Dominic Stone
Muscle fatigue is a common issue that can be caused by a variety of factors. It is characterized by a decrease in the force generated by muscles, leading to feelings of weakness and tiredness. While exercise is a well-known cause of muscle fatigue, there are several other factors that can contribute to this condition. So, what are the three main causes of muscle fatigue?
| Characteristics | Values |
|---|---|
| Definition | Muscle fatigue is a reduced ability to produce force with your muscles. |
| Causes | Exercise, aging, immobility, arthritis, heart failure, dehydration, anemia, depression, hepatitis C, and stroke. |
| Symptoms | Muscle weakness, soreness, localized pain, shortness of breath, muscle twitching, trembling, weak grip, and muscle cramps. |
| Treatment | Rest, hydration, a healthy diet, nutritional supplements, and in some cases, medical attention. |
| Prevention | Proper exercise form, avoiding overtraining, and staying within one's fitness level. |
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What You'll Learn
Exercise and physical activity
When you start exercising, your muscles feel strong and resilient. However, as you repeat movements, your muscles may begin to feel weaker and tired. This is muscle fatigue. It is a decrease in the maximal force or power production in response to contractile activity. It can originate at different levels of the motor pathway and is usually divided into central and peripheral components. Peripheral fatigue is produced by changes at or distal to the neuromuscular junction.
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. Metabolic fatigue is the reduced ability of the muscle fiber to contract. Substrate shortage is one of the causes of metabolic fatigue. Substrates are depleted during exercise or unable to be metabolized, resulting in a lack of intracellular energy sources to fuel contractions.
Additionally, muscle fatigue can be caused by the accumulation of metabolites within muscle fibres, inadequate motor commands in the motor cortex, and precise molecular changes that occur with sustained exercise. For example, the ryanodine receptor in skeletal muscle undergoes a conformational change during exercise, resulting in "leaky" channels deficient in calcium release, contributing to muscle fatigue and decreased exercise capacity.
It is important to note that muscle fatigue can be a symptom of other health conditions, and if it is unrelated to exercise, it is important to consult a doctor.
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Health conditions, e.g. anaemia, arthritis, heart failure
Muscle weakness can be a symptom of many health conditions, ranging from acute infections to chronic illnesses. It can also be caused by certain medications and can be a side effect of some treatments.
Some of the health conditions that can lead to muscle weakness and fatigue include:
Anaemia
Anaemia occurs when a person's haemoglobin levels are low, often due to an iron deficiency. This can result in muscle weakness and fatigue as haemoglobin plays a crucial role in carrying oxygen throughout the body, including to the muscles.
Arthritis
Arthritis is an inflammatory condition that affects the joints, causing pain, stiffness, and swelling. While arthritis primarily affects the joints, it can also lead to muscle weakness and fatigue. This may be due to the extra effort required to move inflamed joints, as well as the potential for arthritis to affect muscles and surrounding tissues.
Heart Failure
Heart failure is a chronic condition where the heart is unable to pump enough blood to meet the body's needs. This can lead to muscle fatigue as the muscles are not receiving the oxygen-rich blood they need to function properly. Heart failure can also cause muscle weakness, and this is often a result of the condition's impact on the nervous system.
Chronic Fatigue Syndrome
Chronic fatigue syndrome, also known as myalgic encephalomyelitis, is a complex condition that can affect various systems and functions in the body. It is characterised by extreme fatigue that does not improve with rest and can be worsened by physical or mental activity. Muscle weakness and pain are common symptoms of this syndrome.
Neurological and Neuromuscular Disorders
Some conditions that affect the nervous system, such as cervical spondylosis, Guillain-Barré syndrome, and botulism, can cause muscle weakness. Neuromuscular disorders affect the nerves that control voluntary muscles, and when these nerves become unhealthy or die, communication between the nervous system and muscles breaks down, resulting in muscle weakness and fatigue.
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Dehydration and lack of nutrients
Dehydration is a common cause of muscle fatigue. Fluids are lost through exercise, and failing to replenish them can lead to dehydration, which can cause muscle fatigue and muscle weakness. Dehydration can also worsen delayed onset muscle soreness (DOMS) after exercise.
Drinking water is essential for rehydration, but it does not replace electrolytes lost through sweat. Electrolystes such as magnesium, potassium, and sodium are essential for normal muscle function and contractions. A lack of these nutrients can hinder muscle contractions and lead to fatigue.
Research has shown that dehydration can exacerbate skeletal muscle damage, particularly during or after exercise when fluid loss is high. This can lead to structural, contractile, and enzymatic protein denaturation, as well as myofiber and connective damage. Dehydration can also contribute to muscle cramps, which are painful contractions of the muscles.
