Morgan Thorson's "Still Life" at the Weisman Art Center calls to mind the marathon dance craze that swept the nation in the 1920s and '30s. Taking place over the course of five hours, three afternoons a week — plus a three-hour performance another day — the work transforms its rotating casts of dancers into marathoners, testing their endurance in a piece that explores the nature of time.

Like runners, the dancers begin the performance alert and focused but their bodies and spirits grow weary as the dance progresses. Along the four gallery walls are strips of chalkboard, with hashmarks that the dancers scratch throughout the piece, as if trapped in a prison cell, waiting endlessly for release.

Being staged all summer long, the piece is a project of the National Performance Network (NPN), co-commissioned by New York's PS 122 in partnership with the Weisman Art Museum, the Cowles Center, American Dance Institute and the University of Wisconsin-Milwaukee's Institute of Visual Arts. The benefit of the long-term residency, according to Thorson, is that the work is always changing, "decaying and growing simultaneously," she said in an e-mail. "It is never complete or finished. It is conceived to move like time — continuously and without notice. It also contains moments that surprise and startle — that are gone forever."

The choreography, by Thorson in collaboration with the 19 rotating performers and with additional research by Judith Howard, has a very calculated structure, with precise gestures and intricate repetition. Yet even in its dense intricacy, the movement appears extemporaneous, in part because of the kinesthetic response conjured as movement from one performer spurs the movement of another, as if the dancers were personifying chaos theory.

At times, the dancers have spurts of exertion that counteract the trajectory of decay. More than an hour into last Sunday's performance, as the labor of dancing began to take its toll on the dancers, performer Pareena Lim burst into a sprint of a dance, repeating a sequence of movements very quickly as her breath became quicker and more urgent. She continued in a desperate frenzy, finally coming to a resting place, barely able to catch her breath.

The physical reality of bodies growing fatigued is mirrored in the choreography, as Thorson finds ways to evoke decay. In one example, the dancers stand in a diagonal with their arms raised above their heads. As time progresses, their arms begin to grow limp, eventually coming down to their sides.

Even watching "Still Life" is an act of endurance. Without a narrative arc or the traditional performance signals of beginning, middle and end, the experience of being in the audience demands a particular focus and attention. Of course, witnessing the entire performance is not required.

The sound score, by New York-based composer Dana Wachs, acts as a kind of metronome. Phrases of sound and music repeat, grounding the dancers with pulsating rhythms. The score often feels like it is in a standstill, creating a sense of waiting and anticipation.

Viewing "Still Life" in a museum offers an alternative to the experience of watching dance in a theater. Museumgoers are free to wander in and out of the gallery, sit in one of the chairs around the playing space or move to get a different view. Doors to the other galleries are open, so you can overhear conversations, or a docent giving a guided tour.

Last Sunday's performance saw visitors come in for an hour or so, while others, with some trepidation, watched from the gallery door. One woman wandered into the gallery without realizing she had become part of the performance. As the spotlight shined on her face, she sat down to watch the dance unfold.

"Still Life" is actually being presented as a part of an exhibition, "Local Time," curated by the Weisman's Diane Mullin, that features work by Alexandros Lindsay, Pritika Chowdhry, Sam Gould and Marcus Young in the adjacent galleries.

As institutions that preserve and catalog time — presenting the world as a series of objects with corresponding dates — museums are a ripe contextual framework for examining time. That context provides imagery that Thorson's work seems to have adapted.

Dinosaur skeletons drawn on the walls, part of the visual design by Thorson and Joel Sass, allude to natural history, which in early museums was shown alongside art and other "curiosities." The dancers even become dinosaurs at certain points, holding out their claws and standing in a wide stance.

They also appear as frozen or slow-moving statues. Like characters from the "Night at the Museum" movies, they become unleashed from their dioramas to wreak havoc in the gallery.

Sheila Regan is a Minneapolis writer.