DEFINITION OF PHYSICAL FITNESS

In 2006, the American College of Sports Medicine (ACSM) defined physical fitness as, “a set of physical attributes that people have or achieve that relates to the ability to perform physical activity” (Wilder et al., 2006).

Until then, physical fitness was defined in several ways, but according to 2013 ACSM guidelines (Thompson, Arena, Riebe, Pescatello, & American College of Sports, 2013), the most well‐accepted definition for physical fitness was “the ability to carry out daily tasks with vigor and alertness, without undue fatigue, and with ample energy to enjoy leisure‐time pursuits and meet unforeseen emergencies”.

AEROBIC AND ANAEROBIC FITNESS

To perform any task, energy is needed. Because physical performance involves muscular work, the degree of performance of the task at hand (i.e., the degree of fitness) depends mainly on how the energy is made available and used by the muscles. In specific, it is not energy that is delivered to the muscles but the raw materials for example fat or glycogen. From these materials, the muscle cells extract the energy required for the task at hand. Cells can extract the necessary energy in one of two ways: with the use of oxygen and without oxygen. The use of oxygen to extract energy is referred to as aerobic metabolism (from the Greek word “aerobiosis”, air‐dependent living). Conversely, extracting energy without using oxygen is referred to as anaerobic (air‐independent living) metabolism.

The derivation of energy aerobically or anaerobically depends exclusively on the intensity of the activity. In high‐intensity activities energy is mainly derived without oxygen from components that are already stored in our body and are readily available. In low‐intensity activities oxygen is used from the muscles to break down energy sources like fats or glucose to perform the exercise. Thus, activities referred to as aerobic or anaerobic accordingly. In general, in aerobic processes more energy is produced from the use of the oxygen than anaerobic processes, but anaerobic processes release energy faster. Aerobic fitness refers to the degree or ability to provide the required energy for a specific task aerobically. Conversely, anaerobic fitness refers to the body’s ability to provide the required energy for a specific task anaerobically.

Aerobic exercises or activities consist of repetitive, low resistance movements (walking or cycling) that last over a relatively long period of time (generally 5 minutes or more). Anaerobic exercises or activities, on the other hand, are characterized by bursts of intense activity lasting only a short time. Such activities include lifting a very heavy weight, jumping, sprinting, etc. These activities challenge the body to maximum or near maximum efforts. They require a great deal of energy within a short span of time and can only be sustained for a few seconds to minutes. The energy requirements are met predominately without the use of oxygen. For example, no oxygen is required to meet the energy necessary to run 100 meters or lift a heavy weight.

These two energy systems (aerobic and anaerobic) almost always work together harmoniously, sharing the responsibility for providing the energy for the entire body. However, one is likely to be the predominant system and provide most of the energy for the particular activity at hand.

As it will be described in the next paragraphs, physical fitness can be further grouped into health‐ and skill‐related components (Table 4.1) (Thompson et al., 2013).

HEALTH‐RELATED PHYSICAL FITNESS COMPONENTS

Health‐related components of physical fitness include five components of physical fitness:

1. cardiorespiratory fitness,
   
2. body composition,
   
3. muscular strength,
   
4. muscular endurance, and
   
5. flexibility.

CARDIOVASCULAR OR CARDIORESPIRATORY FITNESS

Cardiorespiratory fitness (CRF), also known as cardiovascular fitness, is the ability to perform dynamic exercise using large muscles for prolonged periods, at specific intensities and frequency.

The degree of CRF depends on the coordinated functional state of three systems: (1) the respiratory system (lungs) to provide the necessary oxygen for the muscles, (2) the cardiovascular system (heart and vessels) to deliver the nutrients and oxygen requirements to the working muscles, and (3) the muscular system that uses the oxygen and nutrients delivered to meet the energy demands of the activity at hand. Naturally, the more efficiently these three systems become, the higher the performance will be.

Thus, aerobic fitness can be defined as the ability of the circulatory and the respiratory systems to supply the necessary oxygen for the muscle during prolonged work. Consequently, aerobic fitness is referred to as CRF or endurance. Because cardiovascular, cardiorespiratory, and aerobic fitness or aerobic capacity terms are so closely related, they can be used inter‐changeably.

Low CRF is considered as a independent predictor of cardiovascular disease (CVD) and all‐cause mortality in adults. Today, it is well‐known that being physically active is linked to better health outcomes independent of CRF, a concept that would have been considered unorthodox a few decades ago (Myers, Kokkinos, Arena, & LaMonte, 2021). The role of physical activity in health will be discussed further in Chapter 6.

BODY COMPOSITION

With respect to health and fitness, body composition is used to describe both the fat‐free mass (FFM), which includes the skeleton, muscles, and body water, and the fat mass (FM) in human bodies. Both FFM and FM are very important for the assessment of health status.

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