Rosas danst Rosas, Sadler's Wells, London

Repetition to the point of headbanging tedium is not everyone's idea of a good night out, but the Belgian mistress of minimalism pulls it off

While it would be harsh to call Belgium a byword for boredom, it must tax even its tourist board to list many exciting Belgian exports. Leffe and Hoegaarden, Tintin, the saxophone, that strange pallid vegetable known as witloof .... Or is it just that we have trouble pronouncing the others?

Ann Demeulemeester, for instance, the crowned queen of quirky tailoring. And her namesake, Anne Teresa de Keersmaeker, who has led the field in contemporary Belgian dance ever since she hit on the perfect visual response to musical minimalism in 1982. Her choreographic nemesis, the ebullient American Mark Morris, once sarcastically dubbed her "De Tearjerker". That could have referred to her work's strenuous blanking of any and all emotion, or just as easily to her audiences' blubbing for mercy. Repetition to the point of headbanging tedium comes with the territory with De Keersmaeker. But once you accept that as your baseline, you might even start to enjoy it.

Rosas danst Rosas was only her second public work, made when she was a stripling 23. Yet still it stands as her company's signature, not least because it has been hailed far and wide as a kinetic primer on contemporary creative process. The vast number of teenage girls in the Sadler's Wells audience was less a sign of mass enthusiasm than of the work's status as an A-level text. A quartet lasting not far short of two hours without interval, this is minimalism writ massive: small, humdrum gestures added together in multiple combinations to make a series of choreographic sums. A) Lie on floor + rest on elbow + riffle hand through hair + shoot a glance into the wings followed by B) two rolls across floor + slap floor + rest on elbow + shoot glance into wings. Noting that three dancers are performing A and B while one dancer is apparently asleep, or that two are busy with A and B while two are asleep, adds a peculiarly desperate piquancy.

Mercifully, the exercise divides evenly into four parts, which saves you having to keep checking your watch. The first is on the floor and in silence (other than the music of human breath, and you soon learn to distinguish between nasal intakes and oral exhalations, and vice versa, a skill you are unlikely to need ever again). The second part is seated on chairs, to loud and rhythmic industrial noise like looms clacking. The third is performed standing and the fourth (at last!) actually moves about a bit, repeating the sums of section three but strung out across the floor in circles and diagonal lines to the point of exhaustion.

What keeps you watching – apart from sheer amazement that anyone should undertake such a strenuous yet mind-numbing endeavour – is the gleaming precision of the execution. Anne Teresa herself is pushing 50 (though passes for 23 if you're not sitting close), while Cynthia Loemij, Sarah Ludi and Samantha van Wissen are her juniors by only a few years. Their drab uniform dress channels recalcitrant schoolgirls, yet their stamina is that of the marathon runner, combined with the finest muscular control. During the slowest, potentially most boring, section, your eye naturally homes in on details. The way that, say, the palm of a hand hovers millimetres from the floor, yet appears to be taking the weight of the body. Or the difference between a body toppling to the floor like a felled pine, or a kneeling body softly crumpling by degrees.

Yes, some audience members couldn't take the pace and left, noisily. For the rest, although it felt worthy of a medal, there was satisfaction in seeing the thing through. Like the musical minimalism (Steve Reich) that inspired her, de Keersmaeker is exploring not only gradations of sameness, and our tolerance of it, but the way that boredom ultimately sensitises us to things we would normally miss. Thanks for that insight, Anne Teresa.