As New York City Ballet’s spring season enters its homestretch, program notes indicate — all too often — that the cast has changed due to illness or injury. But no matter how heavy the schedule or how many other dancers are out, City Ballet’s senior ballerina Wendy Whelan seems to forge ahead. At 41, not only does she appear to be still in her prime, but she has shown signs of physical renewal this season. The depth, variety, and extreme quality of her work is worth noting and appreciating before the season ends on June 29, and the company heads upstate for its annual Saratoga Springs engagement.

For while Ms. Whelan can be lauded for her stamina — she’s onstage more often than not — and her wide range, what sets Ms. Whelan in her own league is that she manifests the most rare, least appreciated quality in ballet today: artistry. She is sensitive to the dimensions of different roles, as well as the multiple identities dissolved into the ballerina identity of one particular role. And that is one of the great freedoms extended by the Balanchine repertory: to be anything, to be so many different things, bar by musical bar. What’s more, she is a consistent model of discernment: No step, no matter how familiar, turns generic on her body.

Ms. Whelan is a physical type that could be found only in ballet; her shape — thin and angular in the extreme — relates to no woman on television, film, or even fashion runways. Ballet is an art of line as much as of volume, and yet, no matter how beautiful his or her silhouette may be, a very skinny dancer can take an audience off balance, at least initially. It’s easiest for audience members to relate to performers who look — even marginally — more like themselves, and a bit of extra weight presents an inviting tactility. But Ms. Whelan does not give the audience that comfort: She has a musculature that suggests it could be plucked like a lyre or violin string. And what that results in is balletic imagery in pure, clear refractions matched with a supreme sense of musical understanding.

Though it is fascinating to watch, it does mean that there are roles for which she is less suited. As many roles as she has created for Christopher Wheeldon, you wouldn’t think of her dancing one of Mr. Wheeldon’s whimsically real-life types in “An American in Paris.” (Although I’m sure she could do it, and I’d be interested to watch her do it.)

Ms. Whelan also illustrates the difference between what a dancer looks like standing still and what he or she becomes in motion. With as little as a weight shift, she can work every channel of transformation and sleight of hand. You wouldn’t necessarily imagine, looking at Ms. Whelan, how lyric she can be. But as graphic as her musculature is, she can float, and she can uncoil and uncurl luxuriously.

But no matter how familiar one may become with her dancing (or how sure one may be in one’s assessment of what she can or cannot do), she often turns out performances that offer surprises. When she made her debut in “Bugaku” last month she was able to look as ripely nuptial as one expects from the role. It was nice to see her looking infinitesimally fuller in this 1963 Balanchine ballet, which was one of the highlights of her memorable work this season. Then there was her self-mocking wit as the most unstable of the three women in Jerome Robbins’s “In the Night.” And there was her chic and nonchalance in Robbins’s “In G Major.” And there was her new role last month in Alexei Ratmansky’s “Concerto DSCH.” Here she was the gravitational center balancing the bouncing-off-the-walls technicality of Ashley Bouder’s role. She performed to slower music here than Ms. Bouder; Ms. Whelan’s duet with Benjamin Millepied (Tyler Angle at the last performance) whorled in meditative arcs.

Perhaps this season has offered such riches because, as a veteran artist, Ms. Whelan can draw from so much experience. Each role seems to be constructed meaningfully from the sum total of those she has already danced. In 2006, she created a forlorn character in Mr. Ratmansky’s “Russian Seasons” that somehow informs the quite different but moodily related role she created in “Concerto DSCH.”

A mysterious combination of factors, perhaps unknown even to the artist herself, makes some seasons in a dancer’s life more vivid than others. This spring, Ms. Whelan may have seized the moment or it may have seized her. Last year, Kyra Nichols retired at age 48; Ms. Whelan may go on just as long. Inevitably, at some point she will have to stop, and probably before then she will start to wind down. All the more reason, then, to commemorate her endlessly rewarding work this season.