Fatigue

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 one's muscles. Physiologically, ‘fatigue’ describes the inability to continue functioning at the level of one's normal abilities (Gandevia et al., 1995; Hagberg, 1981; Hawley et al., 1997) due to an increased perception of effort (Enoka & Stuart 1992). Fatigue is ubiquitous in everyday life, but usually becomes particularly noticeable during heavy exercise.

Fatigue has two known forms; one manifests as a local, muscle-specific incapacity to do work, and the other manifests as an overall, bodily or systemic, sense of energy deprivation. Due to these two divergent facets of fatigue symptoms, it has been proposed to look at the causes fatigue from "central" and "peripheral" perspectives (Gandevia, 1992; Kent-Braun, 1999).

Fatigue can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. When one is sufficiently fatigued, one 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 a constructive and 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).

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 nerve-based 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 neural 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, 1995; Newsholme et al., 1987; Newsholme et al., 1995).


 * Peripheral Fatigue Fatigue during physical work is considered an ability for the body to supply sufficient energy to the contracting muscles to meet the increased energy demand. This is the most common case of physical fatigue-effecting 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.

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 metabolic chemical conditions of the local muscle affected, 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 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.

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.


 * Relaxation
 * Lack of sleep(Sleep deprivation)
 * Pregnancy
 * Pneumonia
 * Addison's disease
 * Anemia
 * Arthritis
 * Chronic fatigue syndrome (CFS)
 * Depression
 * Diabetes
 * Driving fatigue
 * Endometriosis
 * Fibromyalgia
 * Headaches
 * Hyperparathyroidism
 * Hypothyroidism
 * Lupus erythematosus
 * Interstitial Cystitis
 * Mitral valve prolapse/Mitral regurgitation
 * Mononucleosis
 * Multiple sclerosis
 * Muscular Dystrophy
 * Myasthenia gravis
 * Parkinson's disease
 * Primary Biliary Cirrhosis (PBC)
 * Chemical dependency
 * Certain medications, e.g. lithium salts, ciprofloxacin
 * Starvation
 * Hyperthyroidism
 * Cancer Syndrome
 * Ejaculatory release