Dominic Stone
Skeletal muscle fatigue is a symptom that decreases your muscles' ability to perform over time. It can be caused by exercise or other physical activity, but it can also be caused by other factors such as improper exercise, long-term combat, military training, or health conditions like anemia, dehydration, depression, or hepatitis C. In terms of cellular mechanisms, skeletal muscle fatigue has been linked to metabolic changes, such as the accumulation of lactic acid, which results in intracellular acidosis and impaired calcium release. Additionally, factors like nerve signal weakening, molecular changes, and dietary deficiencies can also contribute to skeletal muscle fatigue.
| Characteristics | Values |
|---|---|
| Molecular changes | Ryanodine receptor undergoes a conformational change, resulting in "leaky" channels that are deficient in calcium release |
| Intracellular acidosis due to lactic acid accumulation | |
| Inorganic phosphate breakdown | |
| Anaerobic metabolism | |
| Accumulation of K+ | |
| Impaired calcium release from SR | |
| Lack of certain nutrients, e.g., vitamin D | |
| Medications or health conditions like anemia, dehydration, depression, hepatitis C, etc. | |
| Fibromyalgia | |
| Carpal tunnel syndrome |
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What You'll Learn
Anaerobic metabolism and muscle fatigue
Skeletal muscle fatigue is a complex phenomenon influenced by various physiological and biochemical factors. One significant contributor to skeletal muscle fatigue is anaerobic metabolism, particularly during high-intensity exercises.
Anaerobic metabolism refers to the process of generating energy without the use of oxygen. During intense physical activity, the demand for energy, in the form of adenosine triphosphate (ATP), exceeds the capacity of aerobic metabolism, leading to a reliance on anaerobic pathways. This shift to anaerobic metabolism is particularly evident in fast-twitch muscle fibres, which have higher energy demands and rely more heavily on anaerobic energy production compared to slow-twitch fibres.
The breakdown of glycogen, a form of stored carbohydrate, during anaerobic metabolism results in the accumulation of inorganic acids, with lactic acid being the most prominent. Lactic acid dissociates into lactate and hydrogen ions (H+), leading to a decrease in pH, a condition known as acidosis. This acidification has been historically considered the primary cause of skeletal muscle fatigue, as it can impair muscle contractile function.
However, recent studies suggest that the relationship between acidosis and muscle fatigue may be more complex. While acidosis can contribute to decreased muscle performance, its effects might have been overestimated. Additionally, other consequences of anaerobic metabolism, such as increased levels of inorganic phosphate (Pi), could be more significant contributors to impaired muscle function during fatigue.
Furthermore, the early stages of skeletal muscle fatigue involve impairments in myofibrillar functions, including decreased cross-bridge force-generating capacity and reduced calcium (Ca2+) sensitivity. This reduced Ca2+ sensitivity further impairs muscle contractility and contributes to the overall fatigue experienced during intense exercise.
In summary, anaerobic metabolism plays a crucial role in skeletal muscle fatigue, particularly during high-intensity exercises that exceed the aerobic capacity of muscle fibres. The accumulation of lactic acid and its impact on pH, along with other metabolic changes, contribute to the development of fatigue and decreased muscle performance. Understanding these mechanisms is essential for optimising athletic performance and developing effective training strategies.
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Impaired calcium release
Several mechanisms have been proposed to explain impaired calcium release. One theory suggests that the reduction in the amplitude of the action potential may be caused by extracellular K^+ accumulation, which can decrease voltage sensor activation. However, this theory is countered by the observation that intact animals have mechanisms to counteract this effect. Another proposed mechanism is the reduced effectiveness of SR Ca^+2^ channel opening due to decreased intracellular ATP and increased Mg^+2^ concentrations during fatigue. Additionally, reduced Ca^+2^ availability within the SR can occur if inorganic phosphate enters and precipitates with Ca^+2.
Calcium phosphate precipitation in the SR has been shown to reduce action potential-mediated Ca^+2^ release in mammalian skeletal muscle. This process may be influenced by changes in intracellular ATP levels, as indicated in studies on fast-twitch muscle fibres of rats. Furthermore, transverse tubular system depolarization has been linked to impaired calcium release and reduced tetanic force in rat skeletal muscle fibres.
The role of parvalbumin in fatigue-induced changes in force and cytosolic calcium transients has also been explored in mouse myofibers. Additionally, studies on mice subjected to a high-fat and high-sucrose diet revealed that sirtuin 3 overexpression preserved maximal sarco(endo)plasmic reticulum calcium ATPase activity. These findings highlight the complex interplay between various factors influencing impaired calcium release during muscle fatigue.
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Nervous fatigue
Additionally, motoneuron firing rates can decrease due to repetitive activation, leading to a decrease in excitability to excitatory synaptic input. The excitatory drive from the motor cortex or other supraspinal areas to the motoneurons may also be lower, further reducing motoneuron firing rates.
Neural training, such as strength training, can help improve the nerve's ability to generate sustained, high-frequency signals, allowing muscles to contract with maximum force. This neural adaptation can lead to rapid gains in strength over several weeks before levelling off when the nerve generates maximum contractions, and the muscle reaches its physiological limit.
Furthermore, muscle fatigue can be caused by “leaky” channels in the ryanodine receptor present in skeletal muscles. These channels are deficient in calcium release and may contribute to muscle fatigue and decreased exercise capacity.
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Intracellular acidosis
While there is often a close temporal relationship between decreased muscle force and increased intracellular concentrations of lactate and H+, this correlation does not always hold. Recent studies on mammalian muscle show little direct effect of acidosis on muscle function at physiological temperatures. Instead, it appears that inorganic phosphate, which increases during fatigue due to the breakdown of creatine phosphate, is a major cause of muscle fatigue.
The role of reduced pH as a significant cause of fatigue is now being questioned. Several studies show that reduced pH may have little impact on contraction in mammalian muscle at physiological temperatures. It has been suggested that rather than acidification, some other consequence of anaerobic metabolism, such as increased Pi, may be the actual cause of impaired muscle function.
In summary, while intracellular acidosis due to lactic acid accumulation has been traditionally considered the primary cause of skeletal muscle fatigue, recent evidence suggests that its effects may have been overestimated, and other factors, such as inorganic phosphate, may play a more significant role in muscle fatigue.
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Health conditions
Muscle fatigue is a symptom that decreases your muscles’ ability to perform over time. While exercise is a common cause of muscle fatigue, this symptom can be the result of other health conditions, too.
- Anemia
- Dehydration
- Depression
- Hepatitis C
- Thyroid dysfunction
- Vitamin B12 deficiency
- Inflammatory diseases
- Cancer
- Stroke
- Polio
- Fibromyalgia
- Carpal tunnel syndrome
In addition, certain medications can also cause muscle fatigue. If you are experiencing muscle fatigue unrelated to exercise, it is important to consult a doctor to rule out more serious health conditions.
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