“Billy Strings & Don Julin play traditional American string band music with energy levels usually associated with extreme sports,” says the website of this northern Michigan duo. The line sounds good for getting them into the barrooms that, especially in the western half of the state, are bringing in more bluegrass groups, and indeed guitarist Strings and mandolinist Julin do offer some real barn-burners. A five-minute “Rollin’ in My Sweet Baby’s Arms” on YouTube, recorded last year at Traverse City’s Little Bohemia, has an audience member screaming “holy crap!” at the end.

Even that stretch of pure adrenaline, though, works so well because there are interludes in which the power and speed recede and subtle duet work takes over. Strings and Julin offer bluegrass classics like “Rollin’ in My Sweet Baby’s Arms,” originals and standards on the outlaw side, and old-time ballads like “Little Maggie.” Most striking of all, perhaps, are their old-time instrumentals like “Salt Creek.” They break down the alternation of solo and accompaniment, quickly entering a breathtaking realm where the two players seem to be functioning as a single unit in elaborating on the material. It all takes place at very high speed, but Strings is essentially a guitarist in the style of Doc Watson, who prized subtlety and depth of historical resonance over velocity. This is not “newgrass” for barrooms and festivals but music that takes its place in a long tradition.

The heavily tattooed Billy Strings (William Apostol) is only twenty-one. He grew up in Morehead, Kentucky, and in Michigan’s Ionia County, both hardscrabble places. His dad was a musician, and about two years ago Billy got a job at a Grand Traverse area resort and began appearing at open stage nights to the astonishment of the locals. He made a great decision in joining forces with Julin, a journeyman mandolinist in his fifties who is the author of, among other things, the mandolin book in the “For Dummies” series. Intergenerational musical partnerships are rare, and ones like these, in which each member pushes the other beyond what might otherwise have been accomplished, are to be prized.

The last time I heard duets like these was in the heyday of progressive bluegrass instrumentalists like Dan Crary and Byron Berline, who borrowed just enough from jazz to build larger structures convincingly out of bluegrass’s foursquare tunes. But they never took the curves at top speed like these guys do. Strings’ voice, a rough, strong thing that hardly sounds like it belongs to one so young, also connects the music to the wild old hoedown tradition.

Strings and Julin are a phenomenon, building through an appearance at a TEDx event in Traverse City recently and now with an appearance at the Ark’s free Take a Chance Tuesday show on July 22. (Nonperishable food donations are welcome.) Strings and Julin have just been signed to the large Crossover Touring agency in Atlanta, whose last major bluegrass client was the Del McCoury Band. Catch them in Michigan while you still can.