PATRICK J. O’CONNOR and TIMOTHY W. PUETZ
Department of Exercise Science, University of Georgia, Athens, GA
O’CONNOR, P. J., and T. W. PUETZ. Chronic Physical Activity and Feelings of Energy and Fatigue. Med. Sci. Sports Exerc., Vol. 37, No. 2, pp. 299 –305, 2005. This brief review summarizes key epidemiological and experimental evidence concerning relationships between chronic physical activity and feelings of energy (vigor, vitality) and fatigue. The epidemiological studies show a positive association between the amount of typical weekly physical activity reported and the frequency with which people report feeling energetic. The randomized controlled experiments show that 10–20 wk of exercise training is associated with an increase in the frequency and intensity of feelings of energy among fatigued people with medical conditions. The results of longitudinal studies with nonfatigued, healthy adults are mixed. Overtraining by athletes is associated with increased intensity of feelings of fatigue. Additional well-controlled investigations into relationships between physical activity and feelings of energy and fatigue, especially among sedentary and fatigued individuals, are warranted given the available evidence and the importance of these moods to health, work productivity, and quality of life. Key Words: DEPRESSION, EXERCISE, INACTIVITY, MOOD, POMS, QUALITY OF LIFE, SF-36, VIGOR, VITALITY
Exercise scientists and sports medicine practitioners should be interested in the influence of regular phys- ical activity on feelings of fatigue and low energy (lack of vitality or vigor) in part because so many people experience these feelings. Epidemiological surveys of more than one million people indicate that nearly one-quarter of the population feels prolonged and disabling fatigue (30). Feelings of low energy are among the most common com- plaints expressed during pregnancy (10). Feelings of low energy and fatigue are associated with a wide range of major physical and psychiatric conditions and illnesses including heart disease, congestive heart failure, stroke, cancer, HIV/ AIDS, obesity, chronic obstructive pulmonary disease, ane- mias, sleep apnea, multiple sclerosis, rheumatoid arthritis, chronic fatigue syndrome, fibromyalgia, anxiety, and de- pression (11,13,14,16,18,20,26,45,46). Feelings of fatigue and low energy also are common side effects of medicines used to treat psychiatric conditions as well as a variety of other medical concerns including cancer (38). In short, feelings of fatigue and energy are strongly associated with health and quality of life. A better understanding of energy and fatigue feelings ultimately could contribute to improve-
Address for correspondence: Patrick J. O’Connor, Department of Exercise Science, University of Georgia, Athens, GA 30602-6554; E-mail: email@example.com.
Submitted for publication May 2004.
Accepted for publication October 2004.
MEDICINE & SCIENCE IN SPORTS & EXERCISE®Copyright © 2005 by the American College of Sports Medicine
ments in work productivity, quality of life, and the diagnosis and treatment of a myriad of fatigue-related health problems. This review examines relations between chronic physical
activity and feelings of energy and fatigue. Feelings of energy and fatigue have been conceived in various ways including as synonyms of alertness-sleepiness (28), moods (31), symptoms (32), personality traits (23), and dimensions of cognitive effort (47). The focus here will be on energy and fatigue conceptualized as moods.
Mood states are subjective, transient feelings that people report experiencing. The mood of energy can be defined as positive feelings about the capacity to complete mental or physical activities. The mood of fatigue can be defined as negative feelings concerning a reduced capacity to complete mental or physical activities. Theoretically, these mood states are associated with, yet distinguishable from, numer- ous related constructs including muscle fatigue, perception of effort, the trait of self-motivation, and self-efficacy. For example, a person with feelings of low energy could con- comitantly have high self-motivation or high self-efficacy (e.g., a person who thinks that she could successfully per- form a specified task).
Moods of energy and fatigue have been operationalized here using either the vigor and fatigue scales of the Profile of Mood States (POMS) questionnaire or the vitality scale of the SF-36 Health Survey because the key epidemiological and exercise training investigations identified by our liter- ature search used these measures. Although these instru- ments are the most frequently used and highly cited mea- sures of feelings of energy and fatigue, dozens of measures have been created in the last decade to quantify symptoms of fatigue and related concerns associated with medical conditions (1,17,36).
The vitality scale of the SF-36 originally was conceptu- alized as a measure of well-being (50). However, a case has been made that the item content and scoring of the vitality scale combined with the extant literature indicate that it is more precisely defined as a measure of the prior monthfrequency of the bipolar mood of energy-fatigue (36). Bi- polar refers to conceptualizing the presence of energy and fatigue as polar opposites of a single mood continuum. This approach is simpler than conceiving energy and fatigue moods as two separate psychological constructs (i.e., unipo- lar). For instance, the unipolar approach allows for the possibility of a person having simultaneous feelings of en- ergy and fatigue. Although it is unlikely that strong feelings of both energy and fatigue would occur regularly, there are circumstances during which mixed feelings of energy and fatigue reasonably might coexist (e.g., at the birth of a child).
