Saving money for a “rainy day” is good advice and, in our current economy, essential for maintaining quality of life. Having savings in the bank allows someone to continue providing for their family in the event of a lost job or other financial crisis. While this may seem like common sense, many Americans have been caught without enough savings when they needed it and found it difficult to meet basic needs like paying the mortgage.

This principle can also be applied to fitness. When you are healthy, you can maintain a high level of fitness. This makes your day-to-day activities easier and serves as a reserve or “bank” to draw on when you need it. Your good fitness now can get you through a health crisis, just like saving money can help you through a financial crisis. This health crisis could come in the form of an injury or illness that keeps you from being active for several days or a hospitalization that keeps you in bed for a week, month or longer.

The problem with periods of inactivity like bed rest or hospitalization is that there are severe physiological effects that occur within days and get worse over time. You may have noticed this as weakness and fatigue after spending two days in bed with a cold. Muscle strength declines at a rate of more than 1 percent per day of bed rest and can be 50 percent lower following as little as three weeks. That 50 percent reduction in strength could limit a person who was already deconditioned to a point where he or she would have difficulty completing the most basic activities of daily living. A person who was fit and strong when they went into the hospital would certainly be better off when released. Older adults fare worse than younger individuals. According to one study, the decline in strength seen in older men in just 10 days was equivalent to the change measured after 28 days in men 30 years younger.

It’s not just the muscles that are affected; the bones get weaker, too. In fact, 12 weeks of bed rest can reduce bone density by as much as 50 percent, exposing patients to a greater risk of fracture. This is due to the reduced stress on the bone from not standing and walking, as well as the lack of muscle activity. Two of the most effective ways to build bone density are putting weight on bones through weight-bearing activity and the action of the muscles pulling on the bones from resistance training. Because bed rest eliminates both of these stresses, bone density declines rapidly.

One unique study had healthy young men complete three weeks of bed rest back in the 1960s. They all experienced a rapid decline (more than 20 percent) in their VO2max, the best measure of aerobic fitness, but recovered quickly after the experiment ended. These individuals also had their fitness tested again 30 years later. It turns out that the decline in fitness in those young men in three weeks of bed rest was greater than the decline in fitness that occurred over 30 years of aging!

There are many reasons to exercise and be fit, but the most important reason may be to develop a fitness “bank” you can draw on if you are injured or hospitalized. Since the effects of bed rest are seen in people of all ages, everyone can benefit from a good fitness foundation. Finally, most patients are encouraged to move around as much as possible and some receive in-patient physical therapy or rehab, even after major surgery, to help lessen the effects of prolonged bed rest. Take advantage of these opportunities if you or are a loved one are hospitalized.

Brian Parr, Ph.D., is an associate professor in the Department of Exercise and Sports Science at USC Aiken where he teaches courses in exercise physiology, nutrition and health behavior.