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Muscle fatigue is a common phenomenon that can be caused by various factors, including exercise, disease, and certain medications. It is characterised by a decrease in the force or power that muscles can generate during sustained physical activity, leading to a sense of exhaustion and weakness. This fatigue can be influenced by physiological, neurological, and pathological factors, and its impact can range from limiting athletic performance to affecting daily life for individuals with certain health conditions. Understanding and managing muscle fatigue is crucial for optimising physical performance and overall well-being.
| Characteristics | Values |
|---|---|
| Definition | A decrease in the maximal force or power that the involved muscles can produce |
| Cause | Vigorous exercise, medications, or health conditions like anemia, dehydration, depression, hepatitis C, etc. |
| Symptoms | Myalgia (muscle pain), shortness of breath, fasciculations (muscle twitching), myokymia (muscle trembling), muscle cramps, soreness, etc. |
| Treatment | Rest and recovery, staying hydrated, maintaining a healthy diet, hot and cold therapy, and in some cases, medical attention |
| Prevention | Training and conditioning, synthetic products (e.g. amphetamine, caffeine), natural products (e.g. ginseng, rhodiola rosea), and nutritional supplements |
| Mechanism | Neural fatigue (limitations of a nerve's ability to generate sustained signals), Metabolic fatigue (reduced ability of the muscle fiber to contract due to substrate shortage or inability to metabolize fuel), central fatigue (originates in the central nervous system), peripheral fatigue (changes at or distal to the neuromuscular junction) |
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What You'll Learn
Neural fatigue
Most movements require a force far below what a muscle could potentially generate, and neural fatigue is seldom an issue. However, during extremely powerful contractions that are close to the upper limit of a muscle's ability to generate force, neural fatigue can be a limiting factor in untrained individuals. In novice strength trainers, the muscle's ability to generate force is most strongly limited by a nerve's ability to sustain a high-frequency signal. After a period of maximum contraction, the nerve's signal reduces in frequency, and the force generated by the contraction diminishes. There is no sensation of pain or discomfort; the muscle appears to simply 'stop listening' and gradually ceases to contract.
Neural training can cause several weeks of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit. This training involves increasing the nerve's ability to generate sustained, high-frequency signals, allowing a muscle to contract with its greatest force.
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Metabolic fatigue
Muscle fatigue is a common issue, often occurring as a result of vigorous exercise, but it can also be caused by certain medications or health conditions. It is characterised by a decrease in the ability to generate force, which can lead to a loss of grip, the inability to lift or push with arms or legs, or the inability to maintain an isometric position.
Substrates, such as adenosine triphosphate (ATP), glycogen, and creatine phosphate, are essential for muscle contractions. ATP binds to the myosin head, causing the 'ratchetting' that results in contraction. Creatine phosphate stores energy so that ATP can be rapidly regenerated within the muscle cells, allowing for sustained powerful contractions. Glycogen is the intramuscular storage form of glucose, used to generate energy quickly when intramuscular phosphocreatine stores are exhausted. During intense exercise, the demand for these substrates can exceed the rate at which they can be produced, leading to a low ATP reservoir and subsequent metabolic fatigue.
Metabolites are substances, often waste products, produced as a result of muscular contraction. They include chloride, potassium, lactic acid, ADP, magnesium, reactive oxygen species, and inorganic phosphate. The accumulation of these substances can interfere with the release of calcium from the sarcoplasmic reticulum or reduce the sensitivity of contractile molecules actin and myosin to calcium. This interference with calcium release or reduction in sensitivity can lead to metabolic fatigue.
Additionally, metabolic myopathies, which are genetic disorders affecting the normal functioning of muscles due to abnormalities in metabolic pathways, can also contribute to metabolic fatigue. These conditions result in impaired energy production and utilisation within muscle cells, leading to limitations in muscle function and increased fatigue.
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Exercise-induced muscle weakness
Muscle fatigue is a commonly experienced phenomenon that limits athletic performance and other strenuous or prolonged activities. It is often defined as an overwhelming sense of tiredness, lack of energy, and feeling of exhaustion, relating to a difficulty in performing voluntary tasks. Exercise-induced muscle weakness, or muscle fatigue, can be caused by various factors, including:
Neural fatigue
This occurs when there is a limitation in a nerve's ability to generate a sustained signal to the muscle. During extremely powerful contractions that are close to the upper limit of a muscle's ability, neural fatigue (enervation) can be a factor, especially in untrained individuals. The nerve signal weakens, and the muscle's ability to generate force is limited. Strength training can help increase the nerve's ability to generate sustained, high-frequency signals, improving muscle contraction.
