You are running a race on a sports day and seconds before crossing the finishing line, intense pain pierces through your calf muscles and you slow down. You exert yourself maximally and try to keep running.
It is 8 pm and you are in your office cubicle, skimming through pages of a document. You rest your head against the chair, close your eyes momentarily and try to breathe in heavily.
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Fatigue is a lingering tiredness that is constant and limiting. There is unexplained, persistent, and relapsing exhaustion. Nearly everyone is overtired or overworked from time to time. Such instances of temporary fatigue usually have an identifiable cause and a likely remedy. Unrelenting exhaustion, on the other hand, lasts longer, is more profound and isn't relieved by rest. It's a nearly constant state of weariness that develops over time and reduces your energy, motivation, and concentration. Fatigue accumulation, if not resolved, leads to overwork, chronic fatigue syndrome (CFS), overtraining syndrome, and even endocrine disorders, immunity dysfunction, organic diseases, and a threat to human health.
On occasion, fatigue is a symptom of other underlying conditions that require medical treatment. According to its duration, fatigue can be classified into acute fatigue and chronic fatigue. Acute fatigue can be quickly relieved by rest or life-style changes, whereas chronic fatigue is a condition defined as a persistent tiredness lasting more than months that is not ameliorated by rest. Fatigue can also be classified as mental fatigue which refers to the cognitive or perceptual aspects of fatigue, and physical fatigue, which refers to the performance of the motor system. Despite being different, physical, and mental fatigue often occur together.
Physical fatigue or muscle fatigue is a commonly experienced phenomenon that limits athletic performance and other strenuous or prolonged activity. It also increases and restricts daily life under various pathological conditions, including neurological, muscular, and cardiovascular disorders, as well as aging and frailty. Muscle fatigue has several possible causes including impaired blood flow, ion imbalance within the muscle, nervous fatigue, loss of desire to continue, and the accumulation of lactic acid in the muscle.
Long-term muscle use requires oxygen and glucose as substrates for aerobic respiration to occur allowing ATP (Adenosine Triphosphate) to be produced which is needed for muscle contraction. Pyruvate is a compound generated by glycolysis during aerobic respiration. Pyruvate enters the Krebs cycle and is used to produce additional ATPs. At the instance of respiratory and circulatory insufficiency, lack of oxygen shifts respiration to the anaerobic pathway. Due to insufficient oxygen, pyruvate cannot enter Krebs cycle. Rather, it is converted to lactic acid. Lactic acid build-up also occurs as there is less oxygen to breakdown glucose and the stored carbohydrate in muscles, glycogen. This lactic acid accumulation in the muscle tissue reduces the pH, making it more acidic and producing the stinging feeling in muscles when exercising. This further inhibits anaerobic respiration, inducing fatigue.
Muscle contraction requires calcium ions, which bind to troponin molecules present on the thin actin filaments of the muscle contractile tissue. With extensive exercise, the osmotically active molecules outside of the muscle are lost through sweating. This loss changes the osmotic gradient, making it more difficult for the required Ca+ ions to be delivered to the muscle fiber. In extreme cases, this can lead to painful, extended maintenance of muscle contraction or cramp.
The smallest functional and structural unit of a muscle tissue. The thin actin filaments slide in between the thick myosin filaments (in yellow) and the unit shortens as the muscle contracts.
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Nerves are responsible for controlling the contraction of muscles, determining the number, sequence, and force of muscular contractions. However, loss of desire to exercise in the face of increasing muscle soreness, respiration, and heart rate can have a powerful negative impact on muscle activity.
Depletion of required substrates such as ATP or glycogen within a muscle results in fatigue as the muscle is not able to generate energy to power contractions. Accumulation of metabolites from these reactions other than lactic acid, such as magnesium ions or reactive oxygen species, can also induce fatigue by interfering with the release of calcium ions from the sarcoplasmic reticulum or through reduction in the sensitivity of troponin to calcium. With training, the muscle’s metabolic capacity can change and improvements to the circulatory and respiratory systems can facilitate better delivery of oxygen and glucose to the muscle, delaying onset of fatigue. However, with aging, muscle function declines due to decreasing levels of ATP, myoglobin (a compound that binds oxygen in muscles). Muscle fibers shrink or are lost and surrounding connective tissue hardens, making muscle contraction slower and more difficult.
