When you exercise, your blood’s calcium levels drop, which signals your body to produce a hormone that leaches calcium from your bones, the Colorado research explains. That loss of calcium leads to a condition called “bone resorption,” which over time can lead to broken bones, hip and joint issues, and arthritis, studies shows. Even if you don’t exercise, nearly 1% of bone mineral density is lost every year after you turn 65, according to research from Boston’s Beth Israel Hospital.