Muscle Fatigue: Summary for NEET

Muscle fatigue occurs when a muscle's supply of ATP and glycogen is depleted and lactic acid accumulates as a result of repeated contractions at short intervals. As a result, its contractility gradually declines and eventually ceases. Muscle fatigue occurs when muscles are excessively used. Muscle tiredness is a typical occurrence that impairs sports performance and other severe or extended activities. Muscle fatigue results in a reduction in force production caused by activity. When you first begin to exercise, your muscles feel strong. As time passes and you repeat actions, your muscles may become weaker and exhausted.

Muscle weakness may be divided into two categories: central (neural) and peripheral (muscular). Peripheral fatigue affects the motor units and is caused by mechanical and cellular alterations in the muscular system. The physiological processes that occur within the central nervous system are referred to as central tiredness (CNS).

In this article, you will be able to cover all the sub-topics that come under muscle fatigue, such as the definition of muscle fatigue, causes, symptoms, treatment, and recovery. 

Muscle Pain and Fatigue 

Pain and fatigue can happen simultaneously during prolonged tiring muscular activity, especially when one approaches the limit of tolerance, and both can impair muscle performance. Chronic myofascial pain (such as fibromyalgia) or contraction-induced pain is common in patients with neuromuscular diseases (such as metabolic myopathies). These two types of pain may overlap in some patients, and both may impair the central motor drive throughout the exercise.

What Causes Muscle Fatigue?

It is caused by a variety of mechanisms at a number of different sites rather than by a single mechanism. The proportional importance (contribution) of causes varies, depending on the task.

Central Fatigue 

1. Central fatigue is defined as a progressive reduction in voluntary muscle activation. The inability of the CNS to drive the muscles optimally results in a loss of force.

2. Central fatigue is caused by a reduction in the neuronal drive to the muscles. This begins in the central nervous system (CNS). It is triggered by an inhibition that is evoked by nerve signals from receptors (likely chemoreceptors) in tired muscles.

3. Inhibition can affect motor pathways anywhere in the brain, from the voluntary centres to the spinal motor neurons. This type of exhaustion is characterised by a reduction in the outflow of motor impulses to the muscles.

4. In the motor cortex, there are disruptions in motor coordination that interfere with the basal ganglia's motor control circuits and resistance to voluntary effort.

5. Motivation and perceived competence are two psychological aspects to consider, which are the awareness of pain, the ability to tolerate it and neurological issues. 

6. The primary reason that people cease exercising in the latter is because of central tiredness.

 The muscle fatigue path

Peripheral Fatigue   

1. It is characterised as the body's inability to send enough energy or other metabolites to the contracting muscles to meet the increasing demand for energy. It produces contractile dysfunction, which results in the eventual decline or loss of a single muscle or a local group of muscles' ability to perform work.

2. Fatigue (forced loss), caused by abnormalities at or distal to the neuromuscular junction, is characterised as peripheral fatigue. Peripheral fatigue will always be the initial sort of fatigue. 

3. Changes in or around the motor neuron cause peripheral fatigue. The reason might be a lack of essential substance(s) and/or a build-up of catabolites or other compounds that are released by muscular action. 

4. The Krebs cycle converts the pyruvate, produced by glycolysis, into more ATP molecules.

5. Pyruvate, produced in the cytoplasm, is usually sent to the mitochondria, where it is entirely converted to carbon dioxide. Pyruvate cannot enter the Krebs cycle due to a lack of oxygen at the time of high exercise activity. It accumulates in the muscle fibres. 

6. When lactic acid builds up in the muscles, the pH drops to around 6.4 to 6.6, causing a stinging sensation in the muscles. At this low pH, energy generation and muscle contraction begin to slow down at the cellular level. Muscle contractions cease when myosin can no longer bind to actin. Anaerobic respiration is the process of breathing without oxygen.

7. Metabolic fatigue is caused by a lack of substrates, such as adenosine triphosphate (ATP), glycogen, and creatine phosphate.

8. Damaged and poisoned mitochondria will be unable to keep up with the intake of pyruvate if they are not functioning properly. Lactic acid builds up during aerobic respiration when you exercise, and your body breaks down glucose for energy and produces ATP which is needed for muscle contraction.

9. When there is not enough oxygen to finish the process during intense activity, a chemical called lactate is produced.

10.  Thus, the bloodstream is incapable of providing it at the required speed. The Cori Cycle transports lactic acid to the liver, where it can be stored, before being converted to glucose in the presence of oxygen. 

Muscle fatigue can be caused by the accumulation of substances such as chloride, potassium, lactic acid, ADP, magnesium, reactive oxygen species, and inorganic phosphate.

Muscle Fatigue Symptoms 

Muscle fatigue may affect any part of the body. Muscle weakness is an early indication of this illness. Muscle tiredness is also associated with localized pain, soreness, muscle twitching, a weak grip, shortness of breath, trembling, and muscle cramps.

Seek emergency medical assistance if symptoms get worse. This might be a sign of a more significant health problem.

Muscle Fatigue Recovery

The time it takes for the muscles to recover after exercise is determined by your degree of fitness, and the complexity of your activity. 

The volume, intensity, and length of your workout all affect how exhausting it is on your body. A very easy activity may allow your muscles to recuperate in 24 hours, but a more strenuous workout may take two to three days. Workouts that are really strenuous may take much longer.

Muscle Fatigue Treatment

The healthcare practitioner will offer suitable treatment once they have discovered the reason for the muscular weakness. The intensity of symptoms, as well as the underlying reason for the muscular weakness, will determine the treatment approach.

There are some therapy options for disorders that produce muscular weakness, such as physiotherapy and occupational therapy (OT). OTC pain medicines, such as ibuprofen or acetaminophen, can help control pain that is caused by illnesses, such as CFS, peripheral neuropathy, and neuralgia.

Conclusion

This article has given all the useful details about muscle fatigue, with respect to the NEET syllabus. The questions that are provided at the end of this article are useful and important from the exam’s point of view and help in clearing the doubts about this topic. It will also help in qualifying for the exam.