Muscle weakness

Muscle weakness

ICD-10	M62.8
ICD-9	728.87 (728.9 before 10/01/03)
DiseasesDB	22832
MeSH	D018908

Muscle weakness or myasthenia (latter from my- from Greek μυο meaning "muscle" + -asthenia from Ancient Greek ἀσθένεια meaning "weakness" but in exact translation means illness) is a lack of muscle strength. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.

True vs perceived

The term can be divided into two other more specific states, true muscle weakness and perceived muscle weakness.^[1] Muscle weakness can also be caused by low potassium levels.

• True muscle weakness (or neuromuscular) describes a condition where the force exerted by the muscles is less than would be expected, for example muscular dystrophy.

• Perceived muscle weakness (or non-neuromuscular) describes a condition where a person feels more effort than normal is required to exert a given amount of force but actual muscle strength is normal, for example chronic fatigue syndrome.^[2]

In some conditions, such as myasthenia gravis muscle strength is normal when resting, but true weakness occurs after the muscle has been subjected to exercise. This is also true for some cases of chronic fatigue syndrome, where objective post-exertion muscle weakness with delayed recovery time has been measured and is a feature of some of the published definitions.^{[3][4][5][6][7][8]}

Grading

One example of grading for muscle weakness is as follows:^[9]

• Grade 0: No contraction
• Grade 1: Trace of contraction
• Grade 2: Movement with gravity eliminated
• Grade 3: Movement against gravity
• Grade 4: Movement against external resistance
• Grade 5: Normal strength

Differential diagnosis

Muscle weakness can be central, neural and peripheral. Central muscle weakness manifests as an overall, bodily or systemic, sense of energy deprivation, and peripheral weakness manifests as a local, muscle-specific incapacity to do work.^[10][11] Also, neural muscle weakness can be both central and peripheral.

Central

The central component to muscle weakness is generally described in terms of a reduction in the neural drive or nerve-based motor command to working muscles that results in a decline in the force output.^[12][13][14] It has been suggested that the reduced neural drive during exercise may be a protective mechanism to prevent organ failure if the work was continued at the same intensity.^[15][16] The exact mechanisms of central fatigue are unknown although there has been a great deal of interest in the role of serotonergic pathways.^[17][18][19]

Neural

Nerves are responsible for controlling the contraction of muscles, determining the number, sequence and force of muscular contraction. Most movements require a force far below what a muscle could in potential generate, and barring pathology nervous fatigue is seldom an issue. For extremely powerful contractions that are close to the upper limit of a muscle's ability to generate force, nervous fatigue can be a limiting factor in untrained individuals. In novice strength trainers, the muscle's ability to generate force is most strongly limited by nerve’s ability to sustain a high-frequency signal. After a period of maximum contraction, the nerve’s signal reduces in frequency and the force generated by the contraction diminishes. There is no sensation of pain or discomfort, the muscle appears to simply ‘stop listening’ and gradually cease to move, often going backwards. As there is insufficient stress on the muscles and tendons, there will often be no delayed onset muscle soreness following the workout. Part of the process of strength training is increasing the nerve's ability to generate sustained, high frequency signals which allow a muscle to contract with their greatest force. It is this neural training that causes several weeks worth of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit. Past this point, training effects increase muscular strength through myofibrilar or sarcoplasmic hypertrophy and metabolic fatigue becomes the factor limiting contractile force.

Peripheral

Peripheral muscle fatigue during physical work is considered an inability for the body to supply sufficient energy or other metabolites to the contracting muscles to meet the increased energy demand. This is the most common case of physical fatigue—affecting a national average of 72% of adults in the work force in 2002. This causes contractile dysfunction that is manifested in the eventual reduction or lack of ability of a single muscle or local group of muscles to do work. The insufficiency of energy, i.e. sub-optimal aerobic metabolism, generally results in the accumulation of lactic acid and other acidic anaerobic metabolic by-products in the muscle, causing the stereotypical burning sensation of local muscle fatigue, though recent studies have indicated otherwise, actually finding that lactic acid is a source of energy.^[20]

The fundamental difference between the peripheral and central theories of muscle fatigue is that the peripheral model of muscle fatigue assumes failure at one or more sites in the chain that initiates muscle contraction. Peripheral regulation is therefore dependent on the localised metabolic chemical conditions of the local muscle affected, whereas the central model of muscle fatigue is an integrated mechanism that works to preserve the integrity of the system by initiating muscle fatigue through muscle derecruitment, based on collective feedback from the periphery, before cellular or organ failure occurs. Therefore the feedback that is read by this central regulator could include chemical and mechanical as well as cognitive cues. The significance of each of these factors will depend on the nature of the fatigue-inducing work that is being performed.

