Random Dance, Sadler's Wells, London<br/>ENB, Royal Albert Hall, London

What if your brain wasn't connected to your legs? Would you dance?

Neuroscientists are not normally noted for their contribution to contemporary dance, but where Wayne McGregor is involved, normal doesn't come into it. In the past he has imagined mutants from cyberspace and made dances using artificial limbs; he's used pointe shoes as weapons and technology to scramble basic anatomy. So it was a fairly logical next step to think about how the brain tells the body what to do, which in turn led him to the neuroscience department at Cambridge University, where he has just completed a six-month fellowship working with researchers studying brain-body dysfunction.

Neuroscientists are not normally noted for their contribution to contemporary dance, but where Wayne McGregor is involved, normal doesn't come into it. In the past he has imagined mutants from cyberspace and made dances using artificial limbs; he's used pointe shoes as weapons and technology to scramble basic anatomy. So it was a fairly logical next step to think about how the brain tells the body what to do, which in turn led him to the neuroscience department at Cambridge University, where he has just completed a six-month fellowship working with researchers studying brain-body dysfunction.

Ataxia is a condition which interferes with a person's ability to co-ordinate movement. A sceptic might say that McGregor's work for his company Random has always resembled this state, and it's true that McGregor's style is hyperactive, feeding off the fractured stimuli and speed of modern life. But not until AtaXia (see, even his word processor developed a twitch) has he set out to chart the interface between brain and limb. The result is 70 minutes of dense, dark struggle with an invisible assailant. What's not clear is which one wins.

Lucy Carter's stage is seductive, all soft dark spaces divided by gleaming Perspex panels. Fuzzy reflections double, or even treble, the number of dancers you see, and dull silver tunics have a luminous trim that sketches their outline with a violet glow. Singly, in groups, or most tellingly in twos, Random's 10 performers grapple with muscles that seem to have a will of their own. Shins flick up to make contact with foreheads, hands grope and shudder like a shard of film frustratingly stuck on a loop.

Michael Gordon's score, "Trance", played live by the group Icebreaker, churns its round of minimalist riffs in a thrilling crescendo of amplified noise. This is sometimes punctuated by violent bass-drum thwacks whose pulse is distinctly - and very precisely - out of joint. Late in the piece the dancers lie flat and inert while the audience is treated to a series of video images too fast to process. Is this what having ataxia is like? Just as the condition has no cure, so McGregor's piece has no proper ending. For all the dancers' heroic efforts, the struggle just goes on.

There can be few occasions when it pays for a critic to miss the start of a show, but I had reason to be grateful to a dodgy District Line when, arriving at the Albert Hall too late to take my seat in the stalls, I was ushered upstairs to watch from the gallery. Seventy feet up, it turns out, is the best spot from which to view the ENB's in-the-round Swan Lake, now making its fourth visit to the cavernous rotunda that spells death to most kinds of dance production.

The Busby Berkeley effect is what gives Derek Deane's staging its buzz: all those white tutus, all those rows of white dots tessellating into diamonds and wedges and whorls; the lake of dry ice; the spooky blue light. The moment when several squads of bourréeing girls converge into a giant quivering "V" and collapse neatly over an outstretched leg deservedly brings down the house. This isn't Swan Lake as conceived in 1895, but it's a valid response to Tchaikovsky's score - compellingly played under Martin West.

Viewed from the expensive seats the dance perspective is less flattering. It's not just that you miss the patterns, but the peculiar logic that makes arena ballet both vast and intimate means that you often find yourself with the least talented cast-members under your nose. ENB's dancing is perfectly up to the job, but at least 40 extras have been drafted in, and some of those swans dance like puddings. Their arms flap stiffly, the eyes are glazed: they'd make a good advert for the evils of foie gras production, but any likeness to wild birds evades them.

Once the principals get stuck in, you relish being close to the action. Roberto Bolle is a model of classical finish. Not for nothing do his fans call him il divino. His acting may be dull but his stage manners are perfect, which makes him the ideal foil for Polina Semionova, the 19-year-old guest ENB is touting as the next big thing. And yes, she is magnificent. Amazonian of figure and with a temperament to match, she surrenders body and soul to the venue's hungry spaces. Her feet and arms could use a little polish, and some will say she shouldn't be pushed so hard so soon. But Semionova has already the unforced grandeur of the greatest Russian swan queens, the quality that must make the ducklings in the corps wonder why they bother. I do hope someone, and preferably not a Moscow oil baron, is watching out for her - someone with her best interests at heart, which may, alas, involve not dancing too many arena Swan Lakes.

jenny.gilbert@independent.co.uk

'Swan Lake': Royal Albert Hall, London SW7 (020 7838 3100), to Saturday