Background
Description of the condition
In 2005 over 925 million people worldwide were 55 years of age or older according to the population database of the United Nations (WPP 2006). It is predicted that in 10 years this age group will grow to over 1.4 bilion. Subjective complaints about cognitive capacities increase with (older) age (Martin 2003; Newson 2006) and an objective decline in cognitive performance accelerates around the age of 50 (Salthouse 2003; Verhaeghen 1997), with the exception of cognitive skills with a large knowledge component. Research has shown that a regular exercise programme can slow down or prevent functional decline associated with ageing and improve health in this age group. The benefits for older people who regularly participate in endurance, balance and resistance training programmes are well established. Such health benefits include improved muscle mass, arterial compliance, energy metabolism, cardiovascular fitness, muscle strength, overall functional capacity (Lemura 2000), and the maintenance or even enhancement of cognitive function (Colcombe 2003).
Description of the intervention
Research using animal models has provided insight into the possible cellular and molecular mechanisms that underlie the effects of physical activity on cognitive function. Increased aerobic fitness is found to increase cerebral blood flow, oxygen extraction and glucose utilisation (Churchill 2002) as well as an activation of growth factors which mediate structural changes such as capillary density (Cotman 2002). A preliminary survey of the literature on human research points towards the same possible physiological mechanisms that could explain the association between physical activity and cognitive vitality (Aleman 2000; McAuley 2004; Prins 2002; Rogers 1990). The assumption is made that improvements in cardiovascular (aerobic) fitness mediate the benefits in cognitive capacity (McAuley 2004; Kramer 1999). This cardiovascular fitness hypothesis therefore implies that changes in cognitive function are preceded by changes in aerobic fitness.
How the intervention might work
Evidence for the link between physical activity, cardiovascular fitness and cognitive function in older individuals is provided by several longitudinal studies (Abbott 2004; Barnes 2003; Laurin 2001; Richards 2003; Sturman 2005; van Gelder 2004) and randomised controlled trials (Bakken 2001; Binder 1999; Emery 1998; Fabre 2002). Results from training studies performed by Hill et al. (Hill 1993) and Blumenthal et al. (Blumenthal 1991), however, failed to correlate changes in aerobic power (VO
Why it is important to do this review
Although these meta-analyses report a robust effect of physical activity on cognitive function, it remains unclear whether improvement in cardiovascular fitness (reflected by cardiovascular parameters such as VO
Objectives
To assess the effectiveness of physical activity, aimed at improving cardiorespiratory fitness, on cognitive function in older people without known cognitive impairment.
Methods
Criteria for considering studies for this review
Types of studies
All randomised clinical trials (published in journals) which met the given inclusion criteria were considered even if the participants were non-blinded. No language restrictions were applied.
Types of participants
Studies in older people and studies examining frail participants with age-related illnesses (such as osteoporosis, arthrosis, etc.) or specific disorders (like Chronic Obstructive Pulmonary Disease (COPD), heart failure, etc.) were included. Participants had to be aged 55 or older and not cognitively impaired in any way (e.g. suffering from dementia in any form, postcerebrovascular accident (CVA), or suffering from depression). Studies including participants just recovering from any surgical treatment were not included in this systematic review, nor those including participants with any co-morbidity that precludes successful participation in exercise programmes.
Types of interventions
Physical activity was considered to be any programme of exercise of any intensity, duration or frequency which was aimed at improving cardiorespiratory fitness. Acceptable comparison interventions were: no programme or treatment; a strength or balance programme; a programme of social activities or mental activities.
Types of outcome measures
The primary outcome measurement was cognitive function, either tested with a neuropsychological test battery (a combination of several tests sensitive to changes in cognitive function in adults) or tested with the Mini Mental Status Examination (MMSE) which gives a global measure of cognitive function. At the same time at least one of the following parameters reflecting cardiorespiratory fitness should have been reported in the article: maximal VO
Search methods for identification of studies
We searched MEDLINE, EMBASE, PEDro, SPORTDiscus, PsycINFO, CINAHL, Cochrane Controlled Trials Register (CENTRAL), Dissertation Abstracts International and ongoing trials registers on 15 December 2005 with no language restrictions. References in the included studies and in reviews of the literature were screened. Contact was made with experts and associations.
