Fitness is a concept that supports many sports science meanings. In its most general application, fitness describes the current levels of both physical health and physical capabilities present in an athlete. Athletes have an innate understanding of what fitness is through personal experience; physical fitness is the expression that is also used to describe the optimal physical condition or "shape" of an athlete at a given time.

The traditional definition of physical fitness as employed by sport experts until the mid-twentieth century concerned the range of the physical capacity of an athlete; current definitions have evolved to include a greater focus upon the general health of every bodily system that might influence fitness. The foundation question in any determination of physical fitness is the assessment of the athlete's ability to perform athletic activities vigorously, a process that involves a consideration of five distinct benchmarks. These benchmarks include: aerobic or endurance fitness, with regard to the function of both the cardiovascular system and the cardiopulmonary system; muscular endurance, representing the ability to generate sustained muscle output; muscular strength, the maximum available power; flexibility, defined as the range of motion achieved in the movement of the joints, the combined effect of the elasticity of muscles, tendons, and ligaments; and body composition, determined by the percentage of body fat in contrast to bone and muscle structure.

A physically fit athlete may possess greater degrees of fitness in one or more of the five individual fitness headings than another: fitness is a cumulative measure. As an example, a world-class soccer player may not possess formidable muscular strength, but this deficit, which is usually of secondary importance to success in that sport, will be amply compensated in the other four categories, particularly those of endurance and flexibility. Conversely, when an athlete is demonstrably fit in only one of the five areas, it is unlikely that he or she shall possess true comprehensive physical fitness. This phenomenon is often observed in disciplines such as weightlifting, in which the athlete is able to generate remarkable muscular strength, but may possess less measurable fitness in the remaining categories.

Fitness is a concept which also may be understood in contrast to other familiar aspects of human

Regular participation in exercise such as step aerobics can help individuals achieve a strong level of physical fitness.
performance. Physical fitness is commonly linked to considerations of life expectancy and longevity. The questions posed by these considerations seek to answer if physically fit persons tend to live longer than sedentary, unfit persons, or if a commitment to lifelong physical fitness actually extends life expectancy, or if such a commitment simply makes a genetically predetermined allotment of years more pleasurable.

There appears to be little question that healthy living practices, combined with a program of physical fitness, will help reduce the risk of early life-ending diseases such as arteriosclerosis and other potentially fatal conditions of the cardiovascular system. Cardiovascular disease is the cause of more death among women in the Western world than all forms of cancer combined. Exercise contributes to the reduction of excess body weight and lessens all of the strains placed on the various physical systems that are caused by obesity. However, elimination of an early cause of ill health or death is not itself the extension of the limit of life expectancy.

Fitness is sometimes sought by adult persons who have lived a demonstrably unhealthy adolescence or young adulthood. In many circumstances, these persons are seeking to reverse the negative lifestyle practices that have impacted their fitness for many years. No matter how devoted to a more healthful lifestyle in later adulthood a person may become, it is unlikely that the poor fitness choices made by an adult earlier in life can be entirely reversed. If the strength and density of physical structures such as the musculoskeletal system were compromised due to a calcium deficiency, or if the cardiovascular system was subjected to excess quantities of plaque-creating cholesterol, the impact of such negative factors may not be eradicated by subsequent health and fitness choices; however, the harmful effects will be lessened over time.

Since the early 1900s, medical science has developed techniques to eliminate numerous previously fatal conditions, particularly in terms of both the prevention of communicable disease, as well as interventions that preserve life. There exists no empirical evidence to confirm that athletes live longer than non-athletes; athletes are more likely to enjoy a healthier, more desirable quality of life. It would appear that the only provable manner of extending one's life from an otherwise expected limit is to maintain strict control of calorie consumption and limit the related negative impacts of excess weight.

SEE ALSO Cross training; Habitual physical activity; Longevity.