Thomas Hobbes, the 17th century English philosopher, described the natural life of mankind as “solitary, poor, nasty, brutish and short.” With a bit of editing (the addition of the words “impassioned” and “beautiful”) that description might certainly capture the life of professional ballet dancers.

The quote came to mind as I watched “Restless Creature: Wendy Whelan,” the documentary about the New York City Ballet star who remains one of the most admired figures in contemporary ballet. The film, directed by Linda Saffire and Adam Schlesinger (and receiving its Chicago premiere at the Gene Siskel Film Center this week), is an intimate, often heartbreaking chronicle of how Whelan — who enjoyed an exceptionally long and distinguished career — was forced to deal with an emotionally and physically agonizing transition. For in 2013, at the age of 46 — and after 30 years with the company that was her life — she was forced to face retirement and re-imagine her next step.

Wendy Whelan in a scene from the documentary “Restless Creature: Wendy Whelan.” | Abramorama Films

Born in Louisville, Kentucky in 1967, Whelan took her first dance class at the age of three. At 17 she was invited to join the fabled New York City Ballet as an apprentice, and two years later entered the corps de ballet. In 1989 she was promoted to soloist, and in 1991 was named a principal dancer. Although she never worked with the company’s founder, George Balanchine (he died in 1983), she performed many of his most challenging works, as well as ballets by Jerome Robbins, Peter Martins, William Forsythe and many others, and she became something of a muse for both Christopher Wheeldon and Alexei Ratmansky. With her twig-thin body, and extreme flexibility and strength, she brought both a delicacy and jackknife-like angularity to her dancing.

And then it happened. Whelan, notably injury-free over the decades, began having some worrying pain in a hip. She was 46 — well past the age when most ballet dancers leave the stage. And then Martins, artistic director of the company, called her into his office and rather unceremoniously informed her she would not be dancing in “The Nutcracker” as she usually did. He also clearly implied she should think about retiring — a notion that left her stunned and devastated. Almost immediately it also left her feeling more physically crippled. But she was not about to submit so easily, and not prepared to lose her identity.

“If I don’t dance, I’d rather die,” she says at one point. And while this might sound melodramatic, it is the natural response of someone who has devoted body and soul to her art with an almost nun-like discipline.

What follows is a step-by-step account of her sessions with physical therapists, and her decision to undergo surgery for the first time in her life. The surgical procedure is filmed in the most graphic way, with Whelan’s doctor discovering enough damage in her joint to make him wonder how she had managed to dance until then.

Ballerina Wendy Whelan takes her final bow in this scene from “Restless Creature: Wendy Whelan.” | Abramorama

The grueling post-surgery rehab process (from crutches, to stretching and strengthening, to a return to class that finds her pushing herself too hard) is counterpointed by her conversations with other dancers who had already retired. All confess the emotional pain of not dancing never entirely fades.

But Whelan will not go gently into the wings of the theater at Lincoln Center where she spent three decades. She returns to perform during the company’s Spring season, and then plans her farewell performance in October, 2014 which will feature a new two-part work choreographed specially for her by Wheeldon and Ratmansky. It is a grand night, with many bows, a stage full of flowers and the homage of her fellow dancers.

Yet it is not the end, for Whelan was already busy making plans for her next chapter — devising her own project, Restless Creature, a program of new duets by four young male choreographers, all of whom also would be her partners. That program came to Chicago’s Harris Theater for Music and Dance in January 2014, and has since had a second incarnation.

By her own admission, the entire ordeal forced Whelan to “grow up” — mourning the changes that come with age, letting go, and then moving on in the most creative way possible. Throughout, you can see the anguish, relief and joy flashing across her long, angular face — one that Modigliani would have loved to paint. And Whelan hides very little in this portrait of courage.

‘RESTLESS CREATURE: WENDY WHELAN’

★★★1/2

Abramorama presents a film directed by Linda Saffire and Adam Schlesinger. No MPAA rating. Running time: 94 minutes. Opens Friday at the Gene Siskel Film Center.