Answer:
In the given case, anaerobic metabolism is used. In the non-existence of oxygen, energy gets produced in the body by the combustion of carbohydrates and the process is termed as anaerobic metabolism. This generally takes place, when the lungs are not able to provide adequate oxygen into the blood in order to sustain the requirement of energy for the muscles.
It is basically used only for performing brief bursts of activities like cycling, running or lifting heavyweights. In case, when blood is lacking enough oxygen, glycogen and glucose cannot get further dissociated into water and carbon dioxide. Thus, in that place production of lactic acid takes place, which accumulates in the muscles and reduces its activity.