The muscle cell cannot contract again with out relaxing. Magnesium is necessary for the muscle cell to relax. Therefore, both are needed each time a muscle is used. As calcium causes the smooth muscles in the blood vessels to contract, the heart pumps harder and causes a reflexive lowering of blood pressure. However, it does not lower the blood pressure in the kidney so the stronger blood flow can harm the kidney.
Magnesium causes a relaxation of the blood vessels in the kidney, thus preventing kidney damage.
The person most susceptible to osteoporosis is the woman after menopause. However, men often get osteoporosis about ten years later than women. The astronauts have bone loss after time in space because there is no opportunity for weight-bearing exercise in a weightless environment. Young women who exercise excessively and who keep the body fat percentage low are at high risk for osteoporosis. Asians and persons who are fair skinned are at higher risk than blacks.