Strength isn't the thing, as you noticed with muscle mass. It's the challenge in my case not so much to develop strength since I ride a bike as main transportation and I can kick like a goat. The challenge is in the finely tuned control that comes from the calisthenics building the habit structures that only come from repetition. My arms and hands are equally strong from using garden tools and playing guitar and bass and hand drums, so it's a little bit of muscle building to develop stick skills, but it's mostly just the practice to establish the clear neuro pathways that make the thing under consideration into an automatic skill that just happens when it's needed. Endurance is also not just muscle density, it's about cardio, coordination, and for most people in the modern world with all the mental stimulation relaxation is an oft ignored skill and has to be specifically trained as it was for me until Tai Ji and Yoga entered my life decades ago, making my music experiments much easier to develop with only correct practice. Along with the martial arts and physical discipline with mental training of Yoga my encounter with Robert Fripp's Guitar Craft trainees helped me develop a compass to direct my sense for what's good practice with ergonomic care of the human body that does all the work and not just getting lost in wanting to play this or that and maybe getting all the notes in the right place but the feel is off for the tension. I'm still uprooting the tendency to tense up in getting louder and faster.