Physical fitness is a state of health and well-being and, more specifically, the ability to perform aspects of sports, occupations and daily activities. Physical fitness is generally achieved through proper nutrition,^[1] moderate-vigorous physical exercise,^[2] and sufficient rest.^[3]

Before the industrial revolution, fitness was defined as the capacity to carry out the day’s activities without undue fatigue. However, with automation and changes in lifestyles physical fitness is now considered a measure of the body's ability to function efficiently and effectively in work and leisure activities, to be healthy, to resist hypokinetic diseases, and to meet emergency situations.^[4]

Fitness is defined as the quality or state of being fit.^[5] Around 1950, perhaps consistent with the Industrial Revolution and the treatise of World War II, the term "fitness" increased in western vernacular by a factor of ten.^[6] The modern definition of fitness describes either a person or machine's ability to perform a specific function or a holistic definition of human adaptability to cope with various situations. This has led to an interrelation of human fitness and attractiveness that has mobilized global fitness and fitness equipment industries. Regarding specific function, fitness is attributed to persons who possess significant aerobic or anaerobic ability, i.e. endurance or strength. A well-rounded fitness program improves a person in all aspects of fitness compared to practicing only one, such as only cardio/respiratory endurance or only weight training.

A comprehensive fitness program tailored to an individual typically focuses on one or more specific skills,^[7] and on age-^[8] or health-related needs such as bone health.^[9] Many sources^[10] also cite mental, social and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three points, which represent physical, emotional, and mental fitness. Physical fitness can also prevent or treat many chronic health conditions brought on by unhealthy lifestyle or aging.^[11] Working out can also help some people sleep better and possibly alleviate some mood disorders in certain individuals.^[12]

Developing research has demonstrated that many of the benefits of exercise are mediated through the role of skeletal muscle as an endocrine organ. That is, contracting muscles release multiple substances known as myokines, which promote the growth of new tissue, tissue repair, and various anti-inflammatory functions, which in turn reduce the risk of developing various inflammatory diseases.^[13]

The Physical Activity Guidelines for Americans were created by the Office of Disease Prevention and Health Promotion. This publication recommends that all adults should avoid inactivity to promote good health mentally and physically. For substantial health benefits, adults should participate in at least 150 minutes a week of moderate-intensity, or 75 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed in episodes of at least 10 minutes, and preferably, it should be spread throughout the week.

New (July 2011) guidelines in the United Kingdom include the following points: The intensity at which we exercise is key, and light activity such as strolling and housework is unlikely to have much positive impact on the health of most people. For aerobic exercise to be beneficial, it must raise the heart rate and cause perspiration. Everyone should do a minimum of 150 minutes a week of moderate-intensity aerobic exercise, but that really is the minimum for health benefits. If you do so beyond 150 minutes, you will gain even more health benefits. Sedentary time (time spent not standing, such as on when on the toilet or in bed) is bad for everyone's health, and no amount of exercise can negate the effects of sitting for too long. These guidelines are now much more in line with those used in the US, which also includes recommendations for muscle-building and bone-strengthening activities such as lifting weights and yoga.<https://www.nhs.uk/news/lifestyle-and-exercise/major-new-exercise-guidelines-announced/ ^[52] >

The US guidelines continue: For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 minutes (5 hours) a week of moderate-intensity, or 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity. Additional health benefits are gained by engaging in physical activity beyond this amount. Adults should also do muscle-strengthening activities that are moderate or high intensity and involve all major muscle groups on 2 or more days a week, as these activities provide additional health benefits.^[14]

Cardiorespiratory fitness can be measured using VO2 max, a measure of the amount of oxygen the body can uptake and utilize.^[15][16] Aerobic exercise, which improves cardiorespiratory fitness, involves movement that increases the heart rate to improve the body's oxygen consumption. This form of exercise is an important part of all training regiments ranging from professional athletes to the everyday person. Also, it helps increase stamina.

Examples are:

Specific or task-oriented fitness is a person's ability to perform in a specific activity with a reasonable efficiency: for example, sports or military service. Specific training prepares athletes to perform well in their sport.

Examples are:

For physical fitness activity to benefit an individual, the exertion triggers a response called a stimulus. Exercise with the correct amount of intensity, duration, and frequency can produce a significant amount of improvement. The person may overall feel better, but the physical effects on the human body take weeks or months to notice and possibly years for full development. For training purposes, exercise must provide a stress or demand on either a function or tissue. To continue improvements, this demand must eventually increase little over an extended period of time. This sort of exercise training has three basic principles: overload, specificity, and progression. These principles are related to health but also enhancement of physical working capacity.^[24]

High intensity interval training (HIIT) consists of repeated, short bursts of exercise, completed at a high level of intensity. These sets of intense activity are followed by a predetermined time of rest or low intensity activity.^[25] Studies have shown that exercising at a higher intensity has increased cardiac benefits for humans, compared to when exercising at a low or moderate level.^[26] When your workout consists of an HIIT session, your body has to work harder to replace the oxygen it lost. Research into the benefits of HIIT have revealed that it can be very successful for reducing fat, especially around the abdominal region.^[27] Furthermore, when compared to continuous moderate exercise, HIIT proves to burn more calories and increase the amount of fat burned post- HIIT session.^[28] Lack of time is one of the main reasons stated for not exercising; HIIT is a great alternative for those people because the duration of an HIIT session can be as short as 10 minutes, making it much quicker than conventional workouts.^[29]

Physical fitness has proven to result in positive effects on the body's blood pressure because staying active and exercising regularly builds up a stronger heart. The heart is the main organ in charge of systolic blood pressure and diastolic blood pressure. Engaging in a physical activity raises blood pressure. Once the subject stops the activity, the blood pressure returns to normal. The more physical activity that one engages in, the easier this process becomes, resulting in a more ‘fit’ individual.^[30] Through regular physical fitness, the heart does not have to work as hard to create a rise in blood pressure, which lowers the force on the arteries, and lowers the overall blood pressure.^[31]

Centers for disease control and prevention provide lifestyle guidelines of maintaining a balanced diet and engaging in physical activity to reduce the risk of disease. The WCRF/ American Institute for Cancer Research (AICR) published a list of recommendations that reflect the evidence they have found through consistency in fitness and dietary factors that directly relate to cancer prevention.

