Logan Steele
Muscle fatigue is a common experience, but the processes involved are not yet fully understood. The simplest way to determine the onset of muscle fatigue is to measure the time during which an individual can perform a certain task, such as keeping a defined level of static (isometric) contraction. However, this method is dependent on psychological factors like motivation. More advanced methods include measuring maximal voluntary contraction force or power, or the force generated by electrical stimulation. The most common way to measure muscle fatigue is through surface electromyography (sEMG), which is a non-invasive measurement of the myoelectric signal generated by muscle activity.
| Characteristics | Values |
|---|---|
| Definition | Any reduction in the maximal capacity to generate force or power output |
| Methods | Assessing maximal voluntary contraction force or power, or the force generated by electrical stimulation |
| Techniques | Electromyography (EMG), sonomyography (SMG), mechanomyography (MMG), and near-infrared spectroscopy (NIRS) |
| Factors | Central and peripheral factors, mechanical properties, electrolyte shifts, energy turnover, psychological factors, and motivational factors |
| Measurements | Time to task failure (TTTF), median frequency (Fmed), maximum force, and muscle electrical activity |
| Applications | Sports, ergonomics, occupational therapy, and injury prevention |
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What You'll Learn
Mechanical manifestation of muscle fatigue
However, it is important to note that the results obtained through this method are influenced by psychological factors such as motivation and task conditions (Enoka, 1995; Enoka and Duchateau, 2008). In addition, the mechanical manifestation of muscle fatigue may not always accurately reflect the underlying processes. For example, EMG changes during submaximal force contractions do not necessarily indicate a decline in the muscle's mechanical output, but they may reflect the changing metabolic status of the muscle, impacting its ability to generate normal force (Schillings et al., 2003).
To address these limitations, researchers have employed surface electromyography (sEMG) as a non-invasive tool to assess muscle fatigue. sEMG measures the myoelectric signal generated by muscle activity, providing valuable information about the muscle's ability to perform contractions. This technique has been used to study muscle fatigue in various contexts, such as in patients in acute care hospitals, where it helps guide patient-centred exercise prescriptions (Skrzat et al., 2020).
Furthermore, the regression analysis between the rate of decline in MF (motor function) and MVC (maximum voluntary contraction) has shown a significant relationship with endurance time to fatigue (P = 0.0001) (Pääsuke et al., 2007). This indicates that the decline in MF can be a reliable indicator of muscle fatigue, especially when defined as the inability to generate the maximum force that a fresh muscle can produce (Bilodeau et al., 2003).
In conclusion, while the mechanical manifestation of muscle fatigue provides a straightforward approach to understanding muscle fatigue, it is important to consider its limitations and complement it with other methods such as sEMG to comprehensively assess and address muscle fatigue in different populations.
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Invasive vs non-invasive techniques
Muscle fatigue is a common muscle condition experienced in daily life and during exercise. It is a complex physiological and psychological phenomenon that impairs physical performance and increases the risk of injury. It is important to continuously monitor fatigue levels to enable early detection and management.
There are several non-invasive techniques to detect and predict muscle fatigue. These include mechanomyography, electromyography, near-infrared spectroscopy, and ultrasound.
Non-Invasive Techniques
Near-infrared spectroscopy (NIRS) is a non-invasive method that uses the near-infrared part of the electromagnetic spectrum to measure the absorption properties of blood haemoglobin. It can be used to determine the optimal current intensity by monitoring oxygenation changes during electrically evoked contractions. NIRS is very sensitive to movement, which makes it unsuitable for sports and other movement-rich scenarios.
Surface electromyography (sEMG) is a non-invasive measure of muscle activity that is widely used in research and clinical settings. It uses electrodes to detect changes in the muscle as it fatigues, with the output signal indicating fluctuations in muscle performance. sEMG is sensitive to electromagnetic noise, and variations in electrode placement can result in signal inconsistencies.
Ultrasound is another non-invasive technique that can be used to monitor muscle fatigue. A 2022 study by Sheng et al. integrated real-time ultrasound image acquisition and processing into a hybrid knee exoskeleton to monitor muscle responses to electrically evoked contractions. Ultrasound can provide a wealth of information about the targeted muscle in real-time, irrespective of its depth.
Invasive Techniques
Invasive techniques for measuring muscle fatigue were not explicitly mentioned in the sources. However, some studies mentioned the use of electrical stimulation and needle EMG, which may involve some degree of invasiveness.
In summary, there are a variety of non-invasive techniques available for measuring muscle fatigue, each with its own advantages and limitations. These techniques have been developed to improve our understanding of muscle fatigue and enhance performance and injury prevention in various scenarios, such as sports and rehabilitation.
