'This leg must be stretched all the way out. I have to see you struggling to get it all the way," choreographer Nicolo Fonte insists as he pulls Edgar Anido's leg up to demonstrate to the rest of the BalletX dancers. Such were the rigorous demands of rehearsal last week - fine-tuning every gesture, every facial expression.

The Brooklyn native retired from dancing 15 years ago, once his now-international choreographic career began to soar. He last worked with BalletX in 2013, when he set Beautiful Decay, his first full-length evening, on the company. For its multigenerational cast, Fonte mused on aging among dancers.

Beasts, his new piece, which has its world premiere Wednesday at the Wilma Theater, also is based on an unusual concept for a ballet.

"The initial premise was the sociological debate of nature vs. nurture," he said after wolfing down a sandwich during a rehearsal break at the Performance Garage. "Of course, we know this notion has been around for 150 years and is now a bit outdated. We know now that human characteristics are a combination of both - a combination of biology and environmental elements.

"Originally, when I was thinking of that in terms of the ballet, I saw a neatly packaged binary vision - one act is the nature act, one act is the nurture act. But in the very first week, I was very tense. It wasn't happening naturally for me, and I started to let that go. It became a process for me, since I know the sociological debate is outdated, and I sort of adopted the same attitude, recognizing that there is animal instinct within us. So the piece started to become something about what is animal instinct. That's how the title Beasts came about. It's an imaginative use of the word. You're not going to see anyone in monkey suits."

But, I point out, there is some beastly movement, in particular one in which two dancers bend from the waist and place their heads next to each other, arms overlapped, looking like four-legged creatures.

"Yes," Fonte said, "so primarily I let that binary vision go. The rigidity is gone, but in my mind, the first act is still the nature act, and in that act you see it even more, a lot of crawling around, very primordial, a lot of that primal exploration."

There is an intermission - "sort of a theatrical device that I like. I used it in Beautiful Decay, as well. We'll do a scene change," he said, "so it's really logistical."

Mimi Lien, who just won a MacArthur fellowship and did the set for Beautiful Decay, is designing this set, as well. Lien's architectural sets are best known locally because of Pig Iron's devised production of Love Unpunished, in which she used empty staircases; Pig Iron's Welcome to Yuba City; and her set for BalletX founder Matthew Neenan's Carmina Burana, which he choreographed for the Pennsylvania Ballet.

But Fonte declines to describe Lien's set for Beasts. "It's really special. So let's let that be a total surprise."

Is Martha Chamberlain, who designed the costumes for his gorgeous Grace Action, which was premiered in June by the Pennsylvania Ballet, doing these?

"No," Fonte said. "This time I'm working with Christine Darch," a much-in-demand designer for companies around the country. It took them a while to come to an agreement on what the costumes should be, he said, but "I think we've got something really good now."

In the last two of his Philadelphia-premiered pieces, the choreographer used Vivaldi and Philip Glass, but also the ambient music of Icelandic composer Ólafur Arnalds, whom he uses again for Beasts - among others.

"It's a collage," he said, "I think I'm using about eight or nine different composers, not sure. But I was looking for music that was going to serve the idea of the savage beast, and a complete score wouldn't have worked for this idea. For the first part, I'm using a lot of club music by Henrik Schwarz, but the Tokyo Chamber Orchestra did an orchestral version I liked. You can hear a little bit of the club sound, but it's highly orchestrated. Part of it, to me, sounds like the birth of time."

Fonte has an easy camaraderie with this group, even though only five of the roster from Beautiful Decay remain. "We're in week six of rehearsals, but the newer dancers climbed on board with me very quickly."

Back in rehearsal, Daniel Mayo is pulling his foot against his thigh and grimacing. "Danny, you're having some emotional problem?" Fonte asks. " 'Cause that is no concern of mine."

Everyone laughs, but he's all business, turning to an assistant. "Cue the music, please."

DANCE

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BalletX

Wednesday through Sunday at the Wilma Theater, 265 S. Broad St.

Tickets: $22-$45.

Information: 215-546-7824

or balletx.org.

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