Most people think about fitness in terms of what it can do for them right now: improving strength, changing body composition, increasing energy, or reducing the risk of developing chronic disease.

But there is another reason to build and maintain physical fitness that may be even more important.

What happens when something goes wrong?

A serious infection, cancer diagnosis, major surgery, cardiovascular event, injury, or prolonged hospitalization can place enormous demands on the body. While being physically fit certainly cannot guarantee that someone will avoid a health crisis—or survive one—research increasingly suggests that the condition of your body before that crisis matters.

Fitness isn’t armor against disease. It may, however, give your body more capacity to respond when its normal systems are placed under extraordinary stress.

Think of Fitness as Building a Physical Reserve

One useful way to understand this is through the concept of physiological reserve.

Your heart, lungs, muscles, circulatory system, and metabolism all have to respond when the body is placed under stress. Someone who regularly exercises may enter that situation with greater cardiorespiratory capacity, more functional muscle, better mobility, and a greater ability to perform physical work.

That does not mean a physically fit person cannot become critically ill. It means they may be starting from a different physiological baseline.

The American Heart Association has described cardiorespiratory fitness as an important clinical vital sign , noting that low cardiorespiratory fitness is strongly associated with cardiovascular disease and all-cause mortality. The association is significant enough that cardiorespiratory fitness can provide valuable information beyond several traditional health risk factors.

Your Heart and Lungs Matter—But So Does Your Muscle

Cardiovascular fitness tends to receive much of the attention when discussing health and longevity, but strength and muscle tissue are another important part of the equation.

During serious illness, hospitalization, or prolonged inactivity, maintaining normal activity can become difficult. Reduced activity can contribute to loss of strength and physical function, particularly in older adults or people experiencing extended recovery periods.

Starting with greater strength and functional capacity doesn’t prevent these challenges, but it can provide something valuable to lose from.

We Can See This Idea in Surgery: “Prehabilitation”

One of the clearest real-world examples is the growing field of prehabilitation.

Instead of waiting until after surgery to begin rehabilitation, prehabilitation attempts to improve a patient’s physical—and sometimes nutritional and psychological—condition before a major procedure.

In other words: build capacity before the body encounters a predictable health stressor.

A systematic review and meta-analysis published in The BMJ evaluated exercise, nutrition, cognitive, and psychosocial prehabilitation strategies and their relationship with outcomes including postoperative complications, hospital stay, quality of life, and physical recovery.

More recent research continues to investigate this relationship. A 2026 systematic review and meta-analysis of exercise-based prehabilitation in cancer surgery reported reductions in overall postoperative complications and hospital length of stay across the included trials, although outcomes varied by cancer and intervention type.

Importantly, research on prehabilitation is not uniform. Some reviews have found stronger evidence than others, and benefits can depend on the population, type of surgery, exercise program, and outcome being measured. That nuance is important: exercise is not a magic shield against complications.

What About Serious Disease?

The same general principle becomes interesting when we look beyond surgery.

Regular physical activity produces adaptations throughout the cardiovascular, musculoskeletal, metabolic, endocrine, and other physiological systems. Federal physical activity guidelines also associate regular activity with substantially lower long-term risk of premature death.

According to the Physical Activity Guidelines for Americans , even moving from inactivity to relatively modest amounts of physical activity produces meaningful health benefits. You do not have to become an endurance athlete to benefit.

For people dealing with cancer, physical activity is increasingly incorporated into survivorship recommendations as well. The American Cancer Society’s physical activity guidance for cancer survivors encourages avoiding inactivity when possible and gradually returning to regular physical activity, with recommendations adjusted to the individual’s condition and treatment.

Fitness Doesn’t Mean You Won’t Get Sick

This distinction is critical.

A person can exercise regularly, eat well, maintain a healthy body composition, and still develop cancer, suffer a heart attack, contract a severe infection, experience an accident, or develop another serious medical condition.

Health is influenced by genetics, age, environmental exposures, infectious disease, access to medical care, chance, and countless other variables. Fitness is only one part of that picture.

So the goal shouldn’t be to create a false sense of invincibility.

A better question is:

If your body is suddenly asked to fight, heal, compensate, or recover, what physical condition do you want to be starting from?

The Difference Between Lifespan and Healthspan

There’s another reason this matters.

Living longer isn’t necessarily the same as living independently.

Being able to get out of a chair, climb stairs, carry groceries, recover from a fall, tolerate a medical procedure, or regain independence after hospitalization depends heavily on physical function.

That makes strength and cardiovascular exercise about much more than aesthetics.

Every workout is an opportunity to build capacity that you may appreciate today—but may value even more decades from now.

You Don’t Need to Be an Elite Athlete

Perhaps the most encouraging part of the research surrounding physical activity is that health benefits aren’t reserved for extremely fit people.

The greatest opportunity for many people may simply be moving from inactive to active.

A well-rounded approach generally includes:

The objective isn’t to prepare for a specific catastrophe. It’s to continually increase—or at least preserve—the amount of physical capacity available to you.

Fitness Is About Being Prepared for Life

It’s easy to judge fitness by what we can see: weight lost, muscle gained, a faster mile, heavier weights, or changes in a body composition scan.

Some of its most meaningful benefits, however, may never appear in a mirror.

They may show up when your body is asked to do something extraordinarily difficult.

Recover from surgery. Maintain mobility during treatment. Rebuild strength after hospitalization. Preserve independence as you age. Or simply tolerate physical stress better than you otherwise might have.

No workout can guarantee what happens during a health crisis. But building cardiovascular fitness, strength, muscle, mobility, and healthier habits gives your body resources that can matter throughout life.

Fitness isn’t only about performing better when everything is going right. It may also help prepare your body for the moments when everything isn’t.

This article is intended for general educational purposes and is not medical advice. Exercise recommendations may need to be modified during illness, cancer treatment, following surgery, or for individuals with cardiovascular or other medical conditions. Consult an appropriate healthcare professional before beginning or returning to exercise when medical concerns are present.