3 per day. This is definitely noticeable. Where do you have this information from? Do you have other information like this? I'd be interested to know what exactly makes muscle tissue grow.