You are using your body, your eyes, hands, arms and fingers, and sometimes breath to move in fine motor movements to play the instrument. Movement of any kind builds brain muscle and brain connections.
You are memorizing and remembering certain notes, chords, tunes, patterns, rhythms, and even lyrics while you play. Using your brain like this builds brain connections and brain muscle, which helps prevent aging and memory loss.
Modalities like walking or running or weight lifting are great too, but musical instruments offer a chance to use the mind and body at the same time. Choreographed dance and martial arts are just as powerful as music for the same reason- body and mind work together and become stronger.
I wrote a book about the mind and movement:
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