GUEST EDITORIAL Sports Med. 19 (2): 61·85. 1995 0112·1642/95/0002-008 1/$02.50/0

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Brain Function and Exercise Current Perspectives

Jennifer L. Etnier and Daniel M. Landers Department of Exercise Science and Physical Education, Arizona State University, Tempe, Arizona, USA

The dualistic view of the mind and the body being distinct entities has long been abandoned in the research community. The adoption of a holistic approach has resulted in a vast quantity of research designed to define the precise relationships be­tween the brain and the body. An aspect of research with a holistic approach is concerned with the in­fluence of exercise of the body on the brain. Both acute exercise and chronic exercise have been ex­amined, and researchers have examined both the direct effects and the indirect (behavioural) effects of exercise upon the brain. In examining the direct influence of exercise upon the brain, research has primarily been devoted to the influence of exercise upon cerebral blood flow (CrBF), neurotransmitter availability and neural efficiency. The indirect in­fluence of exercise upon the brain has primarily been examined in terms of its influence upon men­tal health and cognitive ability.

1. Direct Relationships

Research that has examined changes in cerebral blood flow in humans during an acute bout of ex­ercise has produced mixed results. Authors who have used the nitrogen oxide technique, and there­fore have measured average blood flow through the entire brain,[I} have typically reported that cerebral blood flow remains stable during exercisep-4} How­ever, authors who used the Xenon clearance tech­nique, which allows for the determination of re­gional cortical blood flow,[I} have typically found that participants who exercise at moderate to high intensities show large elevations in CrBF.[1,5.7} It is

likely that the discrepancy in the findings is a result of the technique which was used to measure CrBF and of the intensity of the exercise which was ex­amined. Future study is needed to elucidate this relationship. This is of particular importance be­cause increases in CrBF with exercise have been suggested as an explanation for the relationship be­tween exercise and cognitive functioning.

Researchers have also examined the influence of exercise upon brain neurotransmitters. Several authors have shown that neurotransmitter levels change after an acute bout of exercise. Gordon et a1.[8} and Mitchell et al.[9} found increases in nor­adrenaline (norepinephrine) levels after an acute bout of exercise. Ebert et aJ.!lO} found increases in the precursors of noradrenaline as a function of ex­ercising for 8 hours. Additionally, research with marathon runners has shown that endorphin levels increase after long periods of exercise.[II} Barchas and Freedman[12} found increases in brain seroto­nin levels following running for 3 hours, and Ja­cobs[13} has recently reported that firing rates of serotonin neurons increased when cats were in an active waking state. These findings with acute ex­ercise are important because of the relationship be­tween an acute bout of exercise and the 'feel good' phenomenon which has been described in the pop­ular literature.

Studies have also shown that plasma levels of noradrenaline increase as a result of aerobic train­ing.[14} Poehlman and Danforth[15} and Poehlman et al.[16} found that, in older adults (average age 64 and 69 years, respectively) who had trained for 8

82

weeks, plasma noradrenaline levels increased by as much as 29%. In animals, studies have shown that training results in higher noradrenaline and seroto­nin levels in the brain.[17·19] These findings with chronic exercise are especially important because the neurotransmitters mentioned (noradrenalin, ad­renalin and serotinin) are thought to be associated with memory storage and retrieval[2o·22] and also with mood state.[13]

Results from the animal studies suggest that chronic exercise may also result in permanent structural changes in the brainJ23-24] In these stud­ies, rats were placed in 1 of 4 conditions: an acro­batic condition, a forced exercise condition, a vol­untary exercise condition and a nonexercising control condition. Results from the study by Black et a1P3] showed that rats who were exposed to ei­ther of the exercise conditions had an increase in vasculature density in the cerebellar cortex as com­pared with control rats and with rats who had per­formed acrobatic exercise. Results from the study by Isaacs et aI.l24] showed that rats in the exercise group had shorter vascular diffusion distances than did rats in either the acrobatic exercise group or the control group. The results of both studies showed that the acrobatic rats had an increase in the number of synapses per Purkinje cell. The au­thors concluded that physical exercise improves vascularisation in the cerebellar cortex while a combination of motor learning with physical activ­ity results in a greater communication network in the brain.f24]

Electroencephalographic (EEG) techniques have also been used to examine the influence of exercise upon neural firing patternsJ14] Dustman et aI.l25] measured EEG in 4 groups of participants who were categorised based upon age and maximal oxygen uptake CV02max). Their results showed that the resting EEGs of the young men and of the older, fit men (average ages 25.2 and 53.8 years, respec­tively) had significantly more slow a-activity (8 to 10Hz) than did the resting EEGs of the older, unfit men (average age 55.9 years). Additionally, corti­cal coupling, which is thought to represent 'func­tional autonomy' of areas within the brain, was

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Etnier & Landers

found to be greater in older, unfit men than in any other group. This suggests that fitness in elderly popUlations may result in a maintenance of func­tional autonomy. Campbell et aI.l26] measured EEG prior to and after a lO-week exercise programme. Their results showed that the exercise programme resulted in increased central nervous system (CNS) inhibitory abilities in 21 of the 22 participants who had initially had poor CNS inhibitory abilities.

Dustman et aI.l27] examined brain activity of older people who were classified as high-fit or low­fit. Results of the study showed that the P300 event-related potential latency (a measure of infor­mation processing) was longer for the older low-fit subjects than it was for the older high-fit subjects. Bashore[28] measured reaction time and P300 la­tency in older men who were categorised as exer­cisers or as nonexercisers. Results showed that the exercisers had faster reaction times and shorter P300 latencies than did the nonexercisers. Thus, research suggests that fitness enhances neural effi­ciency and maintains functional autonomy and that this is especially true with older adults.

