The present meta-analysis evaluated the efficacy of exercise interventions on QOL outcomes among cancer survivors. Overall, results showed that cancer patients of various diagnoses who participated in exercise interventions subsequently reported higher QOL in the studies’ first follow-up assessments, an effect that was significant in relation to simultaneous control groups and in relation to their baseline levels of QOL. Of great import, these effects were still intact on assessments usually taken months later (), a finding not explored in previous meta-analyses, which did not examine extended follow-up [24
]. The main effect sizes for first available follow-up were similar in direction and magnitude—positive and medium—to overall effect sizes reported in previously published meta-analyses examining the efficacy of exercise interventions in improving quality of life in cancer patients [24
]. These effects compare favorably to other health promotion literatures [120
A burgeoning sample of available interventions made it feasible to examine what features of the interventions offer the most benefit for quality of life, something that one prior meta-analysis [22
] had done only to a limited extent. Across bivariate and combined analyses in both high- and low-quality studies, targeted aerobic METs emerged as an important predictor of intervention efficacy. Low amounts of aerobic activity were associated with little or no QOL change, but in studies of longer duration, larger amounts of aerobic activity were associated with substantial QOL change (). Thus, aerobic exercises like moderate intensity bicycling (six METs) were associated with greater QOL increases than lower intensity aerobic exercises like walking (four METs; see 121 for a detailed listing of physical activities and corresponding METs), especially in trials of long duration. The physiological and psychological benefits of aerobic activity, including its impact on QOL, may not appear in just a short period with moderate METs, but rather may take considerable time with relatively high METs to emerge consistently. Thus, to see an impact, an intervention would need to facilitate extended participation in exercise, either through longer intervention duration, or maintain greater fidelity to the exercise routine once the intervention ends, or both. Indeed, our analyses support this assertion: at first available post-intervention follow-up, QOL change was most likely to appear in studies of longer duration whereas higher targeted METs had little impact for studies of shorter duration (). This pattern provides some evidence that the advantage of interventions with lower targeted METs among interventions targeting one to three METs may be illusory or due to non-exercise factors such as social support, and may disappear at longer intervals. Although QOL might improve following this pattern as METs further increased, few interventions in this analysis exceeded six targeted aerobic METs, meaning inferences beyond that value are tenuous and further research is necessary to determine whether these effects hold at higher METs. Additionally, studies that included more women tended to produce greater improvements in QOL. This tendency for the interventions to work better for women than men parallels findings of a recent meta-synthesis of health promotion meta-analyses [120
]. Of note, in the current meta-analysis, the impact of targeted aerobic METs and percentage of women was more marked in higher quality intervention studies.
A prior meta-analysis reported that supervised exercise was marginally linked to QOL improvement [22
]. The current work found mixed results concerning its role in intervention efficacy, such that it was significant in bivariate analyses among controlled but not pre–post comparisons, and was not significant when entered simultaneously with other moderators in pre–post comparisons of all interventions. These patterns suggest that supervision is not directly linked to improvement. Cancer type was previously found not to be a significant moderator [22
]; our bivariate analyses showed a relation, but it did not remain significant when entered simultaneously with other predictors like percentage of women, which is highly correlated. Thus, type of cancer appears to be less directly connected to QOL changes than other variables.
Several other variables related on a bivariate basis, including sample size, length of intervention in weeks, and percentage of breast cancer patients. Yet, when they were entered simultaneously with aerobic METS and sample gender, these dimensions did not retain significance, suggesting they are less directly related to QOL improvements. Number of intervention sessions, minutes per intervention session, targeted resistance METs, training of facilitators, and inclusion of flexibility content did not explain variation. Note that the mean value for targeted resistance METs was quite low (2.5), which limits conclusions concerning this potential moderator, as interventions may not have targeted high enough resistance METs to yield an effect.
The current research has several important implications. First, exercise interventions appear to be a generally efficacious way to improve QOL among cancer patients of various diagnoses, supporting previous research concerning the impact of physical activity on improving cancer survivors’ QOL [6
]. It also supports the current recommendations of cancer patient providers concerning exercise in cancer patients, which encourage physically active cancer patients to continue previously established exercise habits and sedentary cancer patients to adopt a moderate program of exercise [7
Taken together, these results support the development of interventions focusing on levels of aerobic METs of moderate intensity in the range of five to six METs, although do not identify particular types of cancer survivors for whom more moderate intensity aerobic exercise will be most beneficial. In addition, there is limited evidence that supervised exercise sessions and intervention length play a more distal role in moderating efficacy; these moderators did not remain significant in combined analyses. There is also evidence that these types of interventions are more efficacious for women, which highlights not only the benefit of these types of interventions for this group, but also the need for development and refinement of interventions for men. No evidence suggests that number or length of sessions, resistance METS, training of facilitators or flexibility content moderate exercise-induced QOL improvements.
There are several limitations to the present research. First, the present meta-analysis did not examine adherence or contamination within the exercise intervention protocol, as many of the studies did not report such measures. Yet, the current work’s primary aim was to identify interventions that were efficacious in increasing QOL of cancer survivors, rather than to identify whether exercise per se affected QOL. Therefore, adherence to the exercise intervention protocol can be seen as a function of the intervention itself, and failing to control for adherence therefore does not affect the validity of the meta-analysis.
An additional limitation concerns the search strategy. Seven large research databases were searched for relevant studies, but no unpublished literature was obtained other than dissertations and theses, which comprised 10% of the sample. Additionally, the analysis included published studies that yielded non-significant or negative effect sizes. Publication bias was also explored using three statistical techniques, and bias was present to some extent. Yet, the fact that the aerobic METs and gender effects patterns were more marked in higher than lower quality studies () suggests that aerobic exercise genuinely impacts QOL especially for female cancer survivors.