We used a combination of MeSH and text-based terms to find records of physical activity, including: exercise*, motor activit*, leisure activit*, physical fitness, physical endurance, exercise tolerance, exercise test, aerobic, aerobic capacity, physical activity, physical capacity, physical performance, training.
To reduce our findings to those records that measure cognition and cardiorespiratory fitness, we searched using the terms: cognit*, mental process*, maximal VO
To reduce our findings further to those records that deal with the older adult population ,we used relevant MeSH and text-based terms: adult*, middle aged, aged, elderly, old*, geriatric, frail.
Finally, we reduced the results to only RCTs and CCTs using the MEDLINE search strategies for optimal sensitivity in identifying randomised clinical trials (Higgins 2005).
Data collection and analysis
Selection of s tudies
Two reviewers (MA and GA) assessed the titles and available abstracts of all studies identified by the initial search and irrelevant studies were excluded. Two reviewers (MA and GA), using the inclusion criteria described above, independently assessed full paper copies of reports of potentially relevant studies. Disagreements on inclusion were resolved by discussion and through arbitration by a third reviewer (LV).
Quality assessment
Two reviewers (MA and GA) independently evaluated the methodological quality of the selected articles. We used the criteria list for quality assessment of nonpharmaceutical trials (CLEAR NPT) developed using consensus (Boutron 2005). This checklist includes information on sampling method, measurement, intervention and reporting of biases and limitations, as presented in Table 1. A small pilot exercise to clarify the method was performed with some articles that were already excluded from the review process. Cohen's kappa (K) was calculated as a measure of inter-observer agreement, and relied on Landis and Koch's benchmarks for assessing the relative strength of agreement (Landis 1977). Discordance in assessment was resolved after a single round of discussion and arbitration by a third reviewer (LV).
Data e xtraction
Data were extracted from the published reports by two reviewers (MA and GA) working independently and entered into RevMan by the first author, with full agreement of the second author. The summary statistics required for each trial and each outcome for continuous data were the mean change from baseline, the standard error of the mean change and the number of participants for each treatment group at each assessment. In some cases (Hassmén 1997 a; Hassmén 1997 b; Moul 1995; Kramer 2001) the standard error of mean had to be converted in a standard deviation, using SE = SD/sqrtN. For each outcome measure, data was sought on every participant randomised. To allow an intention-to-treat analysis, the data was sought irrespective of compliance, whether or not the participant was subsequently deemed ineligible, or otherwise excluded from treatment or follow-up. If intention-to-treat data were not available in the publications, "on-treatment" data, or the data of those who completed the trial, were sought and indicated as such. For cognitive data in which a higher score denotes worse performance (e.g. reaction times, digit vigilance, trail making part A, trail making part B, Stroop interference data and error rates), the mean change from baseline was entered as a negative variable.
Data a nalysis
Cognitive function was measured with various rating scales in the included articles. The neuropsychological tests found in the included RCTs were categorised (see Table 2) into a number of categories that approximately measures the same construct (Lezak 2004; Kessels 2000). At the same time, some cognitive tests were not admitted since double representation of studies in their cognitive category would have compromised the validity of the outcomes. The Weighted Mean Difference (WMD) was used if studies used the same cognitive tests and if the outcome measurements were on the same scale. In all other cases the Standardised Mean Difference (SMD) was calculated. If just one study reported results on a cognitive function, a fixed effects model was used. In all other cases (multiple studies) the random effects model was used.
The possible effects of aerobic exercise were analysed versus any intervention (strength programme, flexibility or balance programme, social or mental programme) and versus no intervention (controls or waiting list). Fabre et al. (Fabre 2002) subjected a group of participants to mental training and subsequently data from this group of participants was kept out of analyses of memory effects. In order to elucidate potential effects of the various intervention protocols on cognitive outcome measures, analyses were also performed separately (e.g. aerobic exercise versus strength programme).
