Cacketacaketacaketa. Cacketacaketacaketa.

With each tap and stomp on Wolf Trap's stage, a small muscle is tearing in some dancer's calf. With every head flip, someone's neck is going crack out of alignment. Cacketacaketa, a little tibialis anterior stress; thunketa thunk, there goes a piriformis muscle; whappety tap and your quadratus lumborum is lumbering.

Which is why, way beyond the dry-ice misty-moisty Celtic twilight hoo-ha of "Riverdance--The Show," now stomping its way though six performances at the Filene Center, there are three small dressing rooms devoted to body repair.

With 51 dancers tapping, spinning and jumping for eight shows a week, the company staff must include a chiropractor and two massage therapists. They open for business about three hours before curtain time, seeing between nine and 15 "patients" a day, depending on the number of shows.

Martin Brennan winces as Sue Thompson, the chiropractor, pulls off the sticky disks anchoring electrodes to his shin. "Should have shaved your legs," she jokes. Brennan, 23, has a tendency toward shinsplints--tiny muscle tears that produce a distinct but not unbearable pain. He wears support socks and comes to see Thompson for ice and electric stimulation.

"It won't hurt while you're dancing--the buzz carries you through," he says. "But then it hurts later."

Irish dancing, particularly the tricked-up version seen in "Riverdance," was never meant to be done for 2 1/2 hours a night, seven or eight times a week. It was meant for a village festival, an occasional competition, a St. Patrick's Day booze-up at the local pub. Dance your feet off, feel sore the next day, and that's it until the next time.

The style is akin to other stomping-type folk dances--clogging, flamenco, tap--except the arms are generally held still at the sides. It has a lot of foot action, on the balls of the feet, in stiff shoes reinforced with fiberglass and wood (in the old days they hammered in nails to make the noise). So when you take the bog to Broadway, however much it's decked out with wispy poetry, amplified fiddles, candles and moons, you have to expect injuries.

Cacketacaketacaketa.

"Sprains and strains, stress fractures," says Thompson, a Los Angeles native who has been on the road with "Riverdance" for a year. "I expected more knee problems. And more prima donnas."

Eileen Martin, the 23-year-old lead female dancer, comes in for a treatment. The front of her shin hurts and she says it's swollen, a condition imperceptible to a layman. She stretches out a smooth, tanned and shapely leg and Thompson bends over to poke it.

"It's me shopping muscle," Martin jokes. The cause, it seems, is not the awful pounding she gives her body but walking around Georgetown. Thompson applies ice and electric "stim," then begins an elaborate routine of cutting pads and taping them to Martin's feet.

"Eileen likes to pound her toenails until they come off," Thompson says. "The padding seems to help." Ouch. Whether she's losing a nail or not, she dances on her toes part of the time--and she's not wearing toe shoes.

She has custom-made orthotics in each shoe, but like all the dancers bemoans the constant battle with footwear they all go through--about 10 new pairs a year. It seems that--much like ballet shoes--as soon as you get them broken in enough to be comfortable, they're worn out.

How like unto life.

Around the corner from Thompson's small domain, Rodney Squires bends over Gary Healy, who lies face down on the massage table. "I'm trying to loosen the erector spinae," Squires says, pressing down Healy's back. "Then I'll be flushing it with long strokes to get oxygen in." Squires was the first traveling masseur for "Riverdance," hired two years ago in Toronto. "Breathe for me, Gary," he says as he applies his elbow to Healy's posterior. That's the piriformis. "The left is tighter," he observes.

"They say, 'My neck is in bits,' " Squires notes as he greets his next patient, dancer Niall Mulligan, 25, who is suffering a neck injury induced by piano playing. Squires has traveled around the world with this company, loves the work and looks forward to returning to Prince Edward Island when the tour ends in November.

In Hartford, Conn., he recalls, his "office" was right under the stage. "Boom, boom, boom!" he says with a discreet wince. He presses his fingers down on Mulligan's scalenus muscles. Mulligan flinches in pain and his feet flip up in reaction. Squires suggests that he go directly to Thompson for an adjustment.

"Actually, Sue can never crack me," Mulligan says. "I'll take a long shower tonight, also."

