Today exercise is a multi billion-dollar fitness industry, and women account for more than half of all gym memberships, and dominate yoga, pilates and barre studios. But it hasn't always been this way. For much of the twentieth century, sweating was considered unladylike, and physical exercise was discouraged. Women were not allowed to run marathons in case they turned into men, or lesbians, and it was feared that physical exertion could damage a woman's reproductive organs or cause her uterus to fall out. Journalist Danielle Friedman has tracked the fascinating history of women's exercise culture through her book Let's Get Physical and found it was only in the sixties that a few forward-thinking fitness pioneers began to promote exercise for women, and women began to move en masse, setting in motion an exercise revolution.