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Muscle fatigue is a decrease in the maximum force or power that a muscle can generate, which develops after the onset of sustained physical activity. It is not the same as muscle weakness, although weakness is an initial symptom. Muscle fatigue can be caused by a variety of factors, including nerve function, muscle contractions, and the accumulation of lactic acid. It is important to distinguish between central (neural) and peripheral (muscular) fatigue, as well as the impact of age, training, and underlying health conditions. While muscle fatigue due to exercise or aging may not require medical treatment, abnormal or persistent fatigue could indicate a more serious issue and should be assessed by a doctor.
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What You'll Learn
Muscular and neural factors
Muscle fatigue refers to the decline in muscle force generated over time. It is not the same as muscle weakness, although weakness is an initial symptom. It is also not the same as the point of task failure or the moment when the muscles become exhausted. Rather, it is a decrease in the maximal force or power that the involved muscles can produce, and it develops gradually soon after the onset of sustained physical activity.
Neural factors refer to the central fatigue that involves the physiological processes within the central nervous system (CNS). This includes the limitations of a nerve's ability to generate a sustained, high-frequency signal, which allows a muscle to contract with its greatest force. During powerful contractions that are close to the upper limit of a muscle's ability to generate force, nervous fatigue (enervation) can occur, where the nerve signal weakens and becomes a limiting factor.
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Impaired blood flow
Muscle fatigue can be caused by limitations in a nerve's ability to generate a sustained signal (neural fatigue) or by a reduced ability of the muscle fibre to contract (metabolic fatigue). Metabolic fatigue is a common term for the reduction in contractile force due to a shortage of, or inability to metabolise, fuel (substrates) within the muscle fibre, causing a low ATP reservoir. Long-term muscle use requires the delivery of oxygen and glucose to the muscle fibre to allow aerobic respiration to occur and produce the ATP required for muscle contraction. Insufficient oxygen delivery to the muscle fibre can result in the accumulation of pyruvate, which is then processed into lactic acid.
Lactic acid accumulation in the muscle tissue reduces the pH, making the muscle more acidic and producing a stinging sensation. This further inhibits anaerobic respiration, inducing fatigue. Lactic acid also increases cytoplasmic Ca2+ concentration by inhibiting the chemical pump that actively transports calcium out of the cell. This counters the inhibiting effects of potassium on muscular action potentials. Lactic acid has also been found to reduce the inhibition of contraction by chloride ions, leaving potassium ions as the only restricting influence on muscle contractions.
Muscle fatigue can also be caused by peripheral (muscular) factors, such as mechanical and cellular changes in the muscular system. Fast-twitch muscle fibres, for example, generate more power but fatigue rapidly and are more anaerobic with a lower blood supply. Impaired blood flow can thus lead to muscle fatigue by reducing the oxygen and nutrient supply to these muscles.
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Age and gender
Muscle fatigue is a commonly experienced phenomenon that limits athletic performance and other strenuous or prolonged activity. It is also known to increase and restrict daily life under various pathological conditions, including neurological, muscular, and cardiovascular disorders, as well as ageing and frailty.
Age
Age is a factor that influences muscle fatigue. The involuntary loss of muscle mass, strength, and function, termed sarcopenia, is a common change that occurs with ageing. This condition is likely multifactorial, involving a combination of factors such as endocrine and nutritional changes, inactivity, and hormonal alterations. Older individuals may experience impaired muscle cell response to insulin, negatively impacting muscle protein synthesis. Additionally, age-related alterations at the cellular level include a reduction in muscle cell number, muscle twitch time and force, sarcoplasmic reticulum volume, and calcium pumping capacity. These changes contribute to decreased muscle function and increased susceptibility to fatigue.
Gender
Gender also plays a role in muscle fatigue, with differences observed between men and women during fatiguing contractions. Women are typically able to sustain contractions for longer durations, especially at lower intensities. This is attributed to their greater reliance on muscle mass and glycolytic metabolism. Men, on the other hand, tend to activate a larger muscle mass to exert the same relative force, resulting in higher intramuscular pressures and greater occlusion of blood flow. These factors can contribute to increased muscle fatigue in men during sustained contractions.