Though not universally used, ‘metabolic fatigue’ is a common alternative term for peripheral muscle weakness, because of the reduction in contractile force due to the direct or indirect effects of the reduction of substrates or accumulation of metabolites within the muscle fiber. This can occur through a simple lack of energy to fuel contraction, or interference with the ability of Ca²⁺ to stimulate actin and myosin to contract.

Lactic acid

It was once believed that lactic acid build-up was the cause of muscle fatigue.^[21] The assumption was lactic acid had a "pickling" effect on muscles, inhibiting their ability to contract. The impact of lactic acid on performance is now uncertain, it may assist or hinder muscle fatigue.

Produced as a by-product of fermentation, lactic acid can increase intracellular acidity of muscles. This can lower the sensitivity of contractile apparatus to Ca²⁺ but also has the effect of increasing cytoplasmic Ca²⁺ concentration through an inhibition of the chemical pump that actively transports calcium out of the cell. This counters inhibiting effects of K⁺ 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, though the effects of potassium are much less than if there were no lactic acid to remove the chloride ions. Ultimately, it is uncertain if lactic acid reduces fatigue through increased intracellular calcium or increases fatigue through reduced sensitivity of contractile proteins to Ca²⁺.

Pathophysiology

Main article: muscle contraction

Muscle cells work by detecting a flow of electrical impulses from the brain which signals them to contract through the release of calcium by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This causes there to be less calcium available for the muscle cell. In addition an enzyme is proposed to be activated by this released calcium which eats away at muscle fibers.^[22]

Substrates within the muscle generally serve to power muscular contractions. They include molecules such as adenosine triphosphate (ATP), glycogen and creatine phosphate. ATP binds to the myosin head and causes the ‘ratchetting’ that results in contraction according to the sliding filament model. Creatine phosphate stores energy so ATP can be rapidly regenerated within the muscle cells from adenosine diphosphate (ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5–7 seconds. Glycogen is the intramuscular storage form of glucose, used to generate energy quickly once intramuscular creatine stores are exhausted, producing lactic acid as a metabolic byproduct. Contrary to common belief, lactic acid accumulation doesn't actually cause the burning sensation we feel when we exhaust our oxygen and oxidative metabolism, but in actuality, lactic acid in presence of oxygen recycles to produce pyruvate in the liver which is known as the Cori cycle.

Substrates produce metabolic fatigue by being depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions. In essence, the muscle stops contracting because it lacks the energy to do so.

