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Fatigue (physical)

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Name of Symptom/Sign:
Fatigue
ICD-10 R53
ICD-9 780.7
ICD-O:
OMIM
DiseasesDB
MedlinePlus
eMedicine
Two fatigued men in Tehran
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Two fatigued men in Tehran

The word ‘fatigue’ is used in everyday living to describe a range of afflictions, varying from a general state of lethargy to a specific work induced burning sensation within muscle. Physiologically, ‘fatigue’ describes the inability to continue functioning at a prescribed work rate (Gandevia SC et al, 1995; Hagberg M, 1981; Hawley JA et al, 1997) in the presence of an increased perception of effort (Enoka RM et al, 1992). Fatigue is ubiquitous in everyday life, but becomes particularly marked during heavy exercise.

The development of fatigue is characterised by an initial, disproportionate increase in the perception of effort required to maintain or increase the work output before the inability to exert the required force is experienced (Cafarelli E, 1988; Garner SH et al, 1990; Jones LA et al, 1983; Matthews PB, 1982). The seemingly dichotomous nature of fatigue has lead scientists to describe the aetiology of fatigue in terms of peripheral and central components (Gandevia SC, 1992; Kent-Braun JA, 1999).

Enoka and Stuart (Enoka & Stuart 1992) propose that the origin of fatigue depends on the mode of work that is being undertaken; a concept that they called task dependency. St Clair Gibson et al (St Clair Gibson, Lambert, & Noakes 2001) used the task dependency model to examine neural control mechanisms during different types of activities. They concluded that the force output appears to be regulated through inhibitory efferent commands in order to maintain a reserve capacity within the muscle and other organs so that there is always reserve capacity during volitional exercise.

Fatigue can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. When someone is sufficiently fatigued, they may experience microsleeps that can cause them to lose concentration; however, objective cognitive testing should be done to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.

The sense of fatigue is believed to originate in the reticular activating system of the lower brain. However, the brain did not evolve merely to register representations of the world; rather it evolved for adaptive action and behaviour. Musculoskeletal structures co-evolved with appropriate brain structures so that the complete unit functions together in an adaptive fashion (Edelman 1989). The entire systems of muscles, joints, and proprioceptive and kinaesthetic functions plus parts of the brain evolve and function together in a unitary way (Kelso 1995).

Contents

Types

There are seen to be two main types of fatigue; Central and Peripheral.

  • Central Fatigue The central component to fatigue is generally described in terms of a reduction in the neural drive or motor command to working muscles that results in a decline in the force output (Gandevia 2001; Kay et al. 2001; Kent-Braun 1999; Vandewalle et al. 1991). It has been suggested that the reduced central drive during exercise may be a protective mechanism to prevent organ failure if the work was continued at the same intensity (Bigland-Ritchie & Woods 1984; Noakes 2000). The exact mechanisms of central fatigue are unknown although there has been a great deal of interest in the role of serotonergic pathways (Davis JM, 1995; Newsholme EA et al, 1987; Newsholme EA et al, 1995).
  • Peripheral Fatigue Fatigue during physical work is usually modelled from the peripheral context of an inadequate capacity to supply metabolic substrate to the contracting muscles to meet the increased energy demand. This causes contractile dysfunction that is manifest in the inability to maintain or increase work output.

The fundamental difference between the peripheral and central theories of fatigue is that the peripheral model of fatigue assumes failure at one or more sites in the chain that initiates muscle contraction. Peripheral regulation is therefore dependent on the localised accumulation or depletion of substrates within the active muscle. Whereas the central model of fatigue is an intregrated mechanism that works to preserve the integrity of the system by initiating fatigue through muscle derecruitment, based on collective feedback from the periphery, before cellular or organ failure occurs. Therefore the feedback that is assimilated by the central regulator could include chemical and mechanical as well as cognitive cues. The significance of each of these factors is dependent on the nature of the work that is being performed.

Causes

It is typically the result of working, mental stress, jet lag or active recreation, but also from boredom or disease or simply lack of sleep. It may also have chemical causes, such as poisoning or mineral or vitamin deficiencies.

When chronic (meaning of six months or more duration) it is a symptom of nearly 30 different diseases. Post exertional fatigue, also known as exercise intolerance, is however far more rare, and is primarily found in organic brain diseases, mitochondrial disease, and neuromuscular disease.

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See also

External links

References

Edelman, GM 1989, The remembered present : a biological theory of consciousness. Basic Books, New York.

Kelso, JAS 1995, Dynamic patterns : the self-organization of brain and behavior. MIT Press, Cambridge, MA.

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.

Newsholme, E. A. & Blomstrand, E. 1995, 'Tryptophan, 5-hydroxytryptamine and a possible explanation for central fatigue', in SC Gandevia (ed.), Fatigue, Plenum Press, New York, pp. 315-320.