When astronauts are in space, they are weightless. Gravity disappears, and they float around their spacecraft. It sounds fun, but it isn’t great for their bones. Unloading your bones (taking stress off them) comes with consequences. Their bones become thinner, and their immune system gets weaker. This can happen to sedentary individuals on earth, too. The more time a person spends sitting, the less stress they are putting on their bones. When bones are under pressure from walking, standing, jumping, climbing, or weight training, they get stronger. They are forced to adapt to the pressure and respond. Bone strength naturally decreases with age, and activity typically does, too. Reduced activity isn’t the primary cause of lost bone density in older folks, but it can certainly speed up the process. Low bone density and weak bones can lead to osteoporosis and a greater risk for fractures. New research published in Nature …