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Lactic acid build-up
Muscle fatigue is a decrease in the maximum force or power that the muscles can produce, which develops gradually after the start of sustained physical activity. It is not the same as muscle weakness, although weakness is an initial symptom. Once muscle fatigue sets in, an individual's ability to persist with an exercise will gradually worsen, and they will eventually lose their grip or become unable to lift or push with their arms or legs.
One of the main causes of muscle fatigue is lactic acid build-up. Lactic acid is a by-product of fermentation or anaerobic respiration, which occurs when there is insufficient oxygen to convert pyruvate into additional ATP molecules in the mitochondria via the Krebs Cycle. As a result, pyruvate accumulates in the muscle fibre and is processed into lactic acid. This accumulation of lactic acid in the muscle tissue reduces the pH, making the muscle more acidic and producing a stinging sensation. This increased acidity inhibits anaerobic respiration, inducing fatigue.
Lactic acid can increase the intracellular acidity of muscles, which lowers the sensitivity of the contractile apparatus to Ca2+. However, it also increases cytoplasmic Ca2+ concentration by inhibiting the chemical pump that actively transports calcium out of the cell. This counters the inhibiting effects of potassium on muscular action potentials. Lactic acid also has a negating effect on the chloride ions in the muscles, reducing their inhibition of contraction and leaving potassium ions as the only restricting influence on muscle contractions.
The impact of lactic acid on performance is uncertain, and it may either assist or hinder muscle fatigue. With sufficient exercise, the onset of muscle fatigue can be delayed. The metabolic capacity of a muscle can change with training, which can delay the onset of fatigue.
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Lack of training
Muscle fatigue due to lack of training can be attributed to several factors. Firstly, untrained muscles have a reduced capacity for aerobic metabolism. This means that the muscles are less efficient at using oxygen to generate energy, leading to a quicker buildup of fatigue during physical activity. The muscles' decreased endurance capacity results in premature exhaustion and a reduced ability to sustain prolonged contractions.
Additionally, without proper training, the body's neuromuscular system is not conditioned to efficiently recruit motor units. Motor units consist of a motor neuron and the muscle fibres it innervates. Efficient recruitment of these motor units involves activating the necessary muscle fibres in a coordinated manner to produce a desired force or movement. With inadequate training, the body is less effective at activating the correct muscle fibres at the right time, leading to uncoordinated and inefficient movements, and ultimately, premature fatigue.
Furthermore, proper training helps develop the skill and technique required for efficient movement patterns. Efficient movement ensures that the muscles work optimally, reducing unnecessary strain and premature fatigue. Lack of training can result in poor technique, placing unnecessary stress on certain muscles or joints, leading to premature fatigue and increased risk of injury.
To prevent muscle fatigue due to lack of training, it is essential to gradually and progressively engage in physical activity. Starting with low-intensity exercises and gradually increasing the duration and intensity helps condition the muscles and improves their endurance. Consistency in training is key, as it allows the body to adapt and develop the necessary energy systems, motor unit recruitment patterns, and structural changes to withstand fatigue and improve performance.
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Frequently asked questions
Muscle fatigue is a decrease in the maximum force or power that the muscles can produce, which develops soon after the onset of sustained physical activity.
Muscle fatigue can be caused by a variety of factors, including overtraining, undertraining, physical injury, pathological issues, nerve function, and the accumulation of lactic acid in the muscle.
Nerves control the contraction of muscles by transmitting electrical impulses from the brain. During extremely powerful contractions, nervous fatigue can occur, causing the nerve signal to weaken and resulting in a reduced ability of the muscles to contract.
Symptoms of muscle fatigue include a feeling of tiredness or lack of energy, muscle pain, shortness of breath, muscle twitching, muscle trembling, and muscle cramps during exercise. In some cases, it may also lead to an inappropriate rapid heart rate response.
Treatment for muscle fatigue depends on the underlying cause. In cases where it is caused by strenuous activity or aging, treatment may not be necessary. However, if muscle fatigue is caused by an underlying medical condition, it is important to consult a doctor for appropriate treatment.
