With apologies to James Brown, the hardest working people in show business may well be ballet dancers. And at New York City Ballet, none work harder than the dancers in its lowest rank, the corps de ballet. During the first week of the company’s winter season, Claire Kretzschmar, 24, a rising corps member, danced in all seven performances, appearing in five ballets, sometimes changing costumes at intermission to dance two roles in a night.

But her work onstage did not even begin to capture the stamina required to be in the corps. Spending a week shadowing Ms. Kretzschmar was exhausting — she gave new meaning to the idea of being on your feet all day. Twelve-hour days at the David H. Koch Theater, the company’s Lincoln Center home, were hardly unusual: Company class each morning was followed by back-to-back-to-back rehearsals, with occasional breaks for costume fittings or physical therapy, and then by the hair-makeup-costume-dance routine of daily performances.