As an extension to subgroup analyses, a meta-regression would allow the effect of cardiovascular fitness (VO
Results
Description of studies
See: Characteristics of included studies; Characteristics of excluded studies.
The search strategy rendered 31 promising abstracts. The full text of five theses could not be traced in peer reviewed journals (see list of studies awaiting assessment). Twenty-eight studies were excluded for various reasons after screening of the full text articles (see characteristics of excluded studies). The following 11 studies were all suitable for inclusion.
Bakken et al. (Bakken 2001) published a RCT in which 15 participants aged 72 to 91 years were randomly assigned to an aerobic exercise group (n = 8) or a wait list control group (n = 7) for eight weeks. The aerobic exercise group trained in one hour sessions for three sessions per week. Duration and intensity increased over the weeks but the subjects' heart rates did not exceed the upper limit of their target heart rate reserve. The main cognitive outcome parameter was a "tracking test" (visual attention). Five participants were available for analysis in each group. Rate-pressure product (RPP) was used to determine if an aerobic training effect had taken place. Both groups showed slight increases in RPP from pre-test to post-test.
Blumenthal et al. (Blumenthal 1989 a; Blumenthal 1989 b) performed a RCT in which 101 men and women aged 60 to 83 years were randomly assigned to either an aerobic exercise training (n = 33), a yoga/flexibility programme (n = 34) or a waiting list control group (n = 34). The aerobic exercise programme consisted of three supervised sessions for 16 weeks. The aerobic training was based on 70% of maximum heart rate achieved on the baseline exercise test. The yoga/flexibility group trained two times a week for 16 weeks and the control group was instructed not to change their physical activity habits and especially not to engage in any aerobic exercise for the study period. Two subjects were lost from the aerobic group and two were lost from the waiting list group. Blumenthal et al. included tests for cognitive speed, verbal, visual and working memory as well as executive functions, cognitive inhibition, visual and auditory attention and motor function in this study. A summary combination of the scores on both 2&7 test (letters and digits) was calculated and standard deviations were pooled and used in analysis. Subjects in the aerobic training group experienced a significant 11.6% increase in their VO
In the RCT of Emery and Gatz (Emery 1990 a) a total of 48 men and women aged 61 to 86 years were randomly assigned to an exercise programme (n = 15), a social activity group (n = 15) or a control group (n = 18). The exercise programme consisted of stretching exercises, aerobic exercise (at 70% of age-adjusted maximum heart rate = 220-age) as well as rhythmic muscle strengthening exercises (e.g. repeatedly standing up and sitting down), three times a week for 12 weeks. Subjects in the social programme participated in non-physical activities (card games, art projects, political discussion groups, watching films) three times a week for 12 weeks. Because attrition from the social group was comparable to that of the control group, and because attendance for the social group was poor overall (ranged from 10 to 94%), the analyses were conducted with the two groups pooled. One subject was lost from analyses in the exercise group, four from the social group and four from the control group. This study included tests for cognitive speed and auditory attention in its design. Resting heart rate, maximum heart rate and systolic/diastolic blood pressure indicated no significant differences between the groups. Both groups showed a significant time main effect decrease in diastolic blood pressure, other measures indicated no significant effects.
