Dominic Stone
Muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It is a reversible decline in an objective measure of performance that occurs as a result of both submaximal and maximal exercise. It can be caused by vigorous exercise, but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: neural fatigue and metabolic fatigue. Neural fatigue is the limitation of a nerve's ability to generate a sustained signal, while metabolic fatigue is the reduced ability of the muscle fiber to contract.
| Characteristics | Values |
|---|---|
| Definition | Muscle fatigue is the incapacity to maintain the required or expected force. |
| Causes | Muscle fatigue can be caused by vigorous exercise, but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes: neural fatigue and metabolic fatigue. |
| Treatment | Muscle fatigue often improves with rest and recovery. Staying hydrated, maintaining a healthy diet, and doing aerobic cool-downs can aid recovery. Synthetic products like amphetamine and caffeine, natural products like ginseng and rhodiola rosea, and nutritional supplements like vitamins and minerals have also shown some effects in treating muscle fatigue. In some cases, medical attention may be required, and a doctor may prescribe anti-inflammatory or antidepressant medications or recommend physical therapy. |
| Prevention | Prevention of muscle fatigue includes listening to your body, wearing compression stockings during workouts, and doing endurance training. |
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What You'll Learn
Neural fatigue
Neurofatigue is not the same as physical fatigue. Patients with neural fatigue may feel completely drained after performing simple tasks, such as reading a book, responding to emails, or watching TV. They may struggle with concentration, memory, and word retrieval, and experience confusion, irritability, and feelings of being overwhelmed.
The treatment for neural fatigue aims to address the root cause of the problem. Psychotherapy and multidisciplinary treatment programs can help improve mental fatigue and other long-lasting symptoms, such as poor sleep, difficulty concentrating, mental health issues, and sensitivity to light and noise. On average, patients' symptoms improve by 60% after one week of specialized treatment for persistent brain injury issues.
While neural fatigue is often associated with brain injuries and intensive learning, it can also occur due to other factors. For example, in the context of muscle fatigue, neural fatigue refers to the limitations of a nerve's ability to generate a sustained signal, which can be a factor in untrained individuals during extremely powerful contractions.
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Metabolic fatigue
Muscle fatigue is a common issue that can arise from exercise, causing a decrease in the ability to produce force. It can be caused by vigorous exercise, but abnormal fatigue may be caused by interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: neural fatigue and metabolic fatigue. Metabolic fatigue is a characteristic muscle response to intense exercise that exceeds the rate of ATP replacement. This response can be observed in McArdle disease (GSD-V), where the heart rate increases to compensate for the deficit of ATP in skeletal muscle cells.
Endurance exercises, such as long-distance running, are limited by the size of muscle glycogen stores and how efficiently they are used. Carbohydrate metabolism can only support a relatively low rate of exercise, and prolonged exercise can lead to general collapse due to dehydration, hyperthermia, or hypoglycaemia. The mechanisms of muscle fatigue are not yet fully understood, and more research is needed to develop effective treatments.
The symptoms of muscle fatigue include a decrease in force generation and muscle weakness or tiredness. If you are experiencing muscle fatigue, rest and recovery are often recommended. In some cases, medical attention may be required, and a doctor may prescribe anti-inflammatory or antidepressant medications. Physical therapy may also be advised to improve mobility and speed up recovery.
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Lactic acid build-up
During intense exercise, when the body's demand for energy is high, muscles can generate energy anaerobically through a process called glycolysis. In this process, glucose is broken down into a substance called pyruvate, which can then be further metabolised aerobically when oxygen is available. When oxygen delivery is insufficient, as can occur during heavy exercise, the muscle cells continue to produce energy anaerobically, resulting in an increased concentration of lactic acid.
The build-up of lactic acid in the muscles leads to an increase in intracellular acidity, which can impact the muscle's ability to contract efficiently. This increased acidity can lower the sensitivity of the contractile apparatus to calcium ions (Ca2+), which are essential for muscle contraction. Additionally, lactic acid inhibits the chemical pump that actively transports calcium out of the cell, leading to an increase in cytoplasmic Ca2+ concentration. These combined effects result in a disruption to the normal muscle contraction process, contributing to muscle fatigue.
However, it is important to note that lactic acid itself does not cause the burning sensation during exercise or the delayed-onset muscle soreness (DOMS) felt in the days following extreme exercise. Instead, the burning sensation is caused by the hydrogen ions that are produced alongside lactic acid during intense exercise. These hydrogen ions create an acidic environment within the muscle cells, leading to the characteristic burn. The DOMS, on the other hand, is attributed to actual muscle cell damage and the release of various metabolites, resulting in an inflammatory-repair response that causes swelling and soreness.
