“There’s always a sense that the virtuoso is bleeding over into a realm of inappropriateness,” says Ariel Osterweis, a dance and performance studies scholar at the California Institute of the Arts. “Classical forms change due to virtuosos. Because they’re not wholly rejecting a certain style or form, they’re just pushing the boundaries.”

The leg extension to the front, side and back is one of ballet’s most recognizable, debated and gawked-at stances. The beauty and magic is in the unwavering, impassive squareness of the dancers’ hips. Almost anyone can “get a leg up” through some contortion of the pelvis and physical strain. But a ballet dancer’s leg extends as though entirely independent, their torso floating above it all.

Battement développé “is an example of the body truly blossoming,” wrote Russian critic Akim Volynsky. For the body to blossom this way, dancers need good turnout, flexibility and strength that work in a highly coordinated way.

This was not an issue for dancers during the first couple hundred years of ballet’s history. Decorum demanded that the leg extend no higher than the hip. Lifting it further revealed too much leg, and displaying the area “down there”—the crotch—was taboo for women. For men, the issue was the suggested effeminacy (and hence homosexuality) of stretch over strength—an early-19th-century notion with residues to this day.

Dancers have often had to push the limits of social propriety to further movement expression. With her skirts lifted to calf height, Marie-Anne de Cupis de Camargo could perfect the entrechat quatre in the 18th century. Marie Taglioni lifted hers some more in the mid-19th century and bourréed across the stage on pointe. By the end of the 19th century, female ballet dancers were performing in shorter tutus and dancing a variety of virtuosic steps. Yet, the legs were still generally lifted only to hip height.

“In my experience, the highest leg height notated in Stepanov is 90 degrees,” says Doug Fullington, the education programs manager and assistant to the artistic director at Pacific Northwest Ballet, who has reconstructed work of Marius Petipa from Stepanov Notation. “This doesn’t mean the leg never went higher in practice. However, body mechanics change as the leg goes above waist height; one gains the ability to do certain things but loses the ability to do others.”

He points out that the 19th-century ballet dancers were more compact, less flexible and less turned out than those we see today. Ballet was about nimble footwork and showing elegant angles of the body and torso, better achieved with the legs below 90 degrees.

High legs were for the circus or the nightclub where brazen cancan performers kicked to their noses and flashed their undergarments—pantalettes—with or without a crotch. These high kicks might suggest that you were loose, but equally that you were independent and powerful and not to be messed with—a sort of proto-punk attitude that made its way into ballet 100 years later.

Gradually during the first half of the 20th century, seeing women’s legs, albeit trousered, became socially acceptable thanks to the “all-inclusive” labor requirements of political revolutions and world war. Women were included in the gymnastics events at the 1928 Olympic Games in Amsterdam, showing off the strength and agility of female legs. Fashion reformers responded to a growing emphasis on athleticism throughout the culture with accommodating and practical styles for women, such as shorter tennis skirts and all-in-one, tight-fitting bathing suits.

A new realm of physical and expressive potential was on offer to the revolutionary choreographers of the period just as audiences grew hungry for the new and exotic (and erotic) from companies like Diaghilev’s Ballets Russes. Dancers’ flexibility could be developed and exploited with fewer social restrictions.

A major change came from George Balanchine. “His modernist focus on form and the body (as opposed to narrative) created a shift in the way ballet-trained bodies conveyed meaning onstage,” says Osterweis. His fast, fragmented movement generated an exaggerated focus on specific body parts, such as legs—their lines, height and speed.

Ever since, high leg extensions have abounded in contemporary ballet choreography. Everyone from Maurice Béjart and Roland Petit to William Forsythe, Wayne McGregor and Dwight Rhoden has pushed, contorted and extended dancers’ legs for different effects and to mixed reviews. “I don’t apologize for using the body in extreme ways,” says Rhoden, “because I think that the work I like to do reflects the world around us, and we are in an extreme time.”

With ballet companies’ mixed classical and contemporary repertoires, and increasing audience expectations, it was almost unavoidable for leg heights in classical ballets not to follow the upward trend. Sure enough, a 2009 study showed that there was a steady increase in leg elevation angles of codified ballet postures between 1946 and 2004. The researchers also found that a group of ballet-naïve volunteers preferred the more vertical shapes to the earlier forms.

Today, hyperextended extensions are an expected part of most classical dancers’ vocabulary. “Dancers stand at the barre their whole lives to improve their extension, to clarify their line, to amplify their form,” says Rhoden. “We train our bodies to do many different types of movement; high extensions are just one way to show a heightened emotion or change in energy.”

And it feels good. “From the dancer’s perspective, when you can control where your working leg goes, and how you want to use it, there is that feeling like sexual power, but there’s also a pure athletic and kinesthetic joy,” says Osterweis. “What’s interesting about the high leg as used by William Forsythe and Sylvie Guillem is that it’s more of an attack and exclamation than a coy invitation.” As such, it is hearkening back to those rebellious and independent cancan dancers of yore.

However, some critics and audiences admit to feeling a bit weary of hyperflexibility on the ballet stage. One high-profile respite has been Alexei Ratmansky’s reconstructions of Petipa’s ballets. The lines are returned to soft, rounded shapes, knees are bent, retirés low. There’s lots of épaulement, demi-pointe and fast footwork. The dances acquire more texture and the musicality Petipa originally intended.

Judith Mackrell, writing about Ratmansky’s Sleeping Beauty in The Guardian, described how the leggy Bolshoi ballerina Svetlana Zakharova was transformed in this “new/old world” choreography: “Her body seemed to become physically rounder and…the more she restricted the height of her leg extensions, the more energy and lushness began to flow through her back and arms.”

Mastering the 19th-century style—and its hip-height legs—still holds value. The more dancers absorb each phase of ballet’s history, the richer the ballet-scape will be.