Emery et al. (Emery 1998) published a second RCT of 79 COPD patients aged 66.6 ± 6.5 years. Twenty nine participants were randomly assigned to an exercise/education/stress management group (EXESM), 25 subjects to an education/stress management group (ESM) and 25 participants were assigned to a control group. EXESM participants trained during a 10-week period. The training included aerobic exercises as well as strength training on Nautilus equipment, educational lectures on topics relevant for COPD patients and a weekly group meeting for stress management and psychological support. In the first five weeks, EXESM participants followed a programme of daily sessions with 45 min of aerobic exercise, lectures on relevant COPD topics and a stress management meeting. Following the initial five weeks, EXESM participants then participated in five more weeks of exercise for three times a week, 60 to 90 min and one hour a week of stress management. ESM consisted of 16 educational sessions and 10 stress management classes in 10 weeks. Controls were instructed not to alter their physical activities during the study. Four subjects were lost from the EXESM group and two subjects were lost from the ESM group. Tests on cognitive speed, visual attention, executive and motor functions were incorporated in this study. Participants in the EXESM condition achieved a significant 16% gain in VO
The RCT of Fabre et al. (Fabre 2002) presented data from 32 men and women aged 60 to 76 years. Participants were randomly assigned to an aerobic exercise programme (n = 8), a mental training programme (n = 8), a combined aerobic/mental programme (n = 8) or a control group (n = 8); no one was lost to follow-up. Physical training consisted of two supervised exercise sessions per week for two months: walking and running to maintain target heart rate (target heart rate corresponded to the ventilatory threshold). Subjects in the mental training programme met once a week for two months and Israel's method (Israel 1987) was used. The controls met as many times as the other groups for leisure activities such as singing. Fabre and colleagues included tests for verbal and visual memory, perception and executive functions in their study. The physical training resulted in an average significant increase in VO
Hassmén and Koivula (Hassmén 1997 a; Hassmén 1997 b) presented an RCT in which 40 men and women aged 55 to 75 years were randomly assigned to an exercise group (n = 20) or a control group (n = 20), no one was lost to follow-up. Participants in the exercise group walked three times a week during three months with intensities according to "Rating of Percieved Exertion" (RPE, Borg scale) of 9 (very light), 11 (fairly light) and 13 (somewhat hard). The performed exercise gave rise to heart rate responses of 95 to 125 beats/min. Subjects in the control group were given home assignments to be performed three times per week. Cognitive speed, verbal memory, perception and auditory attention were tested in the participants. Heart rate data and data on RPE showed no significant differences between the groups over time. The results for men and women were described separately in this article. In this review the data presented in Hassmén 1997 a reflects the data of the women, Hassmén 1997 b those of the men.
A total of 174 participants, aged 60 to 75 years, were included in the RCT of Kramer et al. (Kramer 2001). After randomisation and testing, 124 individuals remained in analyses; 58 (13 men) in the aerobic walking group and 66 (20 men) in the stretching and toning programme. Participants in the aerobic programme walked briskly in three supervised sessions per week for six months. After warming up they spent 40 minutes on brisk walking (gradually beginning at 10 to 15 minutes up to 40 minutes) and finished with a cooling down. Initial exercise was performed at 50 to 55% of VO
Madden et al (Madden 1989) published an RCT of 85 men and women 60 to 83 years of age. All participants were randomly assigned to either an aerobic exercise group (n = 28), a yoga group (n = 30) or a control group (n = 27). The subjects in the aerobic exercise group trained three times per week for 16 weeks and all exercise was performed in target (training) heart range (70% of maximum during initial exercise test). Subjects in the yoga group participated in supervised sessions twice a week during 16 weeks. The controls were instructed not to change their physical activity habits for the length of the study. A total of six subjects were lost from analyses; three from the aerobic exercise group, two from the yoga group and one from the control condition. All mean error rates for the word comparison task (executive functions) and the letter search task (visual attention) were recalculated into summary scores. Standard deviations of these two tasks were pooled. Aerobic capacity remained constant for the yoga and control groups between pre- and post-test (respectively from 18.8 to 18.6 ml/kg/min and from 19.1 to 18.6 ml/kg/min), whereas the aerobic exercise group showed a significant 11% increase in VO
