<human>:  What is the history of the fitness industry in the United States?
<bot>: The fitness industry in the United States has a long and varied history. Physical fitness first became popular in the late 19th century, when the American pioneer and naturalist George W. Stewart promoted and popularized the notion of personal health and physical fitness. After World War II, mountain biking and other outdoor recreation activities such as rock-climbing and kayaking began to become more popular. In the late 1970s, fitness centers such as Jazzercise and Aerobics began to sprout up in the United States, shortly followed by the first large corporate health clubs such as Bally Total Fitness and 24 Hour Fitness. Along with this expansion came the growth of the aerobics and dance exercise industry, pioneered by organizations such as the American Fitness and Aerobic Association (AFAA). Today, the fitness industry continues to grow with the rise of boutique fitness, yoga classes, and specialized personal training services.