Betley and his colleagues were curious about what happens in the brain as people get stronger through exercise. They decided to focus on the ventromedial hypothalamus, a brain region that regulates appetite and blood sugar. The team then zeroed in on a group of neurons in that region that produce a protein called steroidogenic factor 1 (SF1), which is known to play a part in regulating metabolism. A previous study found that the deletion of the gene that codes for SF1 impairs endurance in mice.
A rare circulatory problem required Emily Wheldon to have her left arm amputated three years ago. Her brain still thinks it's there. "Most days, it just feels like I've got my arm next to me," she says. The perception is so compelling that Wheldon had to train herself not to rely on the missing limb. "When I first had the amputation," she says, "I was trying to put my arm out to stop myself from falling."