Metabolic fatigue
Metabolic fatigue is a common term for the reduction in contractile force due to a shortage of, or inability to metabolize, fuel (substrates) within the muscle fiber, leading to a low ATP reservoir. This can be influenced by a decrease in calcium release from the SR due to high levels of inorganic phosphate. Metabolic fatigue is often associated with intense, metabolically demanding exercises, such as sprinting.
Accumulation of metabolites
The accumulation of metabolites within muscle fibers can contribute to muscle fatigue. This includes the buildup of hydrogen ions, lactate, inorganic phosphate, reactive oxygen species (ROS), heat shock protein (HSP), and orosomucoid (ORM). These metabolic factors affect the biochemical equilibrium within the exercising muscle cells, leading to a decline in performance.
Central fatigue
Central fatigue originates in the central nervous system (CNS) and decreases the neural drive to the muscle. It is associated with changes in neurotransmitter concentrations, such as 5-HT, DA, and NA, which can impact exercise performance.
Other factors
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Muscle damage
When an individual engages in vigorous physical activity, the muscles experience a decrease in their ability to generate force, leading to muscle fatigue. This can be influenced by various physiological factors, such as the accumulation of metabolites within muscle fibers, inadequate motor commands in the motor cortex, or a decrease in calcium release from the SR. The latter is caused by high levels of inorganic phosphate entering the SR and preventing calcium release. This results in a reduction in the force-generating potential of the muscles during high-intensity exercise.
The development of muscle fatigue can be quantified as a decline in the maximal force or power capacity of the muscle. It is characterized by a decrease in the outflow of motor impulses to the muscles, resulting in a reduction of force behind the muscles' movements and a feeling of weakness. This weakness is often the initial sign of muscle fatigue and can lead to an inability to maintain muscle force, muscle weakness, and atrophy.
The experience of muscle fatigue can vary depending on the individual's level of training and conditioning. For instance, in muscles that are repeatedly subjected to fatigue, the free nerve endings become sensitized and develop a lower threshold for firing, resulting in the pain of fatigue at lower exertional levels. This can be managed through training and conditioning, which increase muscle tolerance to fatigue and enhance the strength and resistance of tendons, ligaments, and bones.
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Health conditions
Muscle fatigue is a common issue that can be caused by various factors, including certain health conditions. While it is often associated with vigorous exercise, abnormal or persistent muscle fatigue may indicate an underlying health issue.
Muscle fatigue can be a symptom of several health conditions, including neurological, muscular, and cardiovascular disorders. For example, people with polio or post-polio syndrome (PPS) often experience muscle fatigue, along with cognitive problems, sleep disturbances, and impaired concentration. In addition, certain diseases, such as cancer, stroke, and multiple sclerosis (MS), can contribute to muscle fatigue by speeding up the depletion of energy stocks within the body.
Mental health conditions, such as depression, can also lead to muscle fatigue. Additionally, fatigue associated with depression may be further exacerbated by sleep disturbances, which are common in people experiencing depression.
Other health conditions that can cause muscle fatigue include anemia, dehydration, hepatitis C, vitamin B12 deficiency, thyroid dysfunction, and other inflammatory diseases.
If you are experiencing persistent or abnormal muscle fatigue, especially if it is unrelated to exercise, it is important to consult a doctor. A doctor will be able to evaluate your medical history and symptoms to rule out more serious health conditions and recommend an appropriate treatment plan.
Treatment for muscle fatigue will depend on the underlying cause and may include rest, hydration, a healthy diet, stretching, and in some cases, medical attention.
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Frequently asked questions
Muscle fatigue is a decrease in the maximal force or power that the involved muscles can produce. It is often associated with a state of exhaustion, usually following strenuous activity or exercise.
Other symptoms may include myalgia (muscle pain), shortness of breath, fasciculations (muscle twitching), myokymia (muscle trembling), and muscle cramps during exercise.
Muscle fatigue can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex. There are two main causes: the limitations of a nerve’s ability to generate a sustained signal (neural fatigue); and the reduced ability of the muscle fibre to contract (metabolic fatigue).
There are no formal guidelines for treating muscle fatigue. In many cases, muscle fatigue will improve with rest and recovery. Staying hydrated and maintaining a healthy diet can also improve recovery time.
Muscle fatigue is not the same as muscle weakness, although weakness is often an initial symptom. Muscle fatigue is a decline in the force capacity of a muscle, whereas muscle weakness is a loss of muscle strength.