References

1. ^ Marx, John (2010). Rosen's Emergency Medicine: Concepts and Clinical Practice (7th ed.). Philadelphia, PA: Mosby/Elsevier. p. Chapter 11. ISBN 9780323054720. 
2. ^ Enoka RM, Stuart DG (1992). "Neurobiology of muscle fatigue". J. Appl. Physiol. 72 (5): 1631–48. PMID 1601767. 
3. ^ Paul L, Wood L, Behan WM, Maclaren WM (January 1999). "Demonstration of delayed recovery from fatiguing exercise in chronic fatigue syndrome". Eur. J. Neurol. 6 (1): 63–9. doi:10.1046/j.1468-1331.1999.610063.x. PMID 10209352. http://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=1351-5101&date=1999&volume=6&issue=1&spage=63. 
4. ^ McCully KK, Natelson BH (November 1999). "Impaired oxygen delivery to muscle in chronic fatigue syndrome". Clin. Sci. 97 (5): 603–8; discussion 611–3. doi:10.1042/CS19980372. PMID 10545311. http://www.clinsci.org/cs/097/0603/cs0970603.htm. 
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6. ^ De Becker P, McGregor N, De Meirleir K (September 2001). "A definition-based analysis of symptoms in a large cohort of patients with chronic fatigue syndrome". J. Intern. Med. 250 (3): 234–40. doi:10.1046/j.1365-2796.2001.00890.x. PMID 11555128. http://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0954-6820&date=2001&volume=250&issue=3&spage=234. 
7. ^ Bruce M Carruthers, Anil Kumar Jain, Kenny L De Meirleir, Daniel L Peterson, Nancy G Klimas et al., Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Clinical Working Case Definition, Diagnostic and Treatment Guidelines, A Consensus Document Journal of Chronic Fatigue Syndrome 11(1):7-115, 2003. ISBN 0-7890-227-9
8. ^ Jammes Y, Steinberg JG, Mambrini O, Brégeon F, Delliaux S (March 2005). "Chronic fatigue syndrome: assessment of increased oxidative stress and altered muscle excitability in response to incremental exercise". J. Intern. Med. 257 (3): 299–310. doi:10.1111/j.1365-2796.2005.01452.x. PMID 15715687. http://onlinelibrary.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0954-6820&date=2005&volume=257&issue=3&spage=299. 
9. ^ Page 59 in: Hugue Ouellette (2008). Orthopedics Made Ridiculously Simple (Medmaster Ridiculously Simple) (Medmaster Ridiculously Simple). MedMaster Inc. ISBN 0-940780-86-0. 
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11. ^ Kent-Braun JA (1999). "Central and peripheral contributions to muscle fatigue in humans during sustained maximal effort". European journal of applied physiology and occupational physiology 80 (1): 57–63. doi:10.1007/s004210050558. PMID 10367724. 
12. ^ Gandevia SC (2001). "Spinal and supraspinal factors in human muscle fatigue". Physiol. Rev. 81 (4): 1725–89. PMID 11581501. 
13. ^ Kay D, Marino FE, Cannon J, St Clair Gibson A, Lambert MI, Noakes TD (2001). "Evidence for neuromuscular fatigue during high-intensity cycling in warm, humid conditions". Eur. J. Appl. Physiol. 84 (1–2): 115–21. doi:10.1007/s004210000340. PMID 11394239. 
14. ^ Vandewalle H, Maton B, Le Bozec S, Guerenbourg G (1991). "An electromyographic study of an all-out exercise on a cycle ergometer". Archives internationales de physiologie, de biochimie et de biophysique 99 (1): 89–93. doi:10.3109/13813459109145909. PMID 1713492. 
15. ^ Bigland-Ritchie B, Woods JJ (1984). "Changes in muscle contractile properties and neural control during human muscular fatigue". Muscle Nerve 7 (9): 691–9. doi:10.1002/mus.880070902. PMID 6100456. 
16. ^ Noakes TD (2000). "Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance". Scandinavian journal of medicine & science in sports 10 (3): 123–45. doi:10.1034/j.1600-0838.2000.010003123.x. PMID 10843507. 
17. ^ Davis JM (1995). "Carbohydrates, branched-chain amino acids, and endurance: the central fatigue hypothesis". Int J Sport Nutr 5 (Suppl): S29–38. PMID 7550256. 
18. ^ Newsholme, E. A., Acworth, I. N., & Blomstrand, E. 1987, 'Amino acids, brain neurotransmitters and a functional link between muscle and brain that is important in sustained exercise', in G Benzi (ed.), Advances in Myochemistry, Libbey Eurotext, London, pp. 127-133.
19. ^ Newsholme EA, Blomstrand E (1995). "Tryptophan, 5-hydroxytryptamine and a possible explanation for central fatigue". Adv. Exp. Med. Biol. 384: 315–20. PMID 8585461. 
20. ^ R. Robergs, F. Ghiasvand, D. Parker (2004). "Biochemistry of exercise-induced metabolic acidosis". Am J Physiol Regul Integr Comp Physiol 287 (3): R502–16. doi:10.1152/ajpregu.00114.2004. PMID 15308499. 
21. ^ Sahlin K (1986). "Muscle fatigue and lactic acid accumulation". Acta Physiol Scand Suppl 556: 83–91. PMID 3471061. 
22. ^ Kolata, Gina (February 12, 2008). "Finding May Solve Riddle of Fatigue in Muscles". The New York Times. http://www.nytimes.com/2008/02/12/health/research/12musc.html?_r=2&ref=science&oref=slogin&oref=slogin. 

External links

• Jun Mapili's Muscle Weakness Coding Checklist
• Unexplained Muscle Weakness - Information About McArdle's Disease