The direct research, which has examined the re­lationship between exercise and the brain, has shown that exercise can have a significant effect on CrBF, neurotransmitter availability, brain structure and on neural efficiency. Researchers have also de­voted a great deal of effort to determining the indi­rect relationship between exercise and the brain; that is, to looking at the influence of exercise upon behaviour. In particular, research has focused on the influence of exercise upon mental health and cognitive functioning.

2. Indirect Relationships

The relationship between mental health and physical activity has been studied extensively. This relationship is important in terms of brain function because mental health has been shown to be linked to cognitive function. For example, several re­searchers[29-31] have shown that neuropsychologi­cal dysfunction is evident in depressed patients. Therefore, if exercise is linked to mental health, then this may have an indirect impact upon brain

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Brain Function and Exercise

function. The relationship between exercise or fit­ness level and depression has been studied in over 1100 studiesp2] Morgan[32] conducted a recent narrative review of the literature which has exam­ined the relationship between physical activity, fit­ness and depression. Morgan[32] reported that a large portion of the studies in this area have been plagued by methodological problems such as fail­ure to randomly assign to treatments, inappropriate control groups and improper diagnostic tech­niques. Despite these shortcomings, the author con­cluded that 'exercise has been associated with a decreased level of mild to moderate depression' .

North et aLl33] reviewed the literature on exer­cise and depression using the technique of meta­analysis. This allowed them to look at the effects found in all studies (whether large or small) and to look at the influence of moderating variables upon the effects. The results showed that, whereas larger effects were found with clinically depressed pa­tients, significant decreases in depression were also found with people who were not clinically de­pressed. In fact, the overall effect was such that exercise decreased depression by approximately one-half of 1 standard deviation (ES = - 0.53). These effects were found to be immediate and long-last­ing and occurred with all modes of exercise. Addi­tionally, whereas all lengths of exercise pro­grammes had an impact on depression, the largest impact on depression was found for exercise pro­grammes which were 17 weeks or longer. Several physiological hypotheses have been proposed to explain these results. The amine hypothesis and the endorphin hypothesis both suggest that exercise in­creases levels of certain neurotransmitters in the brain (e.g. serotonin) which are associated with de­creases in depression. As previously stated, this hy­pothesis has received some support from studies which have directly measured neurotransmitter re­sponse to exercise.[13]

Many studies have also been conducted to ex­amine the relationship between exercise and cog­nitive functioning. Many authors have reviewed the literature [34-42] and a variety of conclusions have been made. Some authors have concluded that the

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results are mixed,[39,42] others have concluded that

the results support the contention that exercise en­hances cognitive functioning,[35,36,38] others that exercise is detrimental to performance[40] and still others that the influence of exercise upon cognitive functioning is dependent upon the intensity and duration of the exercise and the type of cognitive functioning.[34,37,41]

In an early review of studies using acute exer­cise, PoweW37] concluded that submaximal exer­cise is beneficial to cognitive functioning while maximal exhaustive exercise may be detrimental to cognitive performance, Tomporowski and Ellis[41] also conducted a review of the literature on acute exercise and cognitive functioning. They classified 27 published studies by the duration and intensity of the exercise protocol and then made general con­clusions based upon the general study findings in each category. The authors concluded that while the results were mixed, it appeared that, in general, exercise did not lead to improved cognitive func­tioning. In a recent review by Dustman et al.,[14] the authors concluded that large differences in cognitive functioning could be found using cross­sectional comparisons of groups with different fit­ness levels. The authors suggested that changes in neurophysiological functioning may be reliant upon long-term training programmes (longer than had been studied in past research) which result in large fitness differences (as can be seen in cross­sectional comparisons).

Synthesising the results from many different studies which use different designs is difficult to do in a narrative review. Thomas et al.l43] have cited preliminary results from 91 studies included in a meta-analysis, and concluded that the influ­ence of exercise upon cognitive functioning is de­pendent upon the exercise paradigm which is used. Studies using acute exercise showed very small ef­fects (n = 44, ES = 0.16, P < 0.05), studies using chronic exercise programmes showed larger, and hence more meaningful, increases in cognitive ability (n = 33, ES = 0.32, P < 0.05), and studies which used cross-sectional designs showed the largest effects (n = 14, ES = 0.75, P < 0.05). These

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statistical findings are in accord with those re­ported by Dustman et al.l 14]

3. Conclusions

Thus, the research suggests that exercise has a direct impact upon the brain and that this direct effect may then indirectly mediate influences of exercise upon brain functioning. In particular, evi­dence shows that exercise has an impact on CrBF, neurotransmitter availability, brain structure and neural efficiency, and that these changes are related to better mental health and to improved cognitive functioning.

To date, although a large amount of research has been done in this area, this research has lacked fo­cus and has used such a wide variety of techniques that conclusions are difficult to reach. For this rea­son, future research needs to adopt a more concen­trated focus, so that the direct and indirect effects of exercise on the brain can be more solidly linked. From a theoretical perspective, studies need to be directed at determining the precise mechanisms un­derlying the improvements in cognition, and, from an applied perspective, future studies should con­centrate on the exact parameters which define the exercise, the types of cognition that can be improved and the popUlations which might benefit the most from the exercise.

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8. Gordon R, Spector S, Sjoerdsma A, et at. Increased synthesis of norepinephrine and epinephrine in the intact rat during exer­cise and exposure to cold. 1 Pharmacol Exp Ther 1966; 30: 440-7

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Correspondence and reprints: Dr Daniel M. Landers, De­partment of Exercise Science and Physical Education, Box 870404, Tempe, AZ 852871-0404, USA.

Sports Med. 19 (2) 1995