Mental fatigue is encountered after sustained attention during a prolonged cognitive task. There is disengagement from the task at hand. Mental fatigue may induce over-activation of the visual cortex (the primary cortical region of the brain that receives, integrates, and processes visual information relayed from the retinas), which is related to impaired cognitive performance. Mental fatigue induces decline in executive functions such as executive attention, sustained attention, goal-directed attention, alternating attention, divided attention, response inhibition, planning, and novelty processing. Contributing factors for mental fatigue can be also physical like poor nutrition, lack of sleep, or hormonal imbalances or cognitive overload. Cognitive overload can take the form of intense focus on a single task over an extended period or spreading your attention across too many things. Furthermore, worrying about a task can be as mentally taxing as actually doing it. That means even while you’re procrastinating, you’re taxing your brain.
Mental fatigue can be alleviated by taking frequent short breaks, reducing the amount of refined sugar you consume, giving your body consistent fuel with mid-morning and mid-afternoon snacks and by staying hydrated. Meanwhile, physical fatigue can be reduced by getting regular physical activity, practicing mindfulness meditation, maintaining a good sleep schedule, and avoiding caffeine in the morning and evening.
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By Unaiza Naeem
References:
1. Khatri,MD, Minesh. “How Tired Is Too Tired.” WebMD, 13 Nov. 2019, www.webmd.com/balance/how-tired-is-too-tired#1.
2. Staff, Mayo Clinic. “Fatigue.” Mayoclinic.org, 22 July 2020, www.mayoclinic.org/symptoms/fatigue/basics/definition/sym-20050894.
3. Weatherspoon, Doborah. “What Causes Fatigue, and How Can I Treat It?” Medical News Today, 4 June 2020, www.medicalnewstoday.com/articles/248002
4. Wan, Jing-jing, and Zhen Qin. “Muscle Fatigue: General Understanding and Treatment.” Nature, 6 Oct. 2017, www.nature.com/articles/emm2017194.
5. “Muscle Fatigue.” Medicine LibreTexts, med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/9%3A_Muscular_System/9.4%3A_Muscle_Metabolism/9.4B%3A_Muscle_Fatigue. Accessed 24 Nov. 2020.
6. Huff, Trevor, et al. “Neuroanatomy, Visual Cortex.” NCBI, 31 July 2020, www.ncbi.nlm.nih.gov/books/NBK482504/#:~:text=The%20visual%20cortex%20is%20the,posterior%20region%20of%20the%20brain.
Tanaka, Masaaki, et al. “Effects of Mental Fatigue on Brain Activity and Cognitive Performance: A Magnetoencephalography Study.” Anatomy & Physiology: Current Research, vol. 5, no. 4, 2015, pp. 4–5. www.longdom.org, www.longdom.org/open-access/effects-of-mental-fatigue-on-brain-activity-and-cognitive-performance-amagnetoencephalography-study-2161-0940-S4-002.pdf
7. Tanaka, Masaaki, et al. “Effects of Mental Fatigue on Brain Activity and Cognitive Performance: A Magnetoencephalography Study.” Anatomy & Physiology: Current Research, vol. 5, no. 4, 2015, pp. 4–5. www.longdom.org, www.longdom.org/open-access/effects-of-mental-fatigue-on-brain-activity-and-cognitive-performance-amagnetoencephalography-study-2161-0940-S4-002.pdf.
8. Kane, Becky. “What to Do When Your Brain Is Too Tired to Think Straight.” Ambition & Balance by Doist, blog.doist.com/mental-fatigue. Accessed 25 Nov. 2020.
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