Modern dance is a tricky term, often used as a catchphrase for nearly every nonclassical (read: nonballetic) theatrical dance style in Europe and America since the early twentieth century. Twyla Tharp challenges even that fragile distinction. The groundbreaking choreographer ignores the codified barriers between modern dance and ballet idioms to forge her own distinctive vocabulary based on versatility, musicality, and heart-stopping technical brilliance — not to mention piquant accessibility.

Tharp started out in the 1960s as a postmodernist, sharing that movement's experimental commitment to everyday steps like walking and an intellectualized, less-is-more aesthetic. But then she decided it was okay to dance — really dance — to music. In 1973 she set Deuce Coupe to a string of fourteen Beach Boys hits for the Joffrey Ballet. Tharp has been in impossibly high demand ever since, choreographing for ballet and modern dance companies alike, as well as, from 1965 to 1988, for her own troupe, the first incarnation of Twyla Tharp Dance. Her work is distinguished by a seamless and often surprising structure of pure movement: complex yet playful, rigorous yet harmonious. It's the kind of giddy dance charge that leaves you bouncing up the aisle afterward.

The reimagined Twyla Tharp Dance, assembled in summer 2000, is made up of top-notch talent from the American Ballet Theater, the Joffrey Ballet, and the New York City Ballet — two women and four men — all of whom contribute their own spirited interpretations and expansive technique to Tharp's inventive vernacular.

Both programs in Twyla Tharp Dance's two-day Power Center residency, on March 23 and 24, include two works choreographed specifically for this sextet. Surfer at the River Styx, an intense dance drama performed on both days, packs a sustained emotional punch aided by Donald Knaack's unconventional percussive score. According to Tharp, it's an examination of hubris loosely based on Euripides' Bacchae. But inhabiting this allegorical shell is signature Tharp: full-out physicality with a driving rhythm and beat. Her dancers attack the eclectic material with enthusiasm and attitude. Four costumed in black represent the river, while the two conflicted souls (one in surfer garb) perform athletic solos on their journey to the other side. Poignant and provocative, the piece concludes on a redemptive note.

It would be a shame not to see the smart new Twyla Tharp Dance before the current cast of dancers is reshuffled, or it grows into a grander style of company altogether. Take someone who claims to be unmoved by the expressive power of modern dance. Such preconceptions should be put to rest straightaway.