Babies are mostly bright eyes and fat rolls, but don’t be fooled. Those rotund legs pack a powerful punch—more than 45 newtons (10 pounds of force)—according to the first study ever to measure babies’ kicks in the womb. To quantify fetal brawn, scientists built computer models from MRI scans that tracked the movements of fetuses from 20 to 35 weeks (above). They found that the kicks become stronger from 20 to 30 weeks. But after 30 weeks, kick force plummets, likely because the fetuses have less and less room to move around as they grow. The kicking exercises benefit babies in two ways, the researchers say: First, it’s literally exercise, which helps develop muscles and bones. Second, the effort increasingly strains joints from mid- to full-term. That strain likely helps their joints form properly, the scientists report today in the Journal of the Royal Society Interface; having normally shaped joints could prevent osteoarthritis later in life.