While lactic acid may not be the sole cause of muscle soreness, it does play a role in the overall process of muscle fatigue. As exercise intensity increases, the body's ability to produce energy aerobically may be outpaced, resulting in a higher reliance on anaerobic energy production and, consequently, a greater accumulation of lactic acid. This build-up can contribute to the overall fatigue experienced by the muscles, impacting their ability to maintain force and endurance. Therefore, while lactic acid may not be the sole culprit, it remains a significant factor in the complex interplay of physiological processes that occur during strenuous physical activity.
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Intense muscle use
Muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It is a reversible decline in an objective measure of performance that occurs as a result of both submaximal and maximal exercise. It is a commonly experienced phenomenon that limits athletic performance and other strenuous or prolonged activity. It can be caused by repeated, intense use of muscles, leading to a decline in performance.
When you experience muscle fatigue, the force behind your muscles' movements decreases, causing you to feel weaker. It can be associated with a state of exhaustion, often following strenuous activity or exercise. Muscle fatigue can also be caused by other factors such as dehydration, depression, medications, or health conditions like anemia, hepatitis C, neurological disorders, muscular disorders (e.g., MND), and cardiovascular disorders.
The development of muscle fatigue is typically characterised as a decline in the maximal force or power capacity of the muscle. This means that submaximal contractions can be sustained, but the ability to generate force or power output is reduced. Muscle fatigue can be quantified by interrupting the fatiguing exercise with brief maximal contractions to estimate the decline in maximal force capacity.
The causes of muscle fatigue are specific to the task being performed. It can be caused by the accumulation of metabolites within muscle fibres, an inadequate motor command in the motor cortex, or physiological processes that enable contractile proteins to generate force becoming impaired. For example, inorganic phosphate can enter the SR and prevent the release of calcium, which is necessary for muscle contraction. Additionally, the accumulation of lactic acid, a by-product of fermentation, can increase intracellular acidity and impact calcium concentrations, potentially influencing muscle contractions.
To prevent and recover from muscle fatigue, it is important to stay hydrated, maintain a healthy diet, stretch, and rest. In some cases, doctors may prescribe anti-inflammatory or antidepressant medications, or recommend physical therapy to increase mobility and speed up recovery.
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Lack of blood flow
Muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It is characterised by a reversible decline in an objective measure of performance, such as maximum voluntary contraction. It can be caused by vigorous exercise, but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction.
One of the important roles of blood flow is to provide oxygen to the working muscles. Blood flow can bring oxygen necessary for aerobic ATP production and remove by-products of metabolic processes in working muscles. This plays a crucial role in maintaining force output. However, muscle voluntary contractions increase mean arterial blood pressure, which consequently decreases net blood flow to the working muscle and induces fatigue.
Despite the decrease in blood flow accompanying the development of muscle fatigue, reduced blood flow does not seem to be a key factor in the development of fatigue. For example, using venous occlusion plethysmography, Wigmore et al. decreased blood flow to the ankle dorsiflexor muscles and found that the decline in MVC force preceded significant changes in blood flow to the muscle.
Breathing hypoxic air can significantly increase muscle fatigue, and enhanced oxygen delivery to the exercising muscles directly attenuates muscle fatigue and increases muscle efficiency. Oxygen uptake and ATP utilisation are increased until the VO2 max is reached. At very high-intensity exercises, the demand for more ATP cannot be met by increases in oxygen delivery, resulting in an imbalance of metabolic homeostasis and leading to fatigue.
In summary, while reduced blood flow can induce fatigue, it does not appear to be a primary cause of muscle fatigue. Other factors, such as ATP utilisation and oxygen delivery, play a more significant role in the development of muscle fatigue during high-intensity exercises.
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Frequently asked questions
Muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It can be caused by vigorous exercise, but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction.
Muscle fatigue symptoms include myalgia (muscle pain), shortness of breath, fasciculations (muscle twitching), myokymia (muscle trembling), and muscle cramps during exercise. Muscle soreness may occur afterward.
Muscle fatigue can be caused by nerve signals weakening during powerful contractions, or by reduced muscle fibre contractions. It can also be caused by a build-up of lactic acid, which can increase the intracellular acidity of muscles.
Muscle fatigue often improves with rest and recovery. Staying hydrated and maintaining a healthy diet can also help. If your muscle fatigue is severe or persists, you should consult a doctor to rule out more serious health conditions.