The vigor and fatigue scales of the POMS differ from the SF-36 vitality scale in that they are unipolar and they mea- sure the intensity of energy and fatigue feelings experienced at the moment or recalled during a day, week, or month (31). A large body of convergent psychological, behavioral, and biological evidence supports the validity of interpreting POMS vigor and fatigue scores as measures of the intensity of the moods of energy and fatigue, respectively (31,36). One biologically oriented example is the observation that reductions in POMS fatigue scores after treatment for de- pression were significantly associated with changes in the metabolism of the frontal cortex as measured by positron emission tomography (r 0.39). Specifically, the improve- ments in fatigue were associated with decreased ventral frontal lobe metabolism, especially in the right lobe (7). An even larger corpus of evidence supports the validity of interpreting SF-36 vitality scores as a measure of the bipolar mood of energy fatigue (36).
The primary purpose of this brief review is to summarize what we consider to be key epidemiological and experimen- tal evidence regarding the relationship between chronic physical activity and feelings of energy and fatigue. Liter- ature was identified by searching the Current Contents,PsycInfo, Web of Science, and PubMed databases using the terms fatigue and energy combined either with exercise and physical activity or with epidemiology, prevalence, or inci- dence. Papers concerning chronic physical activity and feel- ings of energy and fatigue were identified and reference lists of these articles were examined for any additional relevant sources. This literature search revealed 7 epidemiological studies, 82 experimental studies with medical patients, and 15 experimental studies with healthy sedentary adults. In- vestigations that measured feelings of energy or fatigue as a multidimensional concept in which, for example, scores on items that assess feelings of fatigue are combined with scores on items that assess consequences of fatigue (e.g., I can’t think, I can’t work, etc., . . .) were excluded. This was done in order to focus this review on studies that examined feelings of energy and fatigue per se. To keep this review brief, we subjectively identified key investigations as those that had research design strengths compared with other
studies of the same type (e.g., larger vs smaller epidemio- logical studies; randomized, controlled experiments vs quasiexperimental investigations).
All of the cross-sectional epidemiological investigations have shown a positive association between physical activity and feelings of energy. Specifically, as self-reported weekly physical activity increased, the frequency of energy feelings reported tended to increase (9,11,27). To illustrate the find- ings, consider the Women’s Health Australia project (9). Three national cohorts of women between the ages of 18 –23 (N 14,502), 45–50 (N 13,609), and 70–75 yr (N 11,421) were surveyed. Feelings of energy were measured using the SF-36 vitality scale. A physical activity score was derived from two questions: one concerned the number of vigorous exercise bouts of 20 or more minutes in a normal week, and the other queried the number of “less vigorous” exercise bouts of 20 or more minutes in a normal week. The reported weekly exercise bouts were coded (never 0, 1wk1 1, 2–3 wk1 2.5, 4–6 wk1 5, every day 7, and more than 1 d1 10), multiplied by an intensity factor (vigorous 5 or less vigorous 3), and summed. The possible physical activity scores ranged from 0 to 80, and the average (SD) score for all study partici- pants was 15 (13).
A negatively accelerating dose–response relationship was found between typical weekly physical activity and the frequency of energy feelings in the Women’s Health Aus- tralia project. Because the shape of the relationship was similar for the three age groups, Figure 1 depicts the rela- tionship for three cohorts combined. As women reported more weekly physical activity, they also reported more frequent feelings of energy. Sedentary women reported the most frequent feelings of low energy and fatigue.
FIGURE 1—The relationship between typical weekly physical activity and the prior month frequency of energy-fatigue feelings measured by the SF-36 vitality scale among 39,532 Australian women. The mean SD from a representative sample of 1412 adult U.S. women is 58 21 (48); reprinted with permission from Research Quarterly for Exer- cise and Sport, Vol. 71(3), Copyright 2000 by the American Alliance for Health, Physical Education, Recreation and Dance, 1900 Association Drive, Reston, VA 20191.
