You say, you're fine, "fit as a fiddle." You're clearly "at the top of your game." The expression is somewhat confusing, however, given the inanimate nature of the fiddle. Could we possibly have confused the "fiddle" with the "fiddler?" The old time fiddler, after all, was renowned for his stamina — playing from dusk to dawn without missing a beat.
No way! It's the musical instrument we're most like. When we are fit as a fiddle we are properly in tune with life and ourselves.
The correct tension is important for both us, and the fiddle, to achieve the desired results. If the strings of the instrument are too loose, it cannot be properly played. We also need tension in our life, but just the right amount. That's what gets us out of bed in the morning; helping us to address our tasks and set our goals. Managing it properly is the challenge. When it becomes too high, it takes its toll. We get "uptight" and "strung out," often to the point where we feel we will snap.
Whether we play a violin or the body-mind, we achieve maximum performance only through proper care and maintenance of our instrument. One way to achieve a balance is to simply introduce a bit more fun into our life.
It's not without risk however, leaving us open to charges of fiddling around — a leftover from the Puritans' disdain for music, dancing and — what they considered "pointless" — frolicking.
"Fiddlesticks!" we respond. Why take their criticism seriously? It all comes down to just so much fiddle-faddle. And besides, there's not much we can do about it.
Best we just dance away — with a smile on our face and a fiddle-dee-dee on our lips.