Sports science in English
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The body needs energy to move. Muscles (see muscular system) transform chemical energy (derived from food intake) into mechanical work, or movement. Food molecules are broken down to the cellular level through chemical reactions occurring in the mitochondria (see muscle fibre) of the cell. Muscles contain the ATP (adenosine triphosphate) that provides energy for the muscle to contract and relax. Average amounts of ATP are usually found in muscles, but even if in small amounts, they are sufficient to ensure contractions of a few seconds. During these short stages, ATP produces energy and it turns into ADP (adenosine diphosphate). To provide energy for muscle contractions longer than 6.8 seconds, ADP needs to be converted back to ATP (ATP recycling), which can occur through three mechanisms.

• Anaerobic alactacid mechanism: it exploits the presence of phosphocreatine (PCr) inside muscle fibres as energy storage. It is activated in the absence of oxygen (anaerobic actually means «without oxygen») and with no lactic acid build up (alactacid). It is very powerful and it works immediately, but it does not last long, as the ATP and PCr run out quickly. It is exploited for spurt, speed, throwing and jumping activities, in which the power plays a major role.
• Anaerobic lactacid mechanism: it is activated if the effort lasts more than 10 seconds and the anaerobic alactacid mechanism is not sufficient to form the necessary ATP. It does not use oxygen, as it exploits glycogen deposits stored in muscles and liver (obtained from the breakdown of carbohydrates and lipids taken with food). Glycogen combines with ADP to form ATP. Since the process occurs without oxygen, lactic acid is formed in muscles with the energy. When lactic acid exceeds a certain amount, the contraction may become painful and fatigue takes over. Therefore, the use of this mechanism depends on the amount of lactic acid that the muscle can tolerate (maximum effort can last 1-2 minutes). After the effort there is the recovery phase, in which the body tries to compensate for a debt of oxygen in the muscles caused by the fatigue: this is why one feels out of breath! The oxygen introduced during the recovery phase decreases the amount of lactic acid, by converting it back into glycogen, or by burning it completely.
• Aerobic mechanism: it occurs when the effort has been going on from several minutes to several hours. It exploits carbohydrates and lipids in the muscles as fuel and oxygen as oxidizing. When glucose and fats in the muscles come into contact with the oxygen in blood, they burn, thus producing energy required to the transformation of ADP into ATP and leaving carbon dioxide, expelled through breathing, and water, excreted in sweat, as waste products. This mechanism enters into function very slowly, but it produces a large amount of energy. Furthermore, there is no oxygen debt, as the consumed oxygen is in balance with the demands of the body. However, there are factors limiting aerobic activity: the availability of lipids and carbohydrates energy stores and the resistance of the person, which can be improved by training. In particular, when an athlete runs at a fast pace and he/she can manage to keep the balance between oxygen consumption and oxygen need without affecting the lactacid mechanism (anaerobic threshold), he/she will be able to take a strain longer than another athlete who, at the same speed, cannot manage this mechanism.