Question

Why do we get tired of quick exercise?

Answer 1

Hint: Tiredness is related to anaerobic respiration by skeletal muscle cells. One of the end products of fermentation gets accumulated within our muscle cells which results in tiredness.

Complete answer:
Quick exercise is often referred to as aerobic exercise, during this type of exercise the body tries to ensure the rate of $O_2$ delivery and nutrient supply to muscle cells is optimum so that the rate of ATP breakdown during muscular contraction is equal to the rate of ATP synthesis.
The instant energy source of muscle cells is ATP and phosphocreatine, which supply the energy sources initially, but if the duration of exercise extends to more than a few seconds then the body relies on the intramuscular glycogen stores for energy. The glycogen stored in muscle cells is first converted to glucose and then glucose is metabolized anaerobically. The anaerobic breakdown of glucose has a great advantage as it provides energy very quickly but the ATP yield in this process is very low. Rapid depletion of glycogen stores in muscles occurs and therefore energy supply stops. Further lactic acid accumulates within our muscular tissue which needs to be removed. Therefore the limit of quick exercises extends to around one minute after which we start to feel tired.
This nonoxidative or anaerobic mode of glucose metabolism provides almost 100-fold more energy than what is obtained by using ATP and phosphocreatine stored in muscles. Therefore we can understand that when these short term sources of energy get depleted it results in tiredness or fatigue of muscles, and hence we are unable to carry out further activities requiring muscular work.

Note: Long term sources of energy include oxidation of glucose, lactate, and fatty acids, but the rate of supply of energy is slower. Aerobic oxidation of glucose provides us with almost 15 times more energy than obtained from non-oxidative metabolism of glucose.
In the case of prolonged exercises, hepatic gluconeogenesis takes priority as the glycogen stores in liver hepatocytes or liver cells get depleted. So during very long exercise periods, lactate is released into the blood circulation by contraction of skeletal muscles. This lactate is taken up by the liver cells and resynthesized into glucose for reuptake by the skeletal muscle cells. This specific cycle where lactate is recycled is known as the Cori Cycle.