Moul et al. (Moul 1995) presented data of an RCT in which 30 men and women aged 65 to 72 years of age were randomly assigned to a walking condition (n = 10), weight training (n = 10) or control condition (n = 10) for 16 weeks. No one was lost to follow-up. Participants in the walking condition walked five times per week for 30 minutes at 60% of "Heart Rate Reserve" (HRR, as determined by treadmill testing). Walking duration was increased two minutes per week until they reached 40 minutes and HRR values were adjusted after eight weeks of training to 65% of HRR. Weight training consisted of five sessions per week of upper and lower body exercises on alternate days of the week. Abdominal crunches and back extension were performed each session. The weight training group employed a "Daily Adjusted Progressive Resistive Exercise Program" (DAPRE) using weights. Subjects in the control condition met five times per week for mild stretching exercises which challenged the cardiovascular or muscular systems minimally. All participants were kept in analyses. The Ross Information Processing Assessment was used to evaluate changes in cognitive function. Post-test data revealed that the subjects in the walking condition significantly increased their VO
The RCT of Panton et al. (Panton 1990) included data on 49 retired university professionals (23 males and 26 females), aged 70 to 79 years. The participants were randomly assigned to a walk/jog group (n =17), a strength group (n = 20) or a control condition (n = 12) for 26 weeks. A total of eight participants were lost to follow-up; it is unclear from which group these subjects were lost. Subjects in the walk/jog condition participated in three exercise sessions per week for the duration of the study. Initially, participants started walking/jogging for 20 minutes at 50% of their maximal heart rate reserve (HRRmax). The duration was increased by 5 min every two weeks until the subjects walked for 40 minutes. Training intensity was gradually increased until subjects could walk at 60 to 70% of their HRRmax. During the 14th week of training, exercise intensity was further increased by alternating fast walk/moderate walk or fast walk/slow jog intervals. Five participants increased their training intensity by increasing the slope of the treadmill. By the 26th week of training, all subjects performed at 85% of HRRmax for 35 to 45 min. Participants in the strength group had workout sessions for three times a week for 30 min per session. The workouts consisted of one set of 10 variable resistance Nautilus exercises (leg, arm and torso muscles). During the first 13 weeks, subjects used light to moderate weights and performed 8 to 12 repetitions for each exercise. During the last 13 weeks, resistance was increased substantially and subjects were encouraged to train to volitional muscular fatigue. When subjects could complete 12 or more repetitions, the resistance was increased. The controls were asked not to change their lifestyle over the six month duration of the study. Tests for cognitive speed were performed to analyse cognitive function. Aerobic capacity significantly improved by 20.4% (from 22.5 to 27.1 ml/kg/min) for subjects in the walk/jog group; participants in both the strength as well as the control group showed no significant changes in VO
Whitehurst (Whitehurst 1991) published an RCT on 14 females aged 61 to 73 years. The subjects were randomly assigned to an exercise programme (n = 7) or a control condition (n = 7) for eight weeks. No one was lost to follow-up. The exercise condition consisted of three supervised sessions per week. The subjects cycled for 8 to 10 minutes the first week to provide acclimation. Thereafter, 3 to 5 minutes was added to subsequent sessions so that by week 4 all subjects were cycling for 35 to 40 minutes at their target heart rate (70 to 80% of predicted maximum by submaximal bicycle test). The controls were instructed not to engage in any form of vigorous physical activity during the course of the study. Choice reaction times were tested for evaluation of cognitive function. The subjects in the exercise group significantly increased their VO
Risk of bias in included studies
Results of the quality assessment can be found in the characteristics of included studies table and the additional table ( Table 3).
The overall methodological quality score of the included studies ranged from 23 to 39 (minimum possible score of 14 points, maximum possible score of 48 points; lower scores denote a better methodological quality). In most of the trials the blinding procedures of the outcome assessors, treatment providers, and participants frequently scored "no, because blinding is not feasible".
Cohen's kappa (K) was calculated as a measure of inter-observer reliability after the initial screening by two reviewers (MA and GA) and reached 0.81, almost perfect according to Landis 1977.
Effects of interventions
Analyses on the effects of aerobic exercise on cognitive function compared to any other intervention rendered significant positive effects of aerobic exercise on cognitive speed (SMD random effects 0.26, 95% CI 0.04 to 0.48, P = 0.02) and visual attention (SMD random effects 0.26, 95% CI 0.02 to 0.49, P = 0.03). Analyses on the effects of aerobic exercise on cognitive function compared to no intervention (controls or waiting list groups) yielded significant positive effects of aerobic exercise on auditory attention (WMD random effects 0.52, 95% CI 0.13 to 0.91, P < 0.01) and motor function (WMD random effects 1.17, 95% CI 0.19 to 2.15, P = 0.02).
