What Is Physical Fitness? Definition, Components, Benefits and How to Improve It

Ask ten people what “physically fit” means and you will get ten answers: running a mile without stopping, lifting heavy, touching your toes, looking lean. All of them capture part of it, which is why the term is so easy to use loosely and so hard to pin down. Researchers needed a precise definition to study fitness and health, and the one they settled on is more useful in everyday life than you might expect.

This guide gives that definition, explains what it includes, how fitness is actually measured, why it predicts long-term health so strongly, and how much activity it takes to improve it.

Quick Answer

Physical fitness is a set of physical attributes that people have or achieve, which determine how well they can perform physical activity. The standard research definition, from a 1985 US public health paper, divides them into health-related attributes (heart and lung endurance, muscular strength, muscular endurance, flexibility and body composition) and skill-related ones (such as balance, agility, coordination, speed, power and reaction time), each measurable with specific tests. It is different from physical activity, which is what you do, and exercise, which is planned activity done to improve fitness. Fitness matters well beyond sport: in a meta-analysis of 33 studies, each step up in aerobic fitness equal to jogging 1 km/h faster was linked to a 13 percent lower risk of death. Adults improve and keep it with 150 to 300 minutes of moderate aerobic activity a week plus muscle strengthening on two or more days.

In this guide:

• The Definition of Physical Fitness
• Fitness vs Physical Activity vs Exercise
• Health-Related and Skill-Related Fitness
• How Physical Fitness Is Measured
• Why Fitness Matters for Long-Term Health
• Benefits of Good Physical Fitness
• How Much Activity Builds Fitness
• How to Improve Your Physical Fitness
• Physical Fitness in Children and Teens
• The Verdict

The Definition of Physical Fitness

The definition most researchers use comes from a 1985 paper published by the US Public Health Service, written to stop the terms physical activity, exercise and fitness being used interchangeably. In it, physical fitness is a set of attributes that people have or achieve that relates to their ability to perform physical activity. Those attributes are either health-related or skill-related, and the degree to which a person has each one can be measured with specific tests.

Two parts of that definition are worth noticing. First, fitness is something you have, a capacity, not something you do. Second, it is a set of attributes, not one number. Someone can have excellent endurance and poor flexibility, or be very strong with little stamina. Being fit, in the full sense, means having enough of each attribute to meet the demands of daily life with energy to spare, and to keep doing so as you age.

Fitness vs Physical Activity vs Exercise

The same paper defined the two related terms, and keeping them apart makes the whole topic clearer:

Activity and exercise are the inputs; fitness is largely the result, along with genetics and age. In short: you cannot buy fitness directly, only the activity that builds it, which is why the guidelines further down are written in minutes of activity, not fitness scores.

Health-Related and Skill-Related Fitness

Health-related fitness covers the attributes linked to health and disease risk, which everyone needs regardless of whether they play sport:

• Cardiorespiratory endurance: how well your heart, lungs and blood vessels supply oxygen during sustained activity.
• Muscular strength: the most force your muscles can produce.
• Muscular endurance: how long your muscles can keep working without tiring.
• Flexibility: the range of movement around your joints. Some people have far more than average, which is explained in what being double jointed means.
• Body composition: the proportions of fat, muscle, bone and water in your body.

Skill-related fitness covers the attributes that matter most for sport and athletic performance: agility, balance, coordination, speed, power and reaction time. Balance in particular becomes a health issue with age, because it protects against falls.

Each health-related component, with simple tests you can do at home, is covered in the 5 components of physical fitness. A reminder that the components are separate: a normal weight does not guarantee the rest, as explained in can you be skinny and still unfit.

How Physical Fitness Is Measured

Because fitness is a set of attributes, it is measured with a set of tests, one or more per component:

Aerobic fitness is often expressed in METs, multiples of the energy you use at rest. One MET is sitting quietly; brisk walking is around 3 to 4; running is 8 or more. The highest MET level you can reach on a treadmill test is one of the most informative numbers in medicine, for reasons covered in the next section. For body composition at home, the waist-to-height ratio calculator is a quick check, and the max heart rate calculator gives training zones for aerobic work.

Why Fitness Matters for Long-Term Health

Fitness is not only about performance. The evidence linking it to how long and how well people live is among the strongest in preventive medicine.

Aerobic fitness and lifespan. A meta-analysis of 33 studies with more than 100,000 healthy adults found that each 1-MET higher level of aerobic fitness, roughly equal to being able to jog 1 km/h faster, was linked to a 13 percent lower risk of death from any cause and 15 percent lower risk of heart disease events. People with low fitness (under 7.9 METs) had a 70 percent higher risk of death than those with high fitness.