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The shape of the curve shown in Figure 1 implies that fatigued, sedentary individuals who adopt any level of phys- ical activity would reap the greatest psychological benefit of increased feelings of energy. Such implications, although generally consistent with public health recommendations such as those made in the Surgeon General’s report on physical activity (48), are best considered in conjunction with data from experiments. There are a number of alterna- tive explanations for the data depicted in Figure 1, including that the relationship could be the result of chronically ill people being less able to be physically active. This possi- bility was considered in one report from the U.S. Nurses Health Study. In this analysis, the strength of relations was determined between feelings of energy (SF-36 vitality scores) and physical activity level (estimated MET-hours per week) as well as nine other variables potentially related to feelings of energy (i.e., body mass index, smoking status, alcohol consumption, menopause status, minority status, and the presence or absence of arthritis, diabetes, hypercho- lesterolemia, and hypertension). Stepwise regression analy- ses of data from 56,510 women participants (aged 45–71 yr) found that physical activity was the strongest predictor of SF-36 vitality scores (11).
We are aware of only one prospective, epidemiological investigation addressing relations between changes in phys- ical activity and changes in feelings of energy. Adult pa- tients (N 1758) enrolled in the longitudinal portion of the Medical Outcomes Study were tested twice over a 2-yr period (45). All participants had one or more of the follow- ing medical conditions: congestive heart failure, depression, diabetes, hypertension, or a recent myocardial infarction. After statistically controlling for potential confounders (i.e., age, body weight, education, gender, ethnicity, smoking, drinking, disease condition, comorbidity, poverty, method of payment, seasonality, and provider type), the total time spent exercising measured at the baseline time point was a statistically significant predictor of the frequency of energy feelings 2-yr later as measured by the vitality scale of the SF-36. Those who reported a high level of physical activity at baseline reported more frequent feelings of energy 2 yr later compared with those who reported a low level of baseline physical activity. This relationship was strongest for patients with hypertension, Type I diabetes, chronic heart failure, and depression (45). In addition to these epi- demiological observations, experimental evidence is useful in understanding relationships between physical activity and the moods of energy and fatigue. Experiments overcome some of the limitations inherent in large cross-sectional studies (e.g., crude methods used to estimate physical ac- tivity, inability to determine cause–effect relationships).
Physical inactivity. Complementary lines of experi- mental evidence suggest a link between physical inactivity and a worsening of feelings of energy and fatigue. Healthy young men undergoing 20 d of experimentally induced bed rest reported large increases (compared with the preinter-
vention assessment) of 2.1 standard deviations in the inten- sity of fatigue feelings and decreases of 0.67 standard de- viations in the intensity of energy feelings measured by the POMS fatigue and vigor scales (22). In a study using a different physical inactivity paradigm, 2 d of complete ex- ercise deprivation among healthy women and men who typically exercise 6 –7 d·wk 1 for 45 min·d 1 resulted in increases in the intensity of fatigue feelings of 0.43 standard deviations and large decreases of 1.67 standard deviations in the intensity of energy feelings as measured by the POMS and compared with preintervention baseline assess- ments (33).
Fatigued patients. Randomized controlled trials are widely regarded as the gold standard design for determining the efficacy and effectiveness of a medical or health inter- vention such as increased physical activity. Randomized controlled trials conducted with chronic fatigue syndrome and fibromyalgia patients are not considered here because these investigations typically have measured a wide array of fatigue-related symptoms rather than energy or fatigue moods per se (e.g., 51). The effect of 10 –20 wk of increased physical activity on the moods of energy and fatigue among sedentary patients with fatigue-related medical conditions has been examined in at least four randomized controlled trial experiments that now will be summarized.
One randomized controlled trial examined the influence of 10 wk of weightlifting exercise on energy feelings among 32 adults aged 60–84 yr who met the criteria for major or minor depression or dysthymia (42). The participants were randomly assigned to either a health education control con- dition (1 h per session twice weekly) or progressive resis- tance training of large muscle groups (thrice weekly, 45 min per session, at an intensity of 80% of one repetition maxi- mum). Feelings of energy were measured using the SF-36 vitality scale, and the results are illustrated in Figure 2. Before the intervention, the depressed participants in both groups reported less frequent energy feelings compared with age norms (i.e., the mean SD for the relevant vitality norms is 59.9 22.1, and these data stem from a sample of 442 U.S. men and women between the ages of 65 and 74 yr) (49). After the 10-wk intervention, those who exercised reported a large increase in the frequency of energy feelings of 1.3 standard deviations. This increase was significantly
FIGURE 2—The influence of 10 wk of resistance exercise on prior month frequency of energy-fatigue feelings among depressed older adults compared with controls receiving health education; adapted from Singh et al. (42). Copyright © The Gerontological Society of America. Reproduced by permission of the publisher.
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greater than the small increase of 0.14 standard deviations observed for the control group.
A second randomized controlled trial examined the in- fluence of 12 wk of aerobic exercise on energy feelings among 25 heart failure patients (40). Participants were ran- domly assigned to either a no treatment control condition or to cycle ergometry and stepping exercises for 3 h·wk1 at 50% of maximal aerobic power. After the 12-wk interven- tion, the mean SF-36 vitality score for the no treatment control group decreased by a small, statistically insignificant amount. In contrast, those who exercised reported large increases of 1 standard deviation in feelings of energy compared with the preintervention baseline. The magnitude of the increase in the frequency of energy feelings was larger than any of the other quality of life indicators mea- sured by the SF-36.
A third randomized controlled trial examined the influ- ence of daily exercise on the intensity of “current day” feelings of energy and fatigue among 62 cancer patients with solid tumors or lymphomas who were undergoing high-dose chemotherapy (16). Participants were randomly assigned either to a no treatment condition or to 15 min of daily cycle ergometry at a low intensity (mean SD power output of 30 5 W). The number of weeks of the exercise program varied for each patient, and pre- and posttests were conducted on the day of admission and the day of discharge from the hospital. Moods were assessed using the POMS questionnaire. As is common among cancer patients being treated with chemotherapy, the patients in the control group (N 32) reported significantly less intense feelings of energy and significantly more intense feelings of fatigue on the day of discharge compared with the day of admission. The general direction of mood changes was the same in the exercise group. However, these negative mood changes, and especially the energy feelings, were attenuated (i.e., the reduction in energy feelings with chemotherapy was not as large) among the patients who completed the low intensity daily exercise (N 27). The effect of exercise on energy feelings in this experiment was 0.5 standard deviations.
The fourth randomized controlled trial examined the in- fluence of 20 wk of aerobic exercise on energy and fatigue feelings among 228 primary care patients with persistent, unexplained physical symptoms (39). Participants were ran- domly assigned either to 1 h·wk1 of physiotherapist su- pervised aerobic exercise or to the same amount of time stretching. Participants also were asked to perform home- based exercise or stretching for 20 min 3 wk1. At base- line, low SF-36 vitality scores characterized both the aerobic exercise (27.2 22.4) and stretching (26.9 24.6) groups. At the end of the program statistically significant and similar increases in the frequency of energy feelings were reported by both the aerobic exercise (35.9 33.1, improvement of0.39 standard deviations) and stretching (39.7 32.0, improvement of 0.52 standard deviations) groups.
In summary, results from the four randomized controlled trials of the effects of exercise training on feelings of energy and fatigue among people with a fatigue-related medical condition were uniformly positive.
Healthy sedentary. At least 15 experiments have ex- amined the influence of chronic exercise on energy and fatigue moods in healthy sedentary people. A few of these investigations were randomized controlled trials, but most were quasiexperimental in design. The findings of these studies have been mixed. About half of the studies showed a moderate-to-large increase in feelings of energy in asso- ciation with exercise training (e.g., 2,4,6,15,19,24,29,34), whereas the others showed no meaningful change in feel- ings of energy or fatigue (e.g., 3,5,8,12,19,21,25,35,44). A representative example from among the better designed investigations (e.g., large sample, random assignment, con- trol condition) will be summarized. A total of 180 healthy adults (aged 40 – 69 yr) were recruited and randomly as- signed to one of five conditions: a Tai Chi–type physical activity program, moderate-intensity walking, low-intensity walking, low-intensity walking combined with listening to audio taped instructions to relax, and a nonexercise control condition. All the exercise programs were performed 3 d·wk1 for 45 min each session for 16 wk. Mood was measured shortly before and after the conditions. Insignifi- cant condition-by-time interactions were found for the POMS energy and fatigue scale scores (8); thus, feelings of energy and fatigue neither increased nor decreased with exercise training to an extent more than would be expected by chance.
One credible explanation for the failure to consistently find that increased chronic physical activity is associated with improvements in the moods of energy and fatigue among sedentary healthy adults is the presence of high energy and low fatigue at the outset of the investigation. A high level of energy at the outset makes it difficult for an exercise intervention to have a positive effect on feelings of energy.
Athletes. Experiments with endurance athletes have re- vealed that physical activity can both improve and worsen moods of energy and fatigue, and the effect depends on the volume and intensity of the training stimulus. Large in- creases and decreases in the volume and intensity of endur- ance training repeatedly have been found to be associated with changes in POMS vigor and fatigue scores in a dose– response fashion (37). For instance, one investigation mon- itored the mood of 186 female and male college swimmers who progressively increased their training volume from 4 to 12 km·d1 over the first 5 months of the training season (41). Monthly assessments of mood, illustrated in Figure 3, revealed that as the training load progressively increased to a peak in January the mean fatigue scores also progressively increased to a peak in January (the magnitude of the increase over the five months was large, 1.4 standard deviations). The vigor scores showed a progressive and large decrease during the period of increased training (1.1 standard de- viations at the January nadir). These negative mood shifts were reversed when the training stimulus was reduced. These negative mood shifts are not reversed among those athletes who become stale. The athletic staleness syndrome is characterized by impaired athletic performance, increased perceptual effort during standardized exercise bouts and
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FIGURE 3—The influence of increases and decreases in swim training on the intensity of feelings of energy and fatigue on 186 male and female college swimmers. A T-score of 50 represents the norm for college students, and a change of 10 T-score units is equal to a 1 standard deviation change; adapted from Raglin et al. (41); reprinted with permission from George Thieme Verlag KG.
severe and long-lasting mood disturbances, including chronic fatigue (37). The yearly incidence of the staleness syndrome among college swimmers ranges from 5 to 10%, and the lifetime prevalence of staleness among elite distance runners ranges from 60 to 64% (37).
SUMMARY AND RECOMMENDATIONS
Feelings of low energy and fatigue are common, affecting about one-quarter of large samples surveyed. These percent- ages are higher in patients suffering from major medical conditions. Feelings of high energy and low fatigue are beneficial for numerous reasons, including their association with good health, a high quality of life, and optimal cogni- tive and physical performance. An intervention that im- proved energy and fatigue moods, if it was safe and widely adopted, would have a large, positive effect on public health, medicine, and the associated health care costs.
The small body of key research findings reviewed here provides several lines of convergent evidence to support the hypothesis that there is a positive association between par- ticipation in regular physical activity and feelings of high energy/low fatigue. There is evidence that this relationship has negatively accelerating dose–response characteristics within the range of physical activity typically performed by nonathletes. Moreover, there is evidence that the relation- ship is stronger than other variables known to be related to energy and fatigue moods such as body mass index and chronic illness. This evidence implies that increased phys- ical activity would be a more effective method for improv- ing feelings of low energy and fatigue than other strategies that would involve manipulating variables which have been found to be less strongly related to these moods, such as losing weight or reducing either smoking or alcohol consumption.
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The available evidence from epidemiological studies and randomized controlled trial experiments with medical pa- tients suggests that sedentary people who feel fatigued are likely to experience a moderate-to-large increase in feelings of energy if they become regularly physically active. Be- cause there appears to be no randomized controlled trials that included in the design both a no treatment control group and an attention (placebo) control group, the available evi- dence is not strong enough to show that chronic physical activity causes improvements in energy and fatigue mood states among sedentary people who feel fatigued.
It is clear from the available evidence that not all people experience improvements in the moods of energy and fa- tigue in association with increased participation in regular physical activity. Sedentary people characterized by high energy and low fatigue have little room to improve these moods, so exercise adoption would likely have little effect on the energy and fatigue moods of these individuals. It is important to recognize that long-duration or high-intensity exercise, the type of exercise regimen that commonly is completed by endurance athletes training for a competition, can produce feelings of fatigue and low energy.
This review suggests a number of priorities for future research, including: 1) randomized controlled experiments that incorporate both a no treatment and an attention (pla- cebo) type control group in order to learn whether exercise training causes an improvement in energy and fatigue moods states; 2) large randomized controlled experiments conducted with fatigued, sedentary individuals as well as those with medical conditions, especially depression, dia- betes, heart failure, and hypertension; 3) experiments exam- ining the degree of overlap and/or independence of the effect of chronic exercise on feelings of energy and other important mood states such as anxiety and depression; 4) investigations aimed at examining the role, if any, of tran- sient (i.e., lasting 3–5 h postexercise) mood responses to acute bouts of exercise in the longer lasting improvements in feelings of energy that are associated with the adoption of regular physical activity for 10–20 wk; 5) experiments documenting dose–response relations between changes in feelings of energy and increased physical activity or exer- cise training; and 6) experiments aimed at understanding the biological correlates and mechanisms for changes in feel- ings of energy and fatigue that occur with regular exercise (43), and investigations comparing regular exercise with other interventions that might be employed to improve feelings of energy such as weight loss or either reduced smoking or alcohol consumption.
The authors acknowledge the support of the North American Branch of the International Life Sciences Institute.
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