"That's good. But don't go directly into that hotel air conditioning after," Squires cautions.

Most major dance companies travel with a physical therapist or chiropractor, because the field is notorious for its casualties. The more you dance, the more you hurt. "Most dancers are in pain," Squires says. Unlike sports medicine, however, performance-related injuries are not much of a medical specialty and few studies have been done on them.

Thompson and Squires both predict the young, quick-to-heal dancers of "Riverdance" may have long-term problems if they continue dancing too long or don't take care of themselves. "I'm already seeing some arthritic feet," Thompson says.

"Stiffness in the back," Squires predicts.

At 7 p.m., an hour before showtime, the company breaks for a warm-up conducted by the other massage therapist, Juliet Harve, who runs them through yoga, toning and tapping exercises for half an hour, to the music of that famous Irishman, Frank O'Sinatra.

The women are all in makeup by this point, transformed into exotic beauties with big swoops of rouge, huge eyelashes and dark accents on their eyelids. The singers swan around in velvet gowns--so comfy on a humid summer evening--and the galloping fiddler, Liz Knowles, wears a peacock blue robe over a black suit.

Thompson is back at her station, ready for a last-minute rush on Advil. Overhead there is a sound like thunder. Rumblerumble. "It's just Marta," she says--Marta Jimenez-Luis, the lanky flamenco specialist, warming up.

"The number of injuries is related to how hard the stage is," says Thompson. "Chicago is bad. Toronto. Radio City is very rigid." The Filene Center, however, has just the right proportion of cement and wood for the portable overlay the company tours with.

Dancer Lorna Bradley arrives for a treatment as the five-minute call for the opening number, "Reel Around the Sun," comes over the loudspeaker. She sits on the table and bends from the waist.

"When I do this it totally locks up on me," she says. Thompson has her turn over and lie face-down, on a special tissue that won't muss her makeup.

"Right leg a bit longer?" Bradley asks.

"Yeah," says Thompson, taking out her "gun," a chiropractor's gizmo that she clicks on various points of Bradley's lower body to "activate" the muscles.

Bradley hops off after a few minutes and dashes upstairs. The ensemble dancers are in seven of the eight weekly shows, but are rotated to different numbers daily. In addition to the Irish dancers (some of whom are English or American), the troupe includes eight Russian folk dancers, three American tap dancers and one flamenco dancer (plus two understudies). Plus 10 musicians and 10 singers.

"Riverdance--the Show," which is making its third visit to Wolf Trap, should not be confused with "Riverdance--the Video," "Riverdance--the CD," or the new version opening in March, "Riverdance--On Broadway." Three "Riverdance--the Show" companies are traveling the world, each with their own chiropractors and massage therapists.

Thompson stands backstage to watch the opening number Wednesday night, which even after a year she still loves. Cacketacaketa, thunk thunk thunk. There's some wafting and some tapping, through a dry-ice cloud, as a sonorous recorded voice intones:

Out of the dark we came

Out of the sea

Where the long wave broke on the shore

As the day broke

And the night rolled back

There we stood in the land we would call home.

The show continues for 2 1/2 hours. The tapping gets louder and amplified, like gunshots, and then softer and more frolicsome. One number features very loud male thumping--the women in Thompson's office at the time called it the "testosterone dance." Another number pits the American tap dancers against three of the Irish; turns out our guys can do Irish but can't do funky, acrobatic tap.

Tara Barry comes in to Thompson's office to patch up her blisters (new shoes) with Second Skin bandage. During intermission, Andrea Curley, the dance captain, arrives with a hip thing.

All in all, Wednesday's opening is a normal night, Thompson says. Perhaps even a little light.

Martin Brennan, dripping with sweat after the finale and the 15-minute curtain call, returns for a brief review of his shinsplints before heading for the company bus.

Yesterday morning, after a night's sleep, he reports: "I'm a little achy. It never really goes away."

He used to work in a bank. He always thought of dance as a hobby, until it gave him a chance to see the world.

"The biggest thing is that we totally enjoy it," he says. "For me, I'll keep on doing it as long as my legs hold up."