Why does muscle fatigue occur?

Hint: During heavy physical activities such as running, cycling, heavy weightlifting and heavy exercise, anaerobic respiration takes place in muscles instead of aerobic respiration which causes muscle fatigue.

Complete Answer:
Due to vigorous exercise, muscles may begin to feel weaker and tired. This is called muscle fatigue.
- During heavy exercise, running, cycling, weightlifting etc. the demand for oxygen in the body increases but the oxygen demand cannot be fully filled. As a result anaerobic respiration takes place.
- In the absence of oxygen glucose $({C_6}{H_{12}}{O_6})$is converted into${C_3}{H_6}{O_3}$i.e. lactic acid.
- The accumulation of lactic acid in the muscle causes fatigue and pain.
- The symptoms of muscle fatigue are – soreness, pain, shortness of breath, muscle cramps etc.
- The decrease in substrates like ATP (Adenosine triphosphate, glycogen and creatine phosphate) causes metabolic muscle fatigue.

Note: Flow of electrical impulse from the brain signals the muscle cells to contract. Calcium ions play an important role in muscle contraction.
- When glucose molecules are converted into the lactic acid and energy, this process is known as lactic acid fermentation.
- When the body's oxygen requirement is fulfilled, lactic acid is converted into carbon-dioxide and water and energy.
\[\mathop {{C_6}{H_{12}}{O_6}}\limits_{Glu\cos e} \underrightarrow {In\,the\,absence\,of\,oxygen}\;\mathop {2{C_3}{H_6}{D_3}}\limits_{Lactic\;acid} + Energy\]
\[\mathop {2{C_3}{H_6}{O_3}}\limits_{Lactic\;Acid} + \mathop {6{O_2}}\limits_{Oxygen} \to \;\mathop {6C{O_2}}\limits_{Carbon - dioxide} + \mathop {6H{}_2O}\limits_{Water} \; + \;Energy\]