The capacity of the organism to accept, transmit and use oxygen efficiently (especially while engaging in physical activities) can be defined as aerobic fitness. The main effects of this capacity are manifested in the ability of the muscles to perform and endure. A significant side-effect is more efficient cardiovascular activity and, under certain circumstances, also reduction of excessive fat.

If we are to maintain or improve aerobic fitness, it is necessary to engage regularly and for a sufficient time in a physical activity involving big muscle groups (e.g. brisk walking, running, cycling, swimming, cross-country skiing, aerobics, movement activities or sports games). The effort invested in these activities should be reflected in the heart rate (HR), which ranges between 60 – 80% of the maximal heart rate (HRmax). This can be counted using the following formula:

SFmax = 220 pulses/min. minus a person’s age

The strain within the range of 60 – 80% HRmax, when we are forced to breathe deeply and quickly, is called a medium intensity strain (often also aerobic strain). The activity should be at least 10 minutes long, in total the whole-day activity should reach at least 60 minutes for children and at least 30 minutes for adults

The above mentioned medium intensity strain should be supplemented with an everyday activity of lower intensity (under 60% HRmax), following at least the above stated time limits (60 minutes for children, 30 minutes for adults). Ordinary everyday walking is also very important. The World Health Organisation suggests that adults should take at least 10 thousand steps a day, and children at least 12 thousand steps a day.

During high intensity strain (anaerobic strain) the heart rate achieves values over 80% HRmax (in children usually more than 170 - 180 pulses/min.). Such strain in less adapted individuals can result in quick lactate accumulation in muscles, quick muscle fatigue as well as muscle pain and lack of oxygen (we cannot “catch our breathˮ). This state usually occurs in less fit people after 15 - 20 seconds of intensive activity (e.g. sprinting, running up the stairs, riding a bike up a steep hill). Because such a strain can even be dangerous for weak individuals we do not advise children or unfit adults to maintain the maximum intensity of strain for longer than the above mentioned 20 seconds. It must be followed by some rest or physical activity of moderate strain. If we follow this principle, high intensity and lower intensity strain can alternate, which is typical for example in movement games (e.g. playing tag), relay competition games, or sports games done as recreation, etc.

The table below (Tab. 3) illustrates approximate values of the heart rate of a person during physical strain:

Tab. 3: Approximate values of the heart rate during physical strain

Age	Max. HR (pulses/min.)	High anaerobic strain (above 80% HRmax)	Recommended aerobic strain (60–80% HRmax)	Low strain (below 60% HRmax)
10	210	170-210	125-170	< 125
20	200	160-200	120-160	< 120
30	190	150-190	115-150	< 115
40	180	140-180	110-140	< 110
50	170	130-170	100-135	< 100
60	160	120-160	95-130	< 95

Any physical activity but also other bodily functions (e.g. maintaining body temperature) require a certain amount of energy which is continuously released into the human body. The organism therefore needs a regular supply of food, i.e. of chemical compounds which are able to release energy during splitting. The conversion of substances and energies in a living organism is called metabolism.

The immediate source of energy for muscle contraction comes from adenosine triphosphate (ATP), whose chemical energy transforms into mechanical energy. ATP is converted in the process called hydrolysis to adenosine diphosphate (ADP) and adenosine monophosphate (AMP). This process can be expressed by this equation:

ATP → ADP + AMP + energy

In the process of splitting ATP releases energy necessary for the function of muscle fibres (filaments of actin move into the filaments of myosin). Because the amount of ATP in the muscle is available only for a few seconds (depending on one’s adaptive capacity), ATP must be continuously renewed (resynthesized). Energy for the renewal of ATP comes from burning of substances rich in energy, such as glucose, fats and in a smaller amount also proteins.

Unlike other organs, muscles have immediate access to another stock of energy in the form of creatine phosphate (CP) which supplies energy immediately after ATP splits for its resynthesis. At the same time ADT enters the reaction:

CP + ADP → ATP + C

This way energy output can be covered for up to 20 seconds of intensive physical activity when a new supply of ATP and creatine (C) is being formed.

For the renewal of ATP other energy sources are available. The process during which sugars are burnt (glucose and glycogen) in the chemical reaction without the presence of oxygen is called anaerobic glycolysis:

G + ADP + AMP → ATP + LA

Apart from ATP this reaction also generates lactate (LA) whose energy can be used either for direct burning in a muscle cell or, after being transported through the blood circulation system, for the activity of the cardiac muscle, respiratory muscles and other less active muscles. A smaller part of LA (about 40 %) is resynthesised in the liver into glucose and glycogen, which can be reused as a source of energy.

Anaerobic glycolysis starts immediately after the beginning of intensive physical activity. The strength of this reaction gradually grows and culminates between the 20^th and 60^th second. Anaerobic glycolysis allows for several tens of seconds of intensive physical activity, but during this process in less adapted individuals LA accumulates in the muscles and blood circulatory system, which results in muscle fatigue and pain that is very difficult to overcome. Therefore we do not recommend very intensive physical activities (with high heart rate) for a period longer than 15 - 20 seconds for pre-primary or primary school children. Unless the maximum physical performance (i.e. anaerobic process) is maintained by a person’s will on the edge of endurance (as is the case of athletes in the 400 and 800m run), a spontaneous drop in intensity occurs and the organism gradually changes to the aerobic process. This process makes less intensive and longer-lasting physical activities possible, which is beneficial for the cardiovascular activity. These are chemical processes during which substances split with the presence of oxygen. In the process of burning glucose (G), LA, and fats - which are actually free fatty acids (F), the end products generated with the presence of oxygen are phosphates and some proteins, esp. ATP, water and carbon dioxide. The basic chemical reaction is as follows:

G (nebo T) + AMP + ADP + O₂ → ATP + CO₂ + H₂O

We need to stress that it is not possible to isolate aerobic processes from anaerobic processes in the human body. In fact, the body is a system where aerobic and anaerobic processes simultaneously take part in the process of releasing energy, especially in long-term and more intensive activity. In less intensive activity the aerobic process prevails, whereas in more intensive activity the anaerobic process predominates.

When sugars are burnt during a particular physical activity, it is possible regarding their low stock in the organism to perform the activity (depending on its intensity) for about 30 minutes. As sugars get consumed gradually, fats increasingly become the major source of energy. Compared to glycolysis (oxidative sugar splitting), these are more complex chemical reactions, which is the reason why the intensity of physical activity decreases, but thanks to large supplies of fat in the body the activity can last for several tens of hours. Unless the intensity of the physical activity is too high, the movement is not accompanied by any stronger negative perceptions (muscle fatigue or pain) and is beneficial for human health.

Proteins can also be used as a source of energy, but are primarily assigned the task to build and renew tissues. They can be used as a source of energy on a larger scale only when the supplies of sugars and fats are consumed.

For more detailed information see specialist literature.