Loss of bone
The loss of bone matrix and bone mineral density (the amount of mineral deposited in a given area of bone) following a period of reduced activity, intensity or forced immobility appears to be much faster than the formation and mineralization of new bone. Calcium is removed quickly from bone resulting in a net loss of bone mineral content, and can occur after only a few weeks of bed rest.
In the absence of mechanical stress, as occurs during periods of inactivity, immobility or bed-rest, bone absorption outstrips formation. This results in weakened bone. This process starts after the age of 30 in females, and accelerates rapidly after the age of 45 as oestrogen (female sex hormone) levels drop, until up to 30% of calcium is lost by 70 years of age. Generally, this process starts in males after the age of 60. Furthermore, there is a loss of collagen with ageing, which causes the bone to become more brittle and lose its tensile strength. This is why osteoporosis and fractures are more common in the elderly.
Bone density can also be lowered in women who have a very low level of body fat as it can reduce production of oestrogen.