I've always been curious about that too. From what I've gathered over the years, Haruichi Furudate was a volleyball player in high school—not a star, but part of a club. I think that firsthand experience of the grind, the squeak of gym shoes on the court and the sheer frustration of a failed receive, is what bleeds into every page of 'Haikyu!!'. It's not just a sports manga; it's a love letter to the mundane, exhausting, yet utterly euphoric reality of team sports. You don't get that specific texture of pain and joy from just researching it.
There's also a famous anecdote about the inspiration for Hinata and Kageyama's dynamic. Furudate-sensei mentioned seeing a tiny, incredibly energetic player relentlessly attacking against a much taller blocker during a real match. That image of defiance, of the 'small giant,' crystallized into Hinata. For Kageyama, I suspect it's an exploration of raw talent clashing with the loneliness of being too good too early, a problem Furudate might have observed or even felt on the court.