Answer
Muscle fatigue can be defined as weakness and loss of ability of muscles to contract.
There are several factors that contribute to loss of contractility by muscles after prolonged use or in
short term high intensity activity ( exercise or work).
1. Potassium accumulation
Action potentials (AP) in muscles release potassium (K+) ions from the sarcoplasm to the ECF/ISF
(extracellular fluid) this hyperpolarizes the cells and makes them less excitable.
2. ADP/Pi accumulation. The hydrolysis of ATP leads to an increasing number of ADP molecules and Pi ions. This has the effect of slowing down crossbridge cycling . In addition, free Pi ions inhibit the following processes, and mechanisms: calcium release from SR ; calcium sensitivity of the contractile mechanism, and force production by myofibrils .
3. Lactic acid accumulation. The role of lactic acid in causing muscle fatigue is controversial. It is believed in some quarters that lactic acid lowers muscle pH (6.5) and that the lowering of pH changes conformation of proteins. These changes are believed to interfere with troponin binding of Ca++; cross-bridge formation, and ATP hydrolysis by myosin. Some authorities in Physiology think lactic acid has no effect on muscle fatigue.
Work Step by Step
4. Fuel depletion.
When levels of muscle glycogen and blood glucose fall, this leaves insufficient fuel for ATP generation. In athletics this condition is called " hitting the wall". Athletes try to delay the onset of this condition by "loading up" on carbohydrates before an athletic contest.
5. Electrolyte loss:
Sweating during exercise causes the athlete to lose not only fluid, but also electrolytes. The electrolyte loss may be great enough to upset the balance between intracellular fluid ( in sarcoplasm) and extracellular fluid (ECF). When this balance is upset the excitability of of muscles declines.
6. Central fatigue.
Ammonia (NH3) is generated when muscles are worked or exercised. This ammonia is carried by the blood to the where it is absorbed. The absorption of ammonia by the brain results in an inhibition of the activity of motor neurons. As a consequence, the CNS sends fewer signals to skeletal muscles and psychological fatigue, or decrease in the will to carry on with the activity ebsues..
