On a recent afternoon at Radio City Music Hall, a group of Rockettes were being examined with critical eyes.

The 36 dancers practiced one of their signature synchronized numbers, their long limbs seemingly in perfect symmetry. They moved as swans, chins up, arms graceful.

Dance captain Karen Keeler drilled them until the last few moments before another group took the stage. “A one, two, three, four, five, six, seven, eight, one, up two, a three and a four,” she sang out.

Linda Haberman, director and choreographer of the annual “Radio City Christmas Spectacular,” watched from the seats and asked Keeler’s opinion of this group of women. It wasn’t overly positive.

“They were just kind of taking it easy. Do you want me to continue?”

“We have, I think, six seconds,” replied Haberman. “So if you can say it in six seconds, you can do it.”

Keeler looked at her charges. “Do it all better,” she told them.

Haberman laughed. “Details,” she said.

This year is the 85th anniversary of the “Christmas Spectacular” and Haberman is determined to put on a birthday party appropriate for this grand old lady.

When one thinks of Christmas in New York, who doesn’t immediately imagine a line of crystal-bedecked women high-kicking it?

Haberman, who has been choreographing the show at Rockefeller Center since 2006, has managed to pull the show into the Wii era, adding digital projections on LED screens, a 3-D section that requires special glasses and even a sequence inspired by a computer game.

“I try to look at it with new eyes every year,” she said. “It’s really important to honor our past. There are things that are part of the Rockettes’ rich history that I would never let go of.

“But I think it’s important to keep the Rockettes as entertainment relevant to our changing times.”

It was Haberman who added a new favorite routine — the Rockettes dancing on a double-decker tour bus. But there are some things she won’t mess with: the “Nutcracker” minisuite, the woozy fall of the wooden soldiers, the “Living Nativity” and “The 12 Days of Christmas” sequences. Haberman keeps those classics but is always tinkering.

“I never leave anything alone because I can always make it better,” she said. “I don’t just leave something. Even if it’s been in the show for five years.”

This year, in a nod to the anniversary, the show will have a costume retrospective featuring Rockettes wearing some costumes they’ve worn through the decades. “It was kind of a fun way to tie it in and also give little bits of history of where we come from,” she said.

The Rockettes have shrugged off Superstorm Sandy. Rehearsals were canceled for three days and vans had to pick up stranded dancers, but there will be no delays. “It’s like New York. Everyone’s pulling together and working hard and we’ll get it done,” Haberman said.

Haberman is an exacting task master. Calm, but steely. A graduate of the School of American Ballet, she became the youngest cast member in Bob Fosse’s “Dancin’.” Precision is in her blood.

“I’m tough with the ladies,” she said. “But I think they respect me and I think they respect the work they’re doing and that’s really the key.”

Haberman, who will start work on the next show in January, rehearses two casts at the same time — a total of 152 people — who alternate playing the punishing schedule of five shows a day, six days a week. There are 80 Rockettes since each show needs 36 ladies and four standbys.

Why 36? Partly that’s all the women who can happily fit shoulder-to-shoulder along the 70-foot stage. And partly it’s a number that Haberman has grown to adore.

“As a choreographer, you’re always playing with formations, especially with the Rockettes. Thirty-six is a great number because you can divide it by two, you can divide it by three,” she said. “Different choreographers like different numbers. I love 36. It becomes a math project.”

The Rockettes, some 500 applied last spring, come from different backgrounds — some are Broadway dancers, some are Pilates instructors — but they must stand between 5-foot-5 and 5-foot-10 1/2. That’s simply because of Haberman’s attempt to arrange her dancers to make a “pleasing effect.”

Through the years, Haberman has helped hone the increasing strength and physicality of the Rockettes. The requirement to tap dance is the one that makes most stumble, but they often return the next year having nailed tap.

“I look at the line now compared with when I started in the ’90s and they’ve just changed. These women train like crazy, they are all so strong and so tough and such good dancers — it’s great,” she said. “Now I can choreograph anything. I’m not limited.”

On the Net

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