While your second sentence is definitely true, I disagree with your first. Developing muscle strength is partially caused by training your nerves to better engage your muscle fibers; however, simply adding fibers aka raw muscle mass is the other main component of gaining strength. I would argue it's the predominant factor.
You're both right. Improved muscle fiber recruitment is the primary way beginners gain strength. This is a fairly fast process, but plateaus off quickly. After the so-called "noob gains" are through, growing more muscle is the primary way to gain strength.
My knowledge on the subject is a bit out of date as I'm not in a serious training for more than a decade, but if I remember correctly explanation from my coach there're 3 main aspects of strength: nerve paths to activate the muscle fibers, the volume of muscle cells (when you get "big muscles" you actually don't increase significantly the number of cells, but their volume), and the capability of muscle cells to use and store glucose from the bloodstream (cellular uptake, which is also increased by regular training).
This is largely true. Though, as with anything in the body, the entire system reacts. Bones will change density, the circulatory system also changes in relation to the increased mass, the cerebellum will re-wire due to the increased muscle mass and usage cases, the reproductive systems may change depending on many factors, the pancreas will change it's output of glycogen, etc.
When it comes down to building muscle mass and 'dexterity', the physiology is fairly well understood. For most athletes today the major issues are in repair/strain and consistency. AKA: Sleep well, eat well, train well, in that order. It is theorized that a lot of the recent gains in MLB pitching are due to improved tendon repair chemical therapies and traditional steroids that focus on bulk (no data here, I think I read it on 538 a few years back)
Adding fibres. i.e. adding cells is hyperplasia and it's currently unknown if this occurs naturally in adult humans. Most visible growth can be attributed to hypertrophy, where the cells grow bigger.
My apologies. I was referring to "myofibrils" which are the strands within a muscle cell. So I meant to say that hypertrophy is the mechanism of growth. But you are correct, these are simply parts of a single cell and I mistakenly used hyperplasia.
