Michelle Obama’s bare arms may have prompted tutting from protocol watchers when she was First Lady, but all that midlife women around the world cared about was how she got those killer biceps. The 55-year-old former First Lady provided a clue on the London leg of her book tour recently, when she checked into Orangetheory Fitness. It’s a high-intensity, science-backed, training class, where you navigate quick changes between running machines, rowers and floor exercises such as squats and weights, while a heart monitor tracks your progress.