Are you talking about motor units when you talk about the muscle fibers? If you are, then both of you are correct. The number of motor units will increase as the muscle gains mass, it has to, or the muscle will no longer be effective in doing its job, which is to contract. Also, on the subject of fast/slow twitch fibers, a marathon runner will have more slow (or red) twitch so that the muscle does not fatigue as fast, and the sprinter has more fast (white) twitch as he needs that quick burst. Each person has the ability to develop both, it just depends on the type of training you are doing. Exercise by any other name, is good for you regardless of what the end product will be. A good cardiovascular program is what most of us, including me, need. That will help with the circulation and the absorption of oxygen into our cells and tissues. While we do make new blood cells on a regular basis, if they aren’t getting enough oxygen they aren’t working right. I live at a high altitude and I have more blood than I need as it is trying to oxygenate my cells. Air is a tad thin at 7225ft. If you bodybuild then you are working toward mass, I find it amazing that bodybuilders can feed themsevles because of the carrying angle of the arm. Strenght training is low reps, increased weight, endurance training is low weight high reps. If you doubt my knowledge in this subject, go to my profile and check out my occupation.
sunny A day without sunshine is like a day without laughter :sun: