Fatigue

Fatigue  is  an  overwhelming  sustained  feeling  of exhaustion  and  decreased  capacity  to  complete physical  and  mental  work.  It  is  multidimensional with emotional, behavioral, and cognitive components.  Feelings  of  fatigue  are  associated  not  only with disease states but also with healthy functioning.  Approximately  20%  of  adults  worldwide report  current  fatigue.  When  examined  in  terms of  physical  activity,  feelings  of  fatigue  are  related to issues of physical exertion, inadequate rest, and sedentary life style. This entry reviews the evidence regarding the relationship between physical activity and feelings of fatigue and touches on issues of measurement and biological mechanisms.

Conceptualization and Measurement of Fatigue

Italian  physiologist  Angelo  Mosso  (1846–1910) began  to  shift  the  focus  of  research  from  fatigue of the body to fatigue of the mind during the late 19th century. Influenced by the work of Mosso, clinicians began to draw a sharp distinction between objective  and  subjective  fatigue,  thus  broadening  the  conceptualization  of  fatigue.  The  concept of  subjective  fatigue  raised  measurement  issues related to the indirect measure of central nervous system activity.Contemporary theories on physical activity and feelings of fatigue have changed little over the last century and continue to focus on the multidimensional nature of the construct. In addition, the nonspecific,  subjective  experience  of  fatigue  remains difficult  to  operationalize  through  measurement instruments because of the lack of known biological markers. Thus, self-report measures have dominated the clinical and scientific community. These measures  range  widely  in  their  ability  to  offer  a valid interpretation of fatigue. Despite these limitations,  clinicians  and  researchers  recognize  exercise  as  a  valuable  treatment  for  fatigue  in  which the  mechanism  for  the  positive  effect  is  likely  an interaction  among  biological,  psychological,  and psychosocial variables.

Empirical Evidence

Despite the limitations in the measurement of both physical activity and fatigue, the evidence for the effect  of  physical  activity  on  feelings  of  fatigue generally  is  both  positive  and  consistent.  Both epidemiological  and  experimental  research  in  the areas of exercise adoption, exercise cessation, and overtraining support this association.

Population Studies

Epidemiological  evidence  suggests  that  people who are physically active in their leisure time have about  40%  reduced  risk  of  experiencing  fatigue compared  with  sedentary  individuals.  There  is general  agreement  among  population-based  studies of a strong, consistent, temporally appropriate, dose-response relationship between physical activity  and  feelings  of  fatigue.  Furthermore,  feelings of fatigue appear to be alterable in relation to the adoption or cessation of physical activity. The beneficial effects of physical activity appear to be largest for sedentary individuals who initiate and then maintain an exercise program.

Exercise Adoption

Experimental  studies  examining  the  effects  of exercise training on feelings of fatigue suggest that exercise  programs  reduce  fatigue  in  previously sedentary individuals. These effects are consistent across  both  healthy  people  and  patient  groups. Physical  activity  may  be  especially  beneficial  in reducing  feelings  of  fatigue  in  certain  patient groups  with  chronic  diseases  such  as  fibromyalgia,  cardiovascular  disease,  and  cancer.  Exercise may  represent  a  low-cost,  efficacious  adjunctive therapy  that  can  address  other  health  outcomes beyond fatigue in these patient groups.

Exercise Cessation

Experimental  studies  examining  the  effects  of temporary  exercise  cessation  by  people  who  regularly  exercise  suggest  that  exercisers  experience increased  feelings  of  fatigue  during  periods  of exercise  withdrawal.  These  temporary  mood  disturbances related to fatigue appear to be alleviated with the reintroduction of the exercise regimen.

Overtraining

Overtraining is characterized by an overload in frequency, intensity, or duration of exercise such that an individual’s training regime exceeds the recovery capacity. Overtraining can result in decreased performance  and  symptoms  of  under  recovery or  burnout  such  as  fatigue.  Increases  in  training volume  during  overtraining  periods  are  associated  with  increased  feelings  of  fatigue,  whereas decreases  in  training  volume  during  tapering periods  are  associated  with  decreased  feelings  of fatigue. Despite increases in negative overall mood in  response  to  overtraining,  feelings  of  fatigue often  exhibit  the  earliest  and  largest  changes  in mood state.

Biological Mechanisms

Brain  functioning  is  controlled  by  genes,  but social,  developmental,  and  environmental  factors  can  alter  gene  expression.  These  alterations in  gene  expression  can  induce  changes  in  brain functioning  and  behavior.  This  integrative  process  supports  the  conceptualization  of  fatigue as  a  multidimensional  construct  influenced  by  a variety  of  biological,  psychological,  and  psychosocial factors. The specific brain mechanisms that generate  the  feelings  of  fatigue  are  unknown, but  monoamines,  histamine-,  acetylcholine-,  and gamma-aminobutyric acid (GABA)-mediated neurotransmission  have  been  implicated.  There  is evidence that physical activity can alter these neurotransmitters and neuromodulators in key brain areas associated with fatigue, such as the dorsolateral  prefrontal  cortex,  striatum,  cerebellum,  and spinal cord.

Conclusion

Physical  activity  is  a  healthful  behavior  that has  promise  for  combating  feelings  of  fatigue. Historically  clinicians  have  recognized  the  physiological  and  psychological  aspects  of  fatigue  and have consistently recommended exercise as a treatment.  The  subjective  nature  of  feelings  of  fatigue has made conceptualizing and measuring the construct difficult. Despite such limitations, epidemiological and experimental evidence suggests physical activity does reduce feelings of fatigue across both healthy groups and patient populations. However, intensive exercise training can produce feelings of fatigue  in  the  event  of  overtraining.  The  biological  mechanisms  associated  with  physical  activity and fatigue should be considered in relation to the nature of the exercise stimulus.

References:

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