Energy is released when organic substances decay. Organic substances come with food. For the decay of organic substances a person needs oxygen. The volume of oxygen that enters the human body with different types of breathing at rest is sufficient to provide the body with energy. When heavy physical exertion, for example in sports, you need additional oxygen and efficient removal of carbon dioxide from the body.

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https://www.healthguidance.org/entry/3538/1/energy-how-does-my-body-get-it-and-how-does-it-use-it.html

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There is a clear threshold during exercise where the amount of oxygen - delivered by increased ventilation (hyperventilation) - can not supply the organism with sufficient energy. At this level (called anaerob threshold) the organism turns to the anaerob process to deliver energy be burning glucose in the muscle cells (glycolysis). Glycolysis is an oxygen- independent metabolic pathway that converts glucose into pyruvate. The free energy released in this process is used to form the high -energy molecules ATP and NADH. From this point the the CO2 production exceeds the oxygen uptake during exercise resulting an increaser respiratory quotient (RQ) of > 1 (the curves of CO2-production and O2-uptake are crossing) and lactate acidosis in the muscles (this letter serves also as energy source in the mitochondria)

You can assess the anaerob threshold (AT) easily by spiroergometry, and it characterizes the conditional state of a person.

Thank you Professor János for the wonderful answer ... Greetings to you

Thank you Professor Hikmet Abdulkarim for recommending ... Greetings to you