Miller has helped create a new workout they call "Puissance Training" — for singers and others to get into shape for stressful careers.
His athleticism matches roles including the dynamic, high-leaping devil in the Met's production of Rossini's "Armida," or Mozart's quick-moving, jack-of-all-trades Figaro.
At Opera Colorado last year, his agility came in handy for contemporary Mexican composer Daniel Catan's "Florencia en el Amazonas," a Spanish-language work of magical realism in which Miller plays a character who vanishes into the sea and returns as a superhero, flying above the stage on ropes.
"We hired him for his Denver debut because the role required a very commanding voice with a rich, dark tone, and a personality that has a certain charisma on the stage," says Greg Carpenter, Opera Colorado's general director.
Miller was raised in Ovid, Colo., a town of 250 where he helped his father tend to their cattle and crops, waking up before dawn.
"I thought, there must be more to life than cattle," says Miller, who went to Colorado on a football scholarship.
"Keith was a devastating blocker," says Larry Zimmer, the longtime radio voice of Colorado football. "He learned the discipline and focus to memorize all those opera roles from football."
As a singer, Miller hopes, as he puts it, for decades more of "running arias" and "tackling operas."