What happens to your bones if you don't do weight-bearing activities?

Bones weaken: Bones respond to and need weight bearing to remain healthy. Unhealthy bones can become osteopenic whereby the bone mass decreases.

Varies: Most everyone dies at least some weight bearing exercises, walking is a weight bearing exercise for example. But the implication is that placing a stress across the bones in the body can make them strong and not doing it can make them weak. Bed ridden patients can pretty rapidly get what is known as disuse osteoporosis, a weakening of the bones due to inactivity.

Space travel.: For an extreme example of cessation of weight bearing activities one needs to look no further than being in space at zero gravity. Bone density begins to decrease within days and progress to as much as 20 %, depending on the time spent at zero gravity. Without the stress of weight, calcium begins to leave the bone and enter the blood stream, which is consistent with disuse atrophy,