Victor Forge
Muscle fatigue is a decrease in the ability to exert force and/or velocity during exercise or other physical activity. It is often experienced as a feeling of tiredness, lack of energy, or exhaustion. While muscle fatigue is usually short-lasting and reversible, it can become chronic and more serious when associated with certain pathological states, medication, or toxic exposure. The causes of muscle fatigue may differ depending on the intensity and duration of exercise, and can include overtraining, undertraining, physical injury, muscle wasting, and various diseases and disorders. Recent studies have also investigated the role of pH, oxygen availability, and inorganic phosphate in muscle fatigue.
| Characteristics | Values |
|---|---|
| Definition | Loss of capacity to develop force and/or velocity in response to a load that is reversible by rest |
| Common Causes | Overtraining, undertraining/deconditioning, physical injury, pathological states, medication, toxic composites, muscle wasting, immobilization, insulin resistance, diseases associated with systemic inflammation |
| Physiology | Decreased oxygen availability to exercising muscle, decreased intracellular ATP, increased free Mg2+, decreased effectiveness of SR Ca2+ channel opening, decreased force production, decreased shortening velocity, decreased power output, decreased isometric force |
| Biochemistry | Increased intracellular accumulation of inorganic acids, increased hydrogen ion concentration, increased lactic acid, decreased pH, decreased Ca2+ concentration, increased inorganic phosphate |
| Progression | Fatigue accumulation, overwork, chronic fatigue syndrome (CFS), overtraining syndrome, endocrine disorders, immunity dysfunction, organic diseases |
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What You'll Learn
Lack of oxygen
Muscle fatigue can be defined as an exercise-induced decrease in the ability to produce force. It is often associated with an overwhelming sense of tiredness, a lack of energy, and feelings of exhaustion, making it difficult to perform voluntary tasks.
One of the important roles of blood flow is to provide oxygen to the muscles. A lack of oxygen can lead to muscle fatigue. Peripheral artery disease (PAD), for example, is damage to an artery that restricts the flow of blood to the limbs. When a person is active, the muscles are not getting enough oxygen to work well and remain healthy. PAD is a common disorder that most often affects men over 50, but women can also experience it. Symptoms of PAD include pain, achiness, fatigue, burning, or discomfort in the muscles of the feet, calves, or thighs. These symptoms typically appear during walking or exercise and go away after a few minutes of rest.
Additionally, breathing hypoxic air can significantly increase muscle fatigue. Oxygen availability affects the fatigue process at moderate work intensities. During very high-intensity exercise, the demand for more ATP energy cannot be met by increases in oxygen delivery, resulting in an imbalance of metabolic homeostasis and leading to fatigue.
Furthermore, impaired calcium release from the SR has been identified as a contributor to fatigue in isolated skeletal muscle fibers. One of the proposed mechanisms involves high-frequency stimulation, which may lead to extracellular K+ accumulation, decreasing voltage sensor activation and the action potential amplitude.
Finally, muscle fatigue can also be influenced by motoneuron firing rates. During fatiguing maximal contractions, motoneuron firing rates decrease due to repetitive activation, lower excitatory drive from the motor cortex, and increased firing of group III/IV muscle afferents, among other factors.
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Anaerobic metabolism
Muscle fatigue is considered a natural consequence of physical training, and its causes may differ depending on the intensity and duration of exercise. Anaerobic metabolism is a natural part of metabolic energy expenditure. Anaerobic means "without oxygen"; it is the process by which the body creates energy without the use of oxygen. During moderate to heavy muscular exercise, the heart cannot meet the demands of the muscles, and an oxygen debt is incurred, leading to a buildup of lactic acid. This is a by-product of anaerobic metabolism, and it can cause muscles to burn or fatigue.
During exercise, the energy that the body needs depends on the available fuel and oxygen. When exercise exceeds a moderate level, and less oxygen is available compared to the energy demands, the muscles convert to anaerobic metabolism. This type of metabolism relies on a special type of stored carbohydrate for fuel. Carbohydrates are the primary fuel for anaerobic metabolism.
Anaerobic exercises are high-intensity workouts completed over shorter durations. They can be used to build endurance, muscle strength, and power. Fast-twitch muscles, as compared to slow-twitch muscles, operate using anaerobic metabolic systems, meaning that any use of fast-twitch muscle fibres leads to increased anaerobic energy expenditure. Anaerobic energy expenditure is difficult to accurately quantify, but it can be estimated by determining the maximum accumulated oxygen deficit or measuring the lactic acid formation in muscle mass.
Cooling down after hard, anaerobic activities allows the blood to transport lactic acid to the liver, where it is changed back into glucose and brings oxygen back to the muscles for aerobic metabolism. Aerobic and anaerobic metabolisms work together to supply the body with energy.
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Lactic acid accumulation
Lactic acid is a substance produced by the body when its cells use glucose or other carbohydrates for energy. This process also leads to the formation of lactate. Lactic acid is created when the body breaks down glucose and other carbohydrates to create energy.
