You Made Me a Monster, Sadler's Wells, London<br>Anton and Erin, Coliseum, London

The audience are invited to get busy with their hands in the startling opening event of a William Forsythe season

William Forsythe has devoted his life to pushing the limits of dance performance.

The Focus on Forsythe season – with events spread across five London locations, many of them free – shows the extent to which he's pushing audiences too. Only some of these events involve Forsythe's extraordinary dancers. Others turn the focus on spectators. Take City of Abstracts, a time-delay video installation that morphs visitors into humanoid Slinkys on a wall-size screen. Stand and flap your arms and the action barely registers, but walk a few steps and, 10 seconds later, you see yourself relocating in a gloopy flow of particles. It's mind-bending in the way it forces you to think about your physical place in time and space.

Next door, in the Lilian Baylis Studio, several films play on a loop. Solo, from 1995, is a pure-dance manifesto in which Forsythe himself, a streak of spiky energy, powers through a sequence in which he seems to be responding to frantic commands from his nerve-endings. Antipodes I/II is a pair of films, deceptively humorous, which play games with gravity. In one, what looks like a wall (with a poster and a cupboard hanging on it) is actually the floor, so that it sometimes seems as if Forsythe is levitating, or holding a balance on one finger.

In a week dominated by headlines about human cloning, You Made Me a Monster at first strikes as unnaturally prescient, as well as odd. It begins like a Blue Peter handicraft project, as patrons are led to tables set out with curious brown-paper sculptures and press-out kits to make a human skeleton. "Choose a component," instructs an assistant, "fold along the dotted lines, then clip it to the sculpture any way you like". Quickly the unease of indulging a childhood pastime among strangers gives way to the pleasure of manual work, as we press and fiddle and clip, and stand back to admire the accumulating tangle of parts – a sacrum attached to a tibia attached to a ribcage. If you know William Forsythe, though, you're on red alert, because nothing in his world is this easy.

Sure enough, the air soon fills with alarming sounds, and a man with a contorted body and face approaches, apparently "reading" each sculpture for instructions. (Strictly, of course, this must be fake, since no one could literally rearrange their skeleton, but you got the idea.) A similarly afflicted second man, and then a woman, work the tables, sometimes lurching within inches of a spectator's face. With their grunting, gurning, and paraplegic-like movements, you're torn between seeing this as tasteless mimicry and a serious exploration of physical breakdown.

Forsythe guides you towards the latter, as words on a screen spell out the history of his first wife's fatal cancer. That might sound tasteless in itself, but the lucidity of the prose redeems it. "My wife was a kind of dance genius," he writes, disarmingly, and the contrast between beauty and innocence, ugliness and grief begins to crystallise. The monster of the title, of course, is the cellular disturbance that attacks from within. From his too-personal tragedy, Forsythe draws something that speaks to us all.

The Coliseum, meanwhile, entered a sequinned timewarp as Anton and Erin, two of the pros from BBC1's Strictly Come Dancing, glided through a two-hour set in which Erin Boag's frocks (eight of them) easily outdazzled the steps, and Anton Du Beke's stale patter almost puttered to a halt.

The surprise, seeing ballroom on a big stage, is how undramatic it is, and for all the pair's gracious line, many of their dances lacked sustaining interest. Worse, their notionally dynamic numbers, such as a tango to an orchestral arrangement of Sting's "Roxanne", looked tame. Oversexed Latin specialists Chris Marques and Jaclyn Spencer brought a kind of vulgar relief, but it was the taut playing of the British Concert Orchestra under Gavin Sutherland, and Richard Shelton's agreeable Sinatra covers, that were the real treat.

'City of Abstracts': Tate Modern, 29 Apr to 1 May