However, in the comparisons of exercise versus any intervention, nine of the 11 cognitive functions yielded no significant effects. Nine of the 11 cognitive functions showed no significant effects for exercise versus no intervention.
Analyses on the effects of aerobic exercise on cognition compared to a strength programme rendered no significant differences. However, these results should be interpreted with care since they were based on one study (Moul 1995). Analyses on the comparisons of aerobic exercise versus a social/mental programme included three studies (Emery 1990 a; Emery 1998; Fabre 2002). These comparisons were are not shown separately (data of the three were already pooled in the analysis of effects of aerobic exercise versus any intervention).
Discussion
This review examined the effect of physical activity aimed at improving cardiorespiratory fitness on cognitive function in healthy older people without known cognitive impairment. The assumption being made is that improvements in cardiovascular (aerobic) fitness mediate benefits in cognitive capacity (Colcombe 2004; Kramer 1999; McAuley 2004). This would imply that a physically active lifestyle resulting in enhanced fitness could affect people's cognitive abilities in the future and would enable them to partially influence their mental health.
Eight out of 11 studies in this review reported that aerobic exercise interventions resulted in increased cardiorespiratory fitness of the intervention group (an increase in VO
However, there are issues that need further consideration.
Although aerobic exercise rendered significant effects on subcategories of cognition, the majority of comparisons yielded no significant results.
At the same time, choices regarding inclusion criteria and classification of cognitive tests into specific cognitive functions seem to have a profound effect on the results which renders the conclusions of this review (and other reviews on cognition) less transparent and limits the practical implementations of the results. Four meta-analytic studies published data based on very similar hypotheses yet failed to find comparable results:
- Etnier et al. (Etnier 1997 b) included 134 articles in their review. Four studies (Blumenthal 1989 a; Blumenthal 1989 b; Emery 1990 a; Madden 1989; Panton 1990) included in the present review were also included in Etnier's review, whereas, based on publication dates, six were possible (Moul and Whitehurst were not included in Etnier's review). The aim of the review conducted by Etnier and colleagues was to give a comprehensive overview of all literature available with sufficient information to calculate effect sizes. Therefore the review included, besides randomised controlled trials, several cross-sectional studies. This resulted in data on the acute effects of exercise and data of strength and flexibility regimens as well as results for younger age groups and cognitively impaired individuals. Cognition was categorised in terms of reasoning skills, verbal ability, memory, mathematical ability, creativity, motor skills, perception, IQ, dual task paradigms, reaction time and academic achievement. The authors concluded that exercise has a small positive effect on cognition and effect size depended on the exercise paradigm, the quality of the study, the participants and the cognitive tests used as outcome parameters.
- Heyn et al. (Heyn 2004) described the effects of exercise training on cognitive function in people with dementia and related cognitive impairments. "Cognitively impaired in any way" was an exclusion criterion in our review and therefore the results cannot be compared with those of Heyn et al. However, Heyn et al. concluded that exercise training increases fitness, physical function, positive behaviour and cognitive function (measured with MMSE) in people with dementia and related cognitive impairments.
- The meta-analysis presented by Colcombe and Kramer (Colcombe 2003) was based on 18 articles. Colcombe and Kramer searched Educational Research in Completion (ERIC), MEDLINE, PsycLIT and PsycINFO. Four studies (Emery 1990 a; Emery 1998; Madden 1989; Moul 1995) included in the present review were also included in Colcombe's review where, based on publication dates, it would have been possible to include the same articles in both reviews. Considering the aim of the meta-analysis ("to examine the hypothesis that aerobic fitness training enhances the cognitive vitality of healthy but sedentary older adults") and the exclusion criteria (cross-sectional design, no random assignment, unsupervised exercise programme, training lacking in fitness component and an average age below 55) this analysis could have resulted in similar results. Yet, we excluded five studies since they did not present a randomised controlled design but rather a quasi-randomised study (matched controls or assignment to intervention groups). Four studies were excluded from our review due to the fact that they did not present any fitness parameter and two studies provided no quantitative data. Furthermore, one study was excluded as the participants were cognitively impaired, one article provided the same data as Blumenthal et al. (Blumenthal 1989 a; Blumenthal 1989 b) which is included in our review and one study was excluded since participants suffered from depression (an exclusion criterion for our review). Colcombe and Kramer recoded all cognitive tasks into categories of speed (low-level neurological functioning), visuospatial (transforming or remembering visual and spatial information), controlled processes (tasks that require some cognitive control) or executive processes (related to planning, inhibition and scheduling of mental procedures) but do not give insight in the exact coding of the several cognitive tasks over the described categories. Colcombe and Kramer's review concluded that executive functions were significantly and positively related to increased physical activity. Moreover, Colcombe and Kramer stated that physical activity is beneficial for all other analysed cognitive functions.
