Every four years, Seattle Opera puts on Wagner’s Ring Cycle. And every four years, the singers who take to the sky to play the Rhine Maidens sign on for a little extra training.

“The Seattle Opera realized the demands of first and foremost singing, and also moving beautifully,” Alexa Rosenthal said.

Rosenthal, owner of Island Fitness, draws out that last word dramatically, with her arms raised, to illustrate the alchemical mixture of song and fitness required to embody the Rhine Maidens in Das Rheingold, the first of the four operas in the cycle.

These three water nymphs spend up to 20 minutes at a time suspended in the air by means of hip harnesses; they swing, flip, rise and fall, all while wearing mermaid tails and all while singing. Opera.

“It’s very difficult, because you’re hooked up at only one point, your center of gravity, your hip. And you’re constantly fighting to stay in control,” said Ishya Silpikul, Rosenthal’s colleague and co-trainer.

Having first taken the role of Rhine Maiden trainer in 2004, Rosenthal was familiar with the setup. She and Silpikul began working with the performers in Seattle a full year before the show began its August 2009 run. Then, since each singer is based in a different location – Montreal, New York and Boston – and was also engaged in other performances, Rosenthal and Silpikul gave each one a take-home program.

In the opera, the three Rhine Maidens act as a unit, yet each has her own presence. Similarly, each singer had her particular training needs.

One had hip tightness, so softening her hip flexors was critical. Another had chronic knee pain. So within the basic framework of building each one’s core, Rosenthal and Silpikul tailored each session with routines that incorporated Pilates, yoga and strength training, all with an eye toward building the core without tightening the muscles involved in singing – the neck, shoulders and upper back.

They found the performers incredibly committed and in fact, would periodically get emails saying things like, “My back hurts, help” or “I’m in Australia, and I fell. What should I do?”

When the group re-convened in Seattle earlier this summer, they trained together three times a week before the run of the show and two times per week after the show opened, also holding a warm-up session before each performance of “Das Rheingold.”

When the opera opened last month, Rosenthal and Silpikul learned just how many Bainbridge “Ring Cycle” fans are out there; one Island Fitness patron reported his delight when he opened the program to see that “his club” had trained the Rhine Maidens.

Opening night, though, was the most gratifying moment for both trainers, when the pieces all came together.

“To hear them sing…” Rosenthal said.

“It was magical,” Silpikul finished, “to hear them sing.”