Since the 1960s, the nature of recreational physical fitness activity in the United States has changed considerably. By 2000, private fitness centers were ubiquitous features on the American landscape. Increasingly centralized ownership characterized the field from 1970 onward. Men and, more significantly, women, joined, left, and rejoined these ever-more mechanized establishments. Images of health, beauty, professional success, and sexuality emphasized athleticism and muscle tone. Fitness took on an especially powerful meaning to women affected by the burgeoning feminist movement, their new economic roles, the rise of women's sports, and the volatile marital and social environment. Fitness centers also emerged as social centers where people went to see and meet members of the opposite or same sex. Paradoxically, as this greater focus on the body and formalized exercise occurred for some, a larger portion of the population grew increasingly unhealthy and obesity became a problem among all age cohorts.