- Just months after our search for articles, Etnier et al. (Etnier 2006) published their meta-analytic review on the relationship between aerobic fitness and cognitive performance. Their primary goal was "to provide a statistically powerful test of the viability of the cardiovascular fitness hypothesis by examining the dose-response relationship between aerobic fitness and cognition". Searching PubMeD, PsychINFO, Dissertation Abstracts International, Educational Research in Completion and Sports Discus rendered 30 studies which reported data on pre-post comparisons. Except for Bakken et al. (Bakken 2001), Blumenthal et al. (Blumenthal 1989 a; Blumenthal 1989 b), Hassmén and Koivula (Hassmén 1997 a; Hassmén 1997 b), Emery and Gatz (Emery 1990 a) and Panton et al. (Panton 1990), all of the remaining studies included in the present review were incorporated in the meta-analytic study by Etnier et al. as well. Etnier and colleagues included only those studies which assessed aerobic fitness by maximal, submaximal or a composite measure of fitness which included VO
2 max (lacking in Bakken et al. (Bakken 2001), Emery and Gatz (Emery 1990 a) and Hassmén and Koivula (Hassmén 1997 a; Hassmén 1997 b)), whereas we included all measures of aerobic fitness. At the same time, we imposed an age criterion and did not include studies on depressed participants whereas Etnier et al. included all ages and at least one study on depressed subjects. Finally, Etnier et al. included unpublished master theses and doctoral dissertations whereas we only included data published in peer reviewed journals. Etnier et al. recoded all cognitive measures into categories based upon Caroll's structure of cognitive abilities as derived from factor analysis (Caroll 1993) or test descriptions provided by Lezak (Lezak 2004). The following categories were discussed: fluid intelligence, crystallised intelligence, general memory and learning, visual perception, retrieval ability, speediness and processing speed. Post-test comparisons showed no significant relationships between aerobic fitness and cognitive performance. For the exercise groups the change in fitness from pre- to post-test was a significant linear predictor of the change in cognitive performance from pre- to post-test albeit in a negative direction. Age acted as a significant negative moderator variable in the relationship between aerobic fitness and cognition for older adults according to Etnier et al.
It is possible that engagement in aerobic physical activities accounts for improvements in short term cognitive capacities driven by physiological or psychological mechanisms other than aerobic fitness (Etnier 2006). This could explain why the positive effects of physical activity on cognition in longitudinal (Barnes 2003; Laurin 2001; Richards 2003; Sturman 2005; van Gelder 2004) and prospective studies (Abbott 2004; Yaffe 2001; Weuve 2004) are seldom demonstrated for all cognitive functions in randomised controlled trials. This might be another explanation for the incompatible results in the discussed meta-analyses. The relationship between physical activity and cognitive function could be modified by a number of (individual) factors other than cardiovascular function, which suggests that possible subgroups in the population could react differently on aerobic training and may show different effects on their cognitive performance across the age span. The search for possible subgroups has provided some promising results (examples in Podewils 2005; Etnier 2007; Schuit 2001) but more subgroups might have to be identified.
At the same time, there are indications that the intensity rather than duration of physical activities would be beneficial for cognition (Angevaren 2007; van Gelder 2004) which may have implications for the effectiveness of some of the training programmes in the included randomised controlled trials.
