As people age, there is usually some tendency to lose physiologic reserves such as muscle mass, bone mass, and cardiopulmonary capacity.
Frailty is a loss of physiologic reserves that leads to physical disability.
Not every loss of physiologic reserves will lead to disability. If one has built up a large margin of safety in one's youth, some loss of reserves in one's middle and older years will not cause a dip below the threshold of functional loss. For example, if one has been physically active throughout life, eaten a balanced diet rich in calcium, and, for women, taken hormone replacement therapy after menopause, one's bones should be strong and dense enough that a low-energy fall will probably not cause a fracture. Although it is true that many persons with a physical disability are frail, many persons still fully functional are also frail in that they are perilously close to the threshold of strength, bone mass, or cardiac capacity below which they will not be able to function normally. Surgery, infection, or similar stressful events may drop them below that critical threshold.