Nutrient deficiencies can also lead to muscle fatigue. Common nutrient deficiencies include iron, vitamin B12, calcium, vitamin D, magnesium, folate, and potassium. These deficiencies can alter bodily functions at a cellular level, impacting water balance, enzyme function, nerve signaling, digestion, and metabolism. For example, a vitamin D deficiency can cause fatigue, bone pain, muscle aches, and weakness.
It is important to incorporate a variety of nutrient-rich foods into a balanced diet to prevent deficiencies and maintain muscle health. Staying properly hydrated is also crucial for energy levels and muscle function.
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Age and immobility
Muscle fatigue is a common issue that can occur due to exercise or other health conditions. It is characterised by a decrease in the ability to produce force during muscle actions. While exercise is a common cause of muscle fatigue, it can also be caused by age and immobility.
Ageing causes a natural decline in muscle strength and endurance, which can lead to muscle fatigue. This phenomenon is known as sarcopenia, or muscle loss with ageing. Sarcopenia usually begins around the age of 40 and accelerates after 75, becoming a significant cause of disability in older individuals. The process of losing muscle strength starts in our 30s, with a typical loss of about 3-5% of muscle strength each decade. This loss can become more noticeable around age 60, and studies suggest that between 11% and 50% of people over 80 have sarcopenia. Sarcopenia can cause significant difficulties in performing basic daily activities, such as getting out of chairs, opening jars, or carrying groceries.
Several factors contribute to the development of sarcopenia. One key factor is a reduction in nerve cells responsible for transmitting signals from the brain to the muscles to initiate movement. Additionally, lower concentrations of certain hormones, including growth hormone, testosterone, and insulin-like growth factor, play a role. The body also experiences a decreased ability to convert protein into energy with ageing, which may be exacerbated by a low-protein diet. Diets high in ultra-processed foods and low in fruits and vegetables may also contribute to sarcopenia.
Immobility, whether due to ageing, bed rest, or an inactive lifestyle, can significantly impact muscle strength and endurance. The antigravity muscles that facilitate locomotion and maintain an upright posture, such as the quadriceps, glutei, erector spinae, and gastrocnemius-soleus muscles, are particularly affected by immobility. On average, 10-15% of muscle strength is lost each week of immobility, with the greatest loss occurring during the initial period of inactivity. Bed rest for 4-5 weeks can lead to a 20-25% decrease in the strength of lower-limb extensor muscle groups.
In addition to muscle strength loss, immobility causes muscle atrophy, with muscle fibres becoming short and stiff, resulting in fixed joints that are challenging and painful to move. This can further contribute to a decline in endurance and an increased susceptibility to fatigue. Therapeutic exercises, such as progressive resistive exercises and isometric and isotonic contractions, are crucial for improving muscle strength and endurance in individuals with immobility.
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Neural fatigue and metabolic fatigue
Neural fatigue is colloquially referred to as "brain fog". It is often associated with brain injuries such as mild traumatic brain injury (concussion), severe traumatic brain injury, hypoxia, viral infection, stroke, transient ischemic attack, or neurological disorders such as Parkinson's disease, multiple sclerosis, and dementia. It can also occur in people with PTSD.
People with neural fatigue may experience a decrease in concentration, focus, memory, recall, and word retrieval. They may feel confused, irritable, and overwhelmed. Even simple tasks can become immensely challenging and take a long time to complete. For example, a person with neural fatigue may struggle to remember the steps to make a cup of coffee or find themselves rereading the same line in a book repeatedly.
Neural fatigue can be exacerbated by triggers, such as loud noises, bright lights, or crowded places. It is important for individuals experiencing neural fatigue to recognize their symptoms and take breaks when needed. Planning and creating strategies can help mitigate the impact of neural fatigue.
Neural fatigue can be treated with a multidisciplinary approach that addresses the root causes of brain injury. Cognitive FX, for example, offers a treatment program that has shown significant improvements in mental fatigue and other long-lasting symptoms of brain injury.
Metabolic fatigue, on the other hand, is a muscle response to intense exercise that exceeds the rate of ATP replacement. During intense exercise, the body produces metabolic by-products, such as hydrogen ions and diprotonated phosphate, which interfere with actin-myosin interaction. This interference prevents further ATP hydrolysis, effectively preserving muscle ATP levels. It is important to note that muscle force and metabolite concentrations typically return to normal within about 5 minutes.
Metabolic fatigue can also occur during less intense exercise due to a disturbance in excitation-contraction coupling, which requires greater effort to generate submaximal forces. Additionally, endurance exercise is limited by muscle glycogen stores and the efficiency of their use. When muscle glycogen is depleted, exercise can only continue at a low rate supported by lipid metabolism.
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