Processing current literature on physical activity and cognition led to a number of items which intrigued us that much that we felt is was necessary to present them as suggestions for future research.
Firstly, our review consists of results from as many as 33 different cognitive tests, already a smaller sample of tests than the absolute total reported in the included studies (tests were lost from analyses in order to avoid double representation of studies over the cognitive categories as well as the calculation of some summary combinations) but still a great number. A broad battery of tests would be in keeping with the reality in clinical settings and would give insight in the potential specificity of physical activity effects. At the same time, too great a number of cognitive tests used in a scientific setting might obstruct a general view on the intended effects. We recommend that all researchers in the field seek mutual agreement on a smaller battery of cognitive tests to use in order to render research on cognition transparent and heighten reproducibility of results. This smaller core-set of cognitive tests should incorporate cognitive measures of importance for scientific and clinical settings.
Secondly, if aerobic fitness is to act as an effective tool against age-related cognitive decline, the rate of change in cognitive performance over a significant period of time rather than the absolute short-term gain in performance immediately after the intervention would be relevant (as in Salthouse 2007). In this light, a limitation of training studies as used in the included randomised clinical trials is the lack of long-term monitoring (with an average duration of 14 weeks). The hypothesis that cardiovascular fitness would be beneficial as a tool which counteracts age-related cognitive decline would be supported if the differences between the trained subjects and the controls would increase as a function of age (Salthouse 2007). This proposition implies that future research should be directed at long-term intervention trials.
Authors' conclusions
Implications for practice There is evidence that aerobic physical activities which improve cardiorespiratory fitness are beneficial for cognitive function in healthy older adults without known cognitive impairment, with effects observed for motor function, cognitive speed, auditory and visual attention. This benefit adds to the other known benefits of aerobic exercise. |
Implications for research The temporal association between improvements in cardiovascular fitness and in cognitive functions is suggestive of a causal link. Larger studies are still required to confirm whether the aerobic training component is necessary, or whether the same can be achieved with any type of physical exercise. If such efficacy studies were to show that cardiovascular fitness achieved by aerobic exercise is necessary for cognitive improvement in the unimpaired elderly, then studies of effectiveness are needed. Specifically, longer studies are required to see whether the effect on cognitive function of brief programmes of aerobic exercise is sustained, or whether continued aerobic exercise is necessary. The clinical relevance and reproducibility of such data would be enhanced if clinicians and scientists in the field could agree upon the use of a smaller battery of cognitive tests. |
Acknowledgements
With thanks to Judith Hoppesteyn-Armstrong, Consumer Reviewer, for her help.
Data and analyses
- Top of page
- Background
- Objectives
- Methods
- Results
- Discussion
- Authors' conclusions
- Acknowledgements
- Data and analyses
- What's new
- History
- Contributions of authors
- Declarations of interest
- Index terms
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What's new
Last assessed as up-to-date: 31 January 2008.
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History
Protocol first published: Issue 3, 2005
Review first published: Issue 2, 2008
Contributions of authors
MA: drafting reviews, searching for trials, obtaining copies of trial reports, selection of included trials and exclusion of trials, extraction and entry of data, interpretation of data analysis.
GA: searching for trials, obtaining copies of trial reports, selection of included trials and exclusion of trials, extraction of data, interpretation of data analysis.
HV: interpretation of data analysis.
AA: interpretation of data analysis.
LV: selection of in-and exclusion of trials, interpretation of data analysis.
Contact Editor: Linda Clare
Consumer Editor: Judith Hoppesteyn-Armstrong
With special thanks to Rob de Bie, editor of Musculoskeletal Group, for his assistance.
Declarations of interest
None known.
Index terms
Medical Subject Headings (MeSH)
Cognition [*physiology]; Cognition Disorders [prevention & control]; Exercise [*physiology]; Memory [physiology]; Oxygen Consumption [physiology]; Physical Fitness [*physiology]; Randomized Controlled Trials as Topic
MeSH check words
Aged; Humans; Middle Aged
* Indicates the major publication for the study