Calcium makes our bones strong. Bones tend to lose calcium when they stop doing work. Calcium is essential to maintaining strong bones. Its loss weakens bones and causes them to break more easily. Calcium loss can also increase the threat of kidney stones as the mineral is excreted.
Some humans have a tendency to lose calcium in their bones as they age.
Running and jumping are not as effective in space as they are on Earth because a person's weightless limbs can provide little resistance. Even with the straps, the astronaut is using less force than on Earth. Astronauts can also be strapped onto cycling and rowing machines. The changes in bones and muscles seem tied to loading, or the amount of force to which each is subjected. The calcium loss is very site specific, differing not only from bone to bone, but at various sites of the same bone.