Theory:

The process of respiration is of two types based of the presence and absence of oxygen. They are
  1. Aerobic respiration
  2. Anaerobic respiration
Aerobic respiration:
The process of respiration in which the food (carbohydrates or glucose) we eat is broken down into carbon dioxide and water in the presence of oxygen is known as aerobic respiration.
                                    
Glucose inthepresenceofoxygen carbon dioxide + water + energy.
 
In this mode of respiration, a high amount of energy is released in the living organisms. Human beings and most of the animals undergo aerobic respiration.
Anaerobic respiration:
The process of respiration in that occurs in the absence of oxygen is known as anaerobic respiration.
The organisms in which this mode of respiration is observed are known as anaerobes. Yeasts are single-celled organisms that undergo anaerobic respiration and are hence known as anaerobes.
During anaerobic respiration, the food (glucose) consumed by the anaerobes is broken down to alcohol and carbon dioxide as represented below.
                                       
Glucose intheabsenceofoxygen alcohol + carbon dioxide + energy.
 
As yeasts produce alcohol during their respiration they are used in the manufacture of wine and beer.
 
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Anaerobic respiration in humans:
 
The process of anaerobic respiration is also seen in the cells of our muscles when oxygen is deficient. This condition comes into action at the time of heavy exercise, prolonged fasting, cycling, walking for many hours where the demand of energy is very high.
During this lactic acid is produced from glucose in the muscle cells.
 
Glucose intheabsenceofoxygen lactic acid + energy
Muscle cramps and the cure:
It is a condition that indicates the process of anaerobic respiration. It is due to the production of lactic acid from glucose during anaerobic respiration. A hot water bath or massage are used to improve the oxygen flow to the muscle cells. The circulation of blood is improved by a hot water bath or massage, which enhances the supply of oxygen to the muscle cells, causing them to respire aerobically and reduce lactic acid production.
Reference:
https://www.vecteezy.com/vector-art/1424977-alcoholic-fermentation-chemical-equation