Answer
The pull of gravity as well as muscle tension are necessary for osteoblasts to create strong mineralized bone matrix. In conditions of microgravity muscles weaken, and the pull of gravity is low. The osteoclasts of the bone continue to break down bone matrix, normally , but the osteoblasts which respond to the pull of gravity are unable to keep up with the osteoclasts. Consequently, matrix become softer, and bone weakens.
Work Step by Step
Certainly, exercise puts stress on muscles and bones and and this will attenuate the effects of microgravity --somewhat. However, it is not continuous or adequate to reverse the effects of reduced gravity. Therefore, bones demineralize, muscles get weaker and subjects (astronauts) in microgravity lose their strength
