"Take a deeeeeep breath."

"Elongate your spine."

"Feel the expansion of your back."

"Squish your bellybutton to your backbone."

"Exhale!"

"Squish!"

"Exhale!"

"Squish!"

I'm sweating to an oldie and Richard Simmons is nowhere in sight.

And I mean a real oldie. Here I am, along with most of the 115 members of the Mendelssohn Choir, backstage in the cramped rehearsal room of Heinz Hall. They are about to rehearse Mendelssohn's oratorio, "Elijah," first performed in 1846, and the work chosen to kick off the official opening of the choir's centennial celebration Friday.

The man leading the choir members through their warm-up exercises is choral conductor Vance George, for 23 years the director of the San Francisco Symphony Chorus. He has been invited by the choir's music director Betsy Burleigh because "he was in town -- it was one of those happy coincidences."

George warmly accepts the warm applause, then introduces his good friend, Robert Page, the man who will actually conduct "Elijah." More applause, more adulation.

The warm-ups continue, with Page, sitting in the back of the room, sometimes nodding along, sometimes nodding off.

"Take another deeeeeep breath!"

"Feel your arms getting heavy and dropping to the floor."

"Exhale!"

"Squish!"

"Exhale!"

"Squish!"

The warm-ups are heating up. George waves his baton to and fro; 15 or 20 minutes later, the exercises are done, and Mendelssohn magic materializes. Even in a casual atmosphere, even sitting on hard-back plastic chairs, even with the distraction of a photographer clicking away and latecomers crawling in, even after being caught in traffic and dealing with high-pressure jobs, the choir sounds, well, heavenly.

Every once in a while, George interrupts the rehearsal to make a point. His criticisms are just and gentle, and no one fails to find the humor in them. One laugh erupts when George points out that the word "prayer" is repeatedly being slurred. "Finish the words! If not, they fall to the floor, and the janitor has to sweep them up."

A few days after rehearsal, I chat with Burleigh, who manages to squeeze her paid choir gig in with her day jobs as assistant director of choruses for the Cleveland Orchestra and the coordinator of choral and vocal music at Cleveland State University. (I had to ask: Does the Cleveland resident listen to classical music on the radio during her long commute? Her response? A resounding: "No! I make phone calls, listen to a French language review tape and books on tape." For the record, she heartily recommends the last book she "heard," Doris Kearns Goodwin's "Team of Rivals.")

Betsy tells me all this Mendelssohn mania started back in 1908, and that the choir is Pittsburgh's oldest continuing performing-arts organization. There are 115 members, 20 of whom are paid, the rest volunteers. The oldest member? He's Kenneth Davies; he's 72. The youngest? Lauren McKee; she's 19. Sixteen couples met and married during the past 20 years, and, yes, all 16 are still together. They all come together to make music because, Betsy says, "they love performing at such a high level." At the performance of "Elijah,'' a special guest will be part of the crowd: Shirley Roth, the granddaughter of the choir's founder, Ernest Lunt.

It's clear that Betsy loves what she is doing and that she can Handel -- oops! handle -- all the choir is and will become. "I want the choir to be one of the great choruses in the country and recognized as such."

The perfect note for my exit.

I quietly leave the rehearsal room, the robust sounds following me down the hall. Such music of the night! Such drama! Such passion! I can't help but chuckle when I think the janitor will have one less room to clean that night.