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. Researchers are still trying to determine the most efficient exercises for counteracting the effects of weightlessness. Currently, astronauts perform isometric exercises in which they push against resistant surfaces such as the modified treadmill. Calcium is an extremely important mineral for our nerves and blood as well as for our muscles and bones.