“I never loved aerobics,” says Cher, right before she launches into a step class involving swoops, kicks, jumps and a heart-rate check-in. The year is 1991. Cher is resplendent in sheer black tights, a black leotard with what looks like a painfully narrow crotch and a black tutu. On her feet are sneakers topped by scrunched-down black leg warmers.  

Cher’s exercise video constitutes just one in a wave of celebrity fitness videos, kick-started by Jane Fonda’s iconic 1982 VHS, Jane Fonda’s Workout. These videos are the banner product to emerge from a peculiar but perfect storm that hit in the final few decades of the last millennium: new interest in the details of celebrities’ lives and bodies, a surge of interest in cardiovascular activity and—that old chestnut—social pr...