The WCRF/AICR recommendations include the following:

These recommendations are also widely supported by the American Cancer Society. The guidelines have been evaluated and individuals that have higher guideline adherence scores substantially reduce cancer risk as well as help towards control with a multitude of chronic health problems. Regular physical activity is a factor that helps reduce an individual’s blood pressure and improves cholesterol levels, two key components that correlate with heart disease and Type 2 Diabetes.^[33] The American Cancer Society encourages the public to "adopt a physically active lifestyle" by meeting the criteria in a variety of physical activities such as hiking, swimming, circuit training, resistance training, lifting, etc. It is understood that cancer is not a disease that can be cured by physical fitness alone, however, because it is a multifactorial disease, physical fitness is a controllable prevention. The large associations tied with being physically fit and reduced cancer risk are enough to provide a strategy to reduce cancer risk.^[32] The American Cancer Society asserts different levels of activity ranging from moderate to vigorous to clarify the recommended time spent on a physical activity. These classifications of physical activity consider the intentional exercise and basic activities are done on a daily basis and give the public a greater understanding of what fitness levels suffice as future disease prevention.

Studies have shown an association between increased physical activity and reduced inflammation.^[34] It produces both a short-term inflammatory response and a long-term anti-inflammatory effect.^[35] Physical activity reduces inflammation in conjunction with or independent of changes in body weight.^[36] However, the mechanisms linking physical activity to inflammation are unknown.

Physical activity boosts the immune system. This is dependent on the concentration of endogenous factors (such as sex hormones, metabolic hormones and growth hormones), body temperature, blood flow, hydration status and body position.^[37] Physical activity has shown to increase the levels of natural killer (NK) cells, NK T cells, macrophages, neutrophils and eosinophils, complements, cytokines, antibodies and T cytotoxic cells.^[38][39] However, the mechanism linking physical activity to immune system is not fully understood.

Achieving resilience through physical fitness promotes a vast and complex range of health-related benefits. Individuals who keep up physical fitness levels generally regulate their distribution of body fat and stay away from obesity. Abdominal fat, specifically visceral fat, is most directly affected by engaging in aerobic exercise. Strength training has been known to increase the amount of muscle in the body, however, it can also reduce body fat.^[40] Sex steroid hormones, insulin, and an appropriate immune response are factors that mediate metabolism in relation to the abdominal fat. Therefore, physical fitness provides weight control through regulation of these bodily functions.^[41]

Menopause is often said to have occurred when a woman has had no vaginal bleeding for over a year since her last menstrual cycle. There are a number of symptoms connected to menopause, most of which can affect the quality of life of a woman involved in this stage of her life. One way to reduce the severity of the symptoms is to exercise and keep a healthy level of fitness. Prior to and during menopause, as the female body changes, there can be physical, physiological or internal changes to the body. These changes can be reduced or even prevented with regular exercise. These changes include:^[42]

The Melbourne Women's Midlife Health Project provided evidence that showed over an eight-year time period 438 were followed. Even though the physical activity was not associated with VMS in this cohort at the beginning. Women who reported they were physically active every day at the beginning were 49% less likely to have reported bothersome hot flushes. This is in contrast to women whose level of activity decreased and were more likely to experience bothersome hot flushes.^[44]

Studies have shown that physical activity can improve mental health and well-being.^[45][46] This improvement is due to an increase in blood flow to the brain and the release of hormones. Being physically fit and working out on a consistent and constant basis can positively impact one's mental health and bring about several other benefits, such as the following.^[47]

To achieve some of these benefits, the Centers for Disease Control and Prevention suggests at least 30–60 minutes of exercise 3-5 times a week.^[49]

Physical fitness has always been an important part of life. It is theorised that when people left a hunter-gatherer lifestyle and formed fixed communities based around agriculture that physical fitness levels declined. This is not to say that levels of physical labour decreased but that the type of work undertaken was not necessarily as conducive to a general level of fitness. As such, regimented fitness regimes were either invented or became more common. This was especially the case in classical civilisations such as Ancient Greece and Rome. In Greece especially physical fitness was considered to be an essential component of a healthy life and it was the norm for men to frequent a gymnasium. Physical fitness regimes were also considered to be of paramount importance in a nation's ability to train soldiers and field an effective military force. Partly for these reasons, organised fitness regimes have been in existence throughout known history and evidence of them can be found in many countries.

Gymnasiums which would seem familiar today began to become increasingly common in the 19th century. The industrial revolution had led to a more sedentary lifestyle for many people and there was an increased awareness that this had the potential to be harmful for health. This was a key motivating factor for the forming of a physical culture movement, especially in Europe and the USA. This movement advocated increased levels of physical fitness for men, women and children and sought to do so through various forms of indoor and outdoor activity, and education. In many ways it laid the foundations for modern fitness culture.^[50]