Question

Name the chemical that causes fatigue in the muscle

Answer 1

It is a by-product of anaerobic metabolism, in which the body produces energy without using oxygen. This chemical causes fatigue and a burning sensation in the muscles after an intense workout.

Complete answer:
During strenuous exercise, we begin to breathe faster as we attempt to take in more oxygen to our working muscles. The body's auto mode tries to generate most of its energy using aerobic methods (meaning with oxygen).
As heavy weight lifting requires energy production faster than our bodies can adequately deliver oxygen. In that case, the muscles which are at work generate energy anaerobically. The energy source is basically glucose and this energy is produced through a process called glycolysis, in which glucose is broken down or metabolized into a substance called pyruvate.
When the body has plenty of oxygen, pyruvate alternates to an aerobic pathway to be further broken down for more energy. But when limited oxygen is supplied then the body temporarily converts pyruvate into a substance called lactate, which then allows glucose breakdown in the body and thus energy production continues.
This is a type of natural defense mechanism used by our body. It primarily prevents any permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction.
Additionally, since lactic acid is known to be a strong acid therefore it dissociates into lactate ions and H+ions. On one hand, lactate ions would have little effect on muscle contraction but on the other hand, the increase in ${ H }^{ + }$ (i.e., reduced pH or acidosis) is the most common cause of skeletal muscle fatigue.

Additional information:
It was German physician Otto Meyerhof, who through his experiment showed that lactic acid was formed from muscle glycogen in the absence of oxygen using frog legs in an air-tight jar. Along with another pioneer in the field, this research eventually led British physiologist Archibald Hill, receive the Nobel Prize for Physiology or Medicine in 1922.

Note: There are three main metabolic path-ways that provide our muscles with energy:
-Phosphagen pathway: This system uses creatine phosphate (CP) and has a very rapid rate of ATP production.
-Glycolytic pathway: It involves events of enzymatic reactions that break down glucose (glycolysis) into pyruvate, creating the energy sources adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH).
-Oxidative pathway: It is one of the metabolic pathways in which cells utilize various enzymes to oxidize nutrients to release the chemical energy stored within in order to produce adenosine triphosphate (ATP).