During intense exercise, the body may not have enough oxygen available to meet the energy demand, so it creates lactate to produce energy without using oxygen. This results in a temporary rise in lactic acid, which is typically flushed out of the muscles quickly and does not cause pain or damage cells.
However, if the body is pushed beyond its limits during intense physical activity, it can lead to lactic acidosis, a condition where there is too much lactic acid in the bloodstream, and the body cannot break it down fast enough. Lactic acidosis is often a complication of other health conditions, such as kidney or liver failure, infections, or sepsis. It can cause symptoms such as nausea, vomiting, weakness, muscle soreness, and exhaustion.
While lactic acid accumulation has been traditionally considered the primary cause of skeletal muscle fatigue, recent studies on mammalian muscle have shown little direct effect of acidosis on muscle function at physiological temperatures. Instead, inorganic phosphate, which increases during fatigue due to the breakdown of creatine phosphate, appears to be a more significant cause of muscle fatigue.
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Overwork and overtraining
Overtraining can lead to a state of fatigue accumulation, which, if not addressed, can progress to chronic fatigue syndrome (CFS). This is particularly relevant in the context of physical training, where muscle fatigue is considered a natural consequence. The intensity and duration of exercise play a role in the development of muscle fatigue.
High-intensity exercise, such as continuous high-frequency stimulation, can lead to a faster rate of fatigue accumulation. The power output of muscles, which is dependent on both force and shortening velocity, declines during fatigue. This decline in performance may not be immediately apparent during submaximal activity but will eventually result in an inability to maintain the original intensity.
Additionally, overwork and overtraining can lead to muscle wasting, which is a significant contributor to the development of chronic fatigue. This can be mediated by various factors, including aging, immobilization, insulin resistance, and diseases associated with systemic inflammation, such as arthritis, sepsis, and cardiovascular disorders.
Preventing overwork and overtraining is crucial to avoid the negative consequences of muscle fatigue and its potential progression to chronic conditions. Rest and recovery are essential to allow the muscles to restore their capacity to generate force and maintain energy levels.
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Chronic illness
Muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It can be caused by exercise or physical activity, but it can also be caused by certain medications or health conditions. Chronic illnesses such as multiple sclerosis, myasthenia gravis, muscular dystrophy, fibromyalgia, chronic fatigue syndrome, and chronic kidney, liver, or lung disease can lead to muscle fatigue.
Myasthenia gravis (MG) is an autoimmune disease where the immune system releases proteins (autoantibodies) that interfere with the normal communication between muscle and nerve cells, resulting in weakness and fatigue. MG can cause muscle weakness in the legs or arms and can even lead to breathing problems. It is often treated with steroids, immunosuppressants, intravenous immunoglobulin, or a synthetic drug called pyridostigmine, which boosts the connection between muscle and nerve cells.
Muscular dystrophy is a genetic condition that affects the muscles and can lead to serious health issues such as trouble walking and using arms. It can also cause heart problems, swallowing difficulties, and brain-related differences. There are various types of muscular dystrophy, including myotonic, facioscapulohumeral (FSHD), and congenital, which can affect people of all ages and ethnic groups.
Fibromyalgia is a common cause of chronic fatigue and musculoskeletal pain, especially in women. It is characterised by severe fatigue, deep muscle pain, painful tender points, sleep problems, anxiety, and a hazy feeling that makes it hard to concentrate.
Chronic fatigue syndrome, also known as systemic exertion intolerance disease, is characterised by persistent and relapsing exhaustion that interferes with daily functioning and well-being. It is often accompanied by other symptoms such as allergic rhinitis, anemia, depression, or a bacterial or viral infection.
In addition to these specific conditions, chronic illnesses in general can also lead to muscle fatigue. For example, a bad cold, the flu, or a prolonged illness that requires bed rest can cause muscle weakness and fatigue. Furthermore, certain medications can also contribute to muscle fatigue as a side effect. It is important to consult a doctor if muscle fatigue persists or is accompanied by other symptoms, as it could indicate a more serious health condition.
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Frequently asked questions
Muscle fatigue is a decrease in the ability to produce force, often experienced as a feeling of tiredness or lack of energy.
The leading causes of short-lasting muscle fatigue are related to overtraining, undertraining/deconditioning, or physical injury. Chronic muscle fatigue can be caused by muscle wasting due to ageing, immobilization, insulin resistance, or diseases associated with systemic inflammation.
Muscle fatigue can be measured by observing the power spectrum of diaphragmatic surface EMG activity. A sustained reduction of the H/L ratio of the EMG power spectrum to less than 80% of the initial value indicates impending diaphragmatic fatigue.
To prevent or treat muscle fatigue, it is important to understand the underlying causes. For short-lasting muscle fatigue, adequate rest and recovery are crucial. For chronic muscle fatigue, addressing the underlying pathological or illness conditions and preventing muscle loss are key.
