The effects of exercise interventions on quality of life in clinical and healthy populations; a meta-analysis
Introduction
There is considerable and growing evidence that physical activity and/or exercise behaviour plays a role in a person's perception of their quality of life (QoL) (Rejeski et al., 1996, Wolin et al., 2007). In addition to links reported in naturalistic settings, studies measuring QoL before and after an exercise intervention can provide clear evidence of a causal pathway that is necessary before promoting exercise as a means of improving QoL.1 Systematic reviews and meta-analyses have quantified the outcomes of exercise interventions for specific population subgroups such as cancer patients (Oldervoll, Kaasa, Hjermstad, Lund, & Loge, 2004), frail older adults (Schechtman & Ory, 2001), and mental health service users (Lawlor & Hopker, 2001). However, no review was found that compared the outcomes of exercise across disease groups. Furthermore, as QoL is often applied as a secondary rather than primary outcome measure in exercise trials, many such reviews only report on a small group of studies (e.g., Jolly et al., 2006, Oldervoll et al., 2004). Additionally, the mechanisms by which the physical activity/exercise-QoL relationship functions are still the subject of investigation.
Using the search terms listed in the Methods section of the present paper, we retrieved eight meta-analyses reporting on the impact of exercise on QoL published between 2001 and 2007. Five of these only conducted within-group analyses without comparison with a control group; three of these reported no beneficial effect of physical activity on QoL (Jolly et al., 2006, Schmitz et al., 2005, Spronk et al., 2005), and two a positive effect (Oldervoll et al., 2004, van Tol et al., 2006). However, accounting for variation in QoL over time in control populations is particularly important; in clinical populations, perceptions of health and well-being may not be stable, as change in QoL resulting from significant illness or disability may be affected by factors other than the intervention itself. An example is adjustment to diagnosis, or response shift (Schwartz & Sprangers, 2000). Of the three remaining meta-analyses that did calculate pooled effect sizes controlling for the degree of change in no-exercise control groups, one reported no effect (Lawlor & Hopker, 2001), and two a positive effect (Netz et al., 2005, Schechtman and Ory, 2001). However, a lack of standardisation of the nature of the exercise activity intervention (e.g., exercise intensity and setting), and the type of QoL measures used, compromises the comparability of studies within meta-analyses, and consequently the reliability of their findings (Oldervoll et al., 2004).
The aim of the present review was to conduct a meta-analysis to assess the efficacy of exercise interventions across clinical and well populations. It also aimed to explore whether characteristics of either the exercise intervention, or the domain of QoL measured can account for heterogeneity in outcomes across studies. Consistent with previous work it was hypothesised that there would be an improvement in QoL across studies of a small effect size. In addition, six a priori hypotheses were specified to test the potential moderating role of exercise purpose, QoL domain, exercise intensity, and exercise type, on the effect of exercise on QoL. Each of the hypotheses is addressed in turn.
The first aim of the present review was to compare the outcome of exercise interventions between well and clinical populations, and to assess the consistency of the QoL response across disease groups according to intervention purpose. As interventions conducted with well populations typically involve adults with existing health risk factors, and thus participants with an initially low level of fitness, we expected there to be similar responses for both well and clinical samples. However, we expected variation in the responses of clinical populations to be further differentiated by illness severity and the intended goals of treatment. To conduct this comparison, studies were classified into one of three groups according to the following criteria (in brackets); (1) health promotion/prevention (an intervention delivered to a non-clinical population), (2) rehabilitation (an intervention delivered following a health threat, but patients are expected to recover a full or near-full level of functioning), and (3) disease management (an intervention delivered as part of a treatment regimen, for symptom management, or to prevent deterioration where improvement in function is not expected).
We predicted that greater improvement in QoL would be reported by those who could expect to discern benefits from exercise than those exercising to maintain current levels of functioning. Thus, it was predicted that rehabilitation studies (Group 2) would report the greatest gains in QoL. Furthermore, we compared the degree of heterogeneity between studies that could be explained through this method of categorisation than by splitting groups into clinical vs. well populations.
Research suggests that the impact of exercise on QoL outcomes may differ between domains (Taylor et al., 2004). Six QoL domains have been identified as universally important to both sick and well people: physical health, psychological state, level of independence, social relationships, environment, and spirituality, religiousness and personal beliefs (The WHOQOL Group, 1998). However, different health-related measures of QoL are constructed with diverse conceptual emphases. Some largely assess the psychological domain (e.g., The General Well-Being Scale; Dupuy, 1984); many address physical functioning and/or independence (e.g., SF36, Ware & Sherbourne, 1992), and many others report only an overall QoL evaluation without differentiating between domains. Although interpretation is more complex, retaining a multi-dimensional profile for comparison purposes allows for greater specificity, enabling detection of an important range of QoL changes associated with exercise. Pragmatically, it also allows for better targeting of poor QoL (Skevington & O'Connell, 2004). Previous meta-analyses have reported the greatest benefits to exercise be in the psychological rather than the physical domain, for example in self-efficacy (Netz et al., 2005), emotions (Schechtman & Ory, 2001), and self-esteem (Oldervoll et al., 2004). No change or even a deterioration along physical dimensions has been reported with clinical populations (e.g., Schechtman & Ory, 2001). Thus, a second hypothesis stipulated that the greatest gains in QoL would be observed in the psychological, as opposed to the physical domain.