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Muscle biopsies
During a muscle biopsy, the patient may receive a sedative to help them relax, and the procedure may be performed on an outpatient basis or as part of a hospital stay, depending on the patient's condition and the healthcare provider's practices. The site of the biopsy is cleaned with an antiseptic solution, and a local anesthetic is administered to numb the area. If it is an open biopsy, an incision is made to extract a larger sample of muscle tissue, while a needle biopsy involves inserting a needle into the muscle to obtain a smaller sample.
The muscle selected for the biopsy depends on the location of symptoms. The bicep (upper arm muscle), deltoid (shoulder muscle), and quadriceps (thigh muscle) are commonly chosen for sampling due to their accessibility. However, the muscle chosen must also exhibit weakness or other symptoms indicative of a potential underlying condition.
Overall, muscle biopsies are an important tool in the diagnostic process, providing valuable information to establish a definitive diagnosis and guide appropriate treatment for patients experiencing muscle-related symptoms.
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Electromyography
Surface electromyography (sEMG) is a non-invasive method of measuring muscle fatigue. It involves measuring the myoelectric signals generated by muscle activity. Each time a muscle fibre is activated, an action potential is conducted from the nerve, along the muscle fibre, and into the fibre.
SEMG can be used to measure the median frequency (Fmed) of the myoelectric signal of the quadriceps femoris muscles and time to task failure (TTTF) during isometric and dynamic fatiguing contractions. TTTF is the time from the initiation of a force-generating contraction to the termination of contraction. It is a common measure of muscle fatigue.
SEMG has been used to study muscle fatigue in patients in an acute care hospital setting. In one study, sEMG was used to determine differences in muscle fatigue during isometric and dynamic submaximal contractions among hospitalised patients, healthy younger participants, and healthy older participants. The study found that hospitalised patients fatigued faster than the other two groups after both types of contractions.
SEMG has also been used to study the effect of fatigue and contraction intensity on the complexity of biceps brachii surface electromyography. The study found that fatigue reduced the short-term complexity of biceps brachii activity during the last third of the fatiguing contraction.
The calculation of time-domain features is a popular approach for sEMG feature extraction. The typical time-domain features of sEMG signals include the root mean square, integrated EMG, zero-crossing rate, waveform length, variance of electromyography, and mean absolute value. With the occurrence of muscle fatigue, the time-domain features of sEMG show an upward trend over time.
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Central vs peripheral fatigue
Muscle fatigue is a commonly experienced phenomenon that limits athletic performance and other strenuous or prolonged activity. It is also associated with various pathological conditions, including neurological, muscular, and cardiovascular disorders, as well as aging and frailty. Fatigue can generally be classified as peripheral fatigue or central fatigue.
Peripheral fatigue is related to our skeletal muscles. It occurs when the brain sends the correct signals to the muscles to contract, but something at a muscular level prevents or hinders these signals from being translated into the intended fast, forceful contractions. Possible sources of peripheral fatigue include depleted energy stores, accumulation of metabolites, and mechanical stress. It is produced by changes at or distal to the neuromuscular junction and can be assessed by measuring the muscle twitch that occurs when a resting muscle is stimulated.
Central fatigue, on the other hand, is related to the central nervous system (CNS), specifically the brain. It occurs when exercise exertion, lack of sleep, or mental exertion acutely changes the brain's chemical balance. The CNS utilises chemical mediators to communicate effectively with the muscles. Central fatigue results in a decrease in muscle recruitment, leading to fewer muscle fibres being activated, which, in turn, causes slower contraction velocity and reduced force generation despite the same level of exertion. Central fatigue can be measured using the twitch interpolation technique, which involves assessing the muscle twitch in response to stimulation while voluntarily generating a Maximum Voluntary Contraction (MVC).
It is important to note that peripheral and central fatigue often occur together but not at a 1:1 ratio. The intensity of the workout plays a role in the prevalence of each type of fatigue, with higher-intensity workouts tending to result in central fatigue persisting longer than peripheral fatigue.
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Frequently asked questions
Muscle fatigue is defined as any reduction in the ability of a muscle to produce force or power. It is commonly associated with exercise and physical activity.
Muscle fatigue can be measured in several ways, including blood tests, muscle biopsies, and electromyography (EMG). EMG is a non-invasive method that uses electrodes to detect electrical currents created in contracting muscles. It is often used in acute care settings to monitor patients' muscle performance and guide their rehabilitation.
Specific techniques include the use of surface electromyography (sEMG), which measures the myoelectric signal generated by muscle activity. Time to task failure (TTTF) is also measured, which refers to the time from the initiation of a force-generating contraction to the termination of contraction.
EMG is used in various fields, including sports, ergonomics, and occupational therapy. For example, in industries that require manual lifting, EMG can monitor workers' muscle conditions and help determine the maximum lifting load, height, and number of repetitions to avoid overexertion and injuries.