Low fitness as a risk factor. A study of 122,007 adults who had treadmill tests at a US academic medical center, followed for a median of 8.4 years, found that the gap in death risk between the least fit and the most fit was larger than the risk linked to smoking, diabetes or coronary artery disease, and there was no upper limit to the benefit of being fitter. The American Heart Association has called for fitness to be treated as a clinical vital sign, because it predicts mortality better than several traditional risk factors.

Strength counts too. In the PURE study of nearly 140,000 adults in 17 countries, every 5 kg lower grip strength was linked to a 16 percent higher risk of death, and grip strength predicted death from any cause and from heart disease better than systolic blood pressure.

These are observational studies, so fitness may partly stand in for other healthy habits. But the pattern is consistent across countries and populations, and fitness is something almost everyone can improve. How regular exercise strengthens the heart specifically is covered in how exercise improves your heart health.

Benefits of Good Physical Fitness

The studies above are about survival, but most people feel the benefits of being fitter long before that. The 2018 Physical Activity Guidelines for Americans, based on a systematic review of the evidence, sum up what regular activity does: it helps people feel, function and sleep better, supports normal growth in children, and lowers the risk of many chronic diseases. The main benefits:

• Heart and lungs: better aerobic fitness, lower risk of heart disease and death, as in the studies above.
• Strength and independence: stronger muscles make daily tasks easier, and grip strength alone predicts survival.
• Mood and stress: in an overview of 97 reviews covering 1,039 trials and 128,119 adults, physical activity had medium-sized effects on depression, anxiety and psychological distress compared with usual care, and higher-intensity activity helped more. The benefits were seen in healthy people as well as in people with depression or chronic disease.
• Sleep and energy: people who are more active report sleeping and functioning better.
• Balance and falls: in older adults, balance and strength training help prevent falls.

Ways to use activity for stress specifically are ranked in how to reduce stress.

How Much Activity Builds Fitness

The Physical Activity Guidelines for Americans, updated in 2018 after a systematic review of the evidence, set out what adults need:

• Aerobic activity: 150 to 300 minutes a week of moderate intensity, such as brisk walking, or 75 to 150 minutes of vigorous intensity, such as running, or a combination.
• Muscle strengthening: on 2 or more days a week, working the major muscle groups.
• For older adults: add balance training to aerobic and strength work.
• For everyone: move more and sit less. People who do the least gain the most from even small increases.

About 80 percent of US adults and adolescents do not meet these guidelines, so most people have a lot of room to improve. The FITT principle is a simple way to build up frequency, intensity, time and type without overdoing it, and the steps to miles calculator turns a daily step count into distance if walking is your starting point.

How to Improve Your Physical Fitness

Fitness responds to training at any age, and the least active have the most to gain. A practical order of steps:

• Start with what you can do now. If you are inactive, any increase counts; the guidelines are clear that the least active gain the most from modest increases. A daily brisk walk is a complete starting plan.
• Build aerobic fitness first. Work up toward 150 minutes a week of moderate activity (you can talk but not sing), then add some vigorous activity (you can only say a few words). Use the max heart rate calculator for training zones.
• Add strength twice a week. Squats, push-ups, rows, lunges and carrying heavy bags all count. A simple split such as push pull legs works once you are training more often.
• Progress gradually. Increase one thing at a time, how often, how hard, how long or what type, which is the FITT principle.
• Keep flexibility and balance in. A few minutes of stretching after sessions, and balance practice if you are older.
• Retest every 6 to 8 weeks. Repeat the simple field tests in the table above, such as a timed walk, push-ups and a plank, so you can see progress.

If you have a heart condition, are pregnant, or have been inactive with health problems, check with your doctor about what intensity is safe before starting vigorous exercise.

Physical Fitness in Children and Teens

The same components apply to young people, but the targets are different. The Physical Activity Guidelines for Americans recommend that children aged 3 to 5 be active throughout the day, and that children and teens aged 6 to 17 do 60 minutes or more of moderate-to-vigorous activity every day. Most do not: about 80 percent of US adolescents fall short.

Children have also become less fit over time. An analysis of 965,264 children and teens aged 9 to 17 in 19 countries, measured with the 20-metre shuttle run between 1981 and 2014, found that aerobic fitness fell by about 7 percent overall, more in boys than girls. The decline slowed with each decade and had almost stopped by around 2000. The 20-metre shuttle run is the same test as the PACER used in many US school fitness programs.

For parents, the practical point is simple: active play counts toward the 60 minutes, so the goal is more time moving and less time sitting, in whatever form a child enjoys.

The Verdict

Physical fitness is a set of measurable attributes, mainly heart and lung endurance, strength, muscular endurance, flexibility and body composition, plus skill-related qualities such as balance and agility, that determine how well you can do physical activity. It is the result of activity and exercise, not the same thing. And it matters far beyond sport: aerobic fitness and strength each predict long-term survival as strongly as the classic risk factors. The good news is that the recipe is simple and well tested: 150 to 300 minutes of moderate activity a week, strength work twice a week, and less time sitting.