Interventions expose participants to a range of different acute exercise bouts, including intensive aerobic exercise training that results in notable gains in fitness (e.g., using cycle ergometers), low intensity walking programmes, and passive physiotherapy exercises. Drawing from research in acute exercise settings, it would be expected that psychological outcomes, particularly positive mood, may differ as a result of exercise intensity. For example, lower intensity exercise is associated with greater enjoyment and persistence than high intensity, aerobic exercise is more beneficial to mood than resistance (isometric) exercise, and fitter individuals report more positive psychological responses to higher intensity exercise (e.g., Ekkekakis & Petruzzello, 1999). Some previous systematic reviews have assessed QoL outcomes in response to variation in exercise intensity, for example Netz et al. (2005) reported better QoL in response to moderate intensity exercise over light or strenuous intensity in older adults (d = 0.34). However, in clinical populations, the majority of meta-analyses have either not tested this, or have retrieved insufficient numbers of studies to statistically evaluate differences in outcome according to exercise intensity (Oldervoll et al., 2004, Schmitz et al., 2005, Spronk et al., 2005).
Exercise intensity may be particularly pertinent for clinical populations for whom poor physical condition may influence how exercise is experienced. For instance, exercise of the same objective intensity has been associated with greater perceived effort and lesser enjoyment in obese participants than non-obese samples (Ekkekakis & Lind, 2006). This suggests a moderating role for health state or physical condition. Similarly, in a study involving survivors of childhood leukaemia, former patients reported experiencing moderate intensity exercise to be more strenuous than did a healthy control group, even after recovery from cancer (Bell et al., 2006). Further investigation from a wider range of studies across health states is therefore warranted in investigating the importance of exercise intensity on QoL outcomes.
The present review aimed to examine whether physical health moderates the effect of exercise on QoL. Specifically, we tested the hypothesis that greater gains in QoL would be reported for interventions involving light, rather than moderate or strenuous intensity exercise for patients with a chronic physical illness (Hypothesis 3), whereas greater gains in QoL will be reported for moderate, rather than light intensity exercise in well populations (Hypothesis 4).
The type of exercise undertaken has also been considered as a moderator in the exercise-QoL relationship; however, evidence in support of this is variable. In a healthy but elderly sample, Netz et al. (2005) found better QoL for aerobic over resistance training in promoting QoL; in cancer patients, resistance training (isometric exercise) resulted in a better QoL response (Segal et al., 2003), and for depressed patients, exercise type had no differential effect on QoL (Lawlor & Hopker, 2001). Exercise type when crudely differentiated between aerobic or resistance training is not independent of exercise intensity (i.e., resistance training is classified as light intensity exercise), however the two are not synonymous (e.g., walking is light intensity, but is also aerobic). Therefore, a fifth hypothesis was tested that predicted better QoL outcomes from interventions incorporating aerobic exercise than resistance training alone.
Across studies, exercise is reported to take place in a variety of settings; in groups or alone, supervised or unsupervised, and in or out of the home (Netz et al., 2005). In Hypothesis 6, we predicted that greater improvement in the social domain of QoL would result from group-based, rather than individual- or home-based interventions.
The purpose of establishing a firmer basis for our understanding of a potential causal link between exercise interventions and QoL is to assist in the identification of necessary and sufficient factors of either the exercise, or participant group, in order to experience beneficial effects. It is important from an ethical basis to ensure that patients who are already experiencing health problems are not asked to change their lifestyle in ways that they may find difficult or uncomfortable, unless there is a clear evidence base to suggest this. Furthermore, better understanding of likely outcomes of interventions may enable greater cost efficacy through more efficient targeting of resources.
Section snippets
Inclusion criteria
Papers were allocated to the following inclusion criteria:
- 1.
Randomised controlled trials
- 2.
Intervention incorporating an active exercise component
- 3.
Pre- and post-test ratings of QoL for both intervention and control groups
- 4.
Adult populations (i.e., over 18 years)
- 5.
Available in English.
Exclusion criteria
- 1.
Passive receipt of physiotherapy exercise (this served the purpose of restricting the review to a more homogenous set of interventions)
- 2.
Absence of a no-exercise control group.
Search strategy
This systematic review was conducted using the
Results
The 56 studies in the final sample varied in size from 9 to 264 participants, including patients from seven broad disease categories and well people. They totalled 7937 participants and provided follow-up data at points from 1 to 26 months following an intervention. Full characteristics of studies according to the proposed moderating factors are shown in Table 1, and summarised in Table 2.
To enable comparisons to be made at similar time points across studies, the main analyses were conducted on
Discussion
The results of this comprehensive meta-analysis suggest that over the short term of 3–6 months, a small but meaningful improvement in QoL can be brought about by exercise interventions in well populations, and in patients exercising as part of their rehabilitation from a health event. However, there was a small deterioration in overall and psychological QoL for patients involved in exercise interventions as part of the treatment or management of chronic conditions (e.g., people diagnosed with
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