When we get physically active a lot of physiological changes happen. Our body recognises the stress via complex mechanisms and answers accordingly. The most obvious things like increase in breathing frequency and higher pulse rate appear at first. With longer periods of exercise our body adapts itself through changes on a molecular level.
Any muscle contraction is powered through adenosine triphosphate (ATP). ATP is produced via three different mechanisms ATP- PC (phosphocreatine) system, the glycolytic system, or the oxidative system. Which of these three mechanisms is active depends on the duration of exercise. When performing explosive and short movements such as sprints or heavy lifts ATP-PC system can provide a burst of immediate energy for about twelve seconds. First few seconds from that are energised by immediate ATP and in the last few seconds phosphocreatine(PC) steps in until another energy system starts. A rough estimate of what energy this system can produce is roughly 36kcal per minute. When ATP stores are depleted energy source shifts to glycolytic system. Dietary glucose in blood and glycogen in muscles or liver is used to produce further ATP through the process of glycolysis. After the initial 12 seconds of maximum energy, there is 30s period of fast glycolysis in which lactic acid is produced and is followed by fatigue and burning sensation in muscle. After the effort of another 50s muscles enters slow glycolysis in which it depends on oxidative system. Pyruvic acid is a byproduct of glycolysis. In fast glycolysis it converts itself to lactic acid causing immediate fatigue. In slow glycolysis it gets converted to acetyl coenzyme A and is transported to Krebs cycle, which produces more ATP and so delays the onset of fatigue. The Krebs cycle also utilises the triglycerides as fuel. This process is called lipolysis. The byproducts are free fatty acids and glycerol. However before entering the Krebs cycle free fatty acids are mediated through beta-oxidation where they undergo a series of reaction to produce acetyl coenzyme A which can enter Krebs cycle to produce more ATP. In extremely prolonged activities proteins can be used as last resort fuel.
A lot of hormonal and other chemical changes happen when under stress. Discomfort caused by exercise leads to the release of endorphins from pituitary gland. This is also what causes addiction to exercise in some cases or what is described as ‘’runners high’’. Higher levels of serotonin combined with elevation of endorphins is what makes exercise a pleasurable activity. Dopamine is another chemical released during exercise and gives the feeling if satisfaction.
There is a significant rise in prolactin and cortisol levels during exercise. A rise in growth hormone during exercise is also observed and falls in the rest period. Testosterone concentrations rise independently of LH and reach their peak and around 35 min of activity. Prolactin levels inhibit further secretion of testosterone secreted by Leydig cells. It should be noted that testosterone concentration was increased after weightlifting or swimming, prolonged exercises cause a fall in plasma concentration.
Acknowledgment I would like to express appreciation for the support of the sponsor Health Centre Celje.
Åstrand P-O, Rodahl K. Textbook of Work Physiology—Physiological Bases of Exercise, 3rd Edn. McGraw–Hill Book Company, 1986.
Brooks GA, Fahey TD. Exercise Physiology—Human Biogenetics and its Applications. Macmillan Publishing Company, 1985.
McArdle WD, Katch FI, Katch VL. Exercise Physiology: Energy, Nutrition and Human Performance, 5th Edn. Lippincott, Williams and Wilkins, 2001.
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