Another limitation concerns the study population and potential interactions with moderator variables. It is possible that recruitment into trials was selective, such that high-functioning survivors were more likely to elect or be eligible to participate. These high-functioning survivors may tolerate, and benefit from, more intense exercise than would their low-functioning counterparts. Measures of functioning among survivors recruited into studies, or proxy measures of functioning such as time since treatment, are not routinely reported across the literature, making it difficult to evaluate whether high- and low-functioning survivors benefit from different interventions targeting different intensities of aerobic activity. Similarly, although the results suggest that exercise interventions work best for women, there were too few male samples to evaluate whether the aerobic intensity relates to QOL improvements in the same pattern as that shown for women. Until more studies with males and with identifiably lower functioning patients are available, the results of this meta-analysis should be interpreted with caution when generalizing the current results to these target groups.
A related limiting factor is lack of detail about the exercise interventions and their samples [122
]. It is possible that a consideration of such aspects as wellness or social support, which has been shown to play a role in cancer outcomes [124
], and many of the moderating variables could be correlated with social support or other behavioral change tactics, including minutes per session, number of sessions, and length of intervention. Yet, the analyses could not control for social support, as studies did not report the necessary descriptive information for such analyses, such as whether participants in unsupervised interventions engaged in exercise alone or with friends. As stated previously, functioning level of cancer survivors may be another important factor we cannot characterize, similar to any potential benefits of exercise interventions that focus on resistance exercise. Additionally, there may be other intervention characteristics, such as tone of intervention delivery, setting, and cohesion of participants that could affect outcome but are not reported, thus limiting our ability to predict variations in efficacy. These unreported characteristics represent a restriction of range and likely contributed to relatively poor model fit in moderator analyses. In the future, more emphasis should be placed on identifying and publishing characteristics of interventions whether they are efficacious or not, so that this knowledge may be used in the development of novel exercise interventions.
This meta-analysis also highlights the lack of theoretically driven interventions in this domain. The majority of the interventions did not explicitly state whether they were theoretically informed, and as such moderator analyses examining the differential efficacy of theory-based interventions were not possible. Of the few studies that did mention theory, most did not explain how the theory contributed to intervention development, so inclusion of theory-based intervention content could not be evaluated. Interventions that draw on empirically validated theory may be more efficacious in changing behavior than those that do not [125
], and as such, future exercise interventions for cancer survivors may benefit from the use of theory in their development.
Future meta-analyses should examine outcomes in addition to those in this study. Possibilities include fatigue, depression, body mass index, and aerobic capacity. Several recent systematic literature reviews and meta-analyses have, in fact, addressed the efficacy of exercise interventions in reducing fatigue in cancer survivors, although these analyses had strict selection criteria and therefore may not represent the body of literature well enough [29
], or were broader reviews but failed to explore plausible moderators of intervention efficacy [31
It may be of great interest to clinicians and cancer survivors alike to determine whether exercise interventions would provide additional benefits to the currently increasing length or rate of post-cancer diagnosis survival. Although preliminary evidence suggests that exercise increases life expectancy after cancer diagnosis [128
], no studies in the current meta-analysis examined this variable of interest. One potential reason for this absence is that available resources have limited researchers’ ability to retain participants for observation in studies spanning a time period long enough to measure post-diagnosis survival in a meaningful way. The lack of life expectancy outcome measures in this literature highlights the need for additional funding devoted to designing and implementing studies that could capture this outcome reliably. Additionally, absent resources to examine actual survival, researchers could measure surrogate markers of survival, such as maximum VO2
], the gold-standard measure of cardiorespiratory physical fitness. Of note, to date, published studies do not consistently report such statistics. Future studies should incorporate such measures.
Related, there is a lack of consistency across the literature in terms of what outcomes are reported. A large number of studies that involve exercise interventions did not assess QOL, examining instead biological measures such as maximum VO2
or timed distance tests [28
] or other psychological outcomes such as fatigue or depression [27
]. Better standardizing the outcomes of interest would permit more thorough comparisons of intervention impact across the literature. We recommend researchers in this field measure and report a wide and consistent variety of psychological measures, including QOL, fatigue, and depression, as well as measures of health fitness including cardiovascular physical fitness, muscle strength and endurance, body composition, and flexibility. In addition, we recommend that future research incorporate extended follow-up measures of these outcomes in addition to immediately post-intervention outcome evaluations. Future meta-analyses could examine the extent to which improvements on these latter outcomes are related to QOL, among other patterns.
The present study also validates recommendations to develop interventions designed to improve QOL by increasing exercise levels in cancer survivors, and highlights intervention characteristics that may be important to explore further in improving QOL. Future intervention development may benefit from the conclusions of this meta-analysis. Specifically, this analysis supports further exploring optimal levels of targeted aerobic METs and provides limited support for further exploring the role of intervention length and supervised exercise sessions. The development of future interventions designed to ascertain optimal levels of aerobic and resistance METs, intervention length, and supervision is necessary. Though the analysis demonstrates the efficacy of such interventions in improving QOL in female cancer survivors, and thus supports the development of future interventions in this population, it also highlights the lack of efficacious interventions of this type for male cancer survivors. As such, the development and refinement of exercise interventions for male cancer survivors is encouraged. Since the present meta-analysis demonstrates the importance of exercise, and exercise interventions, in improving QOL of cancer survivors, future research should attempt to refine the development of such interventions, guided by recommendations from research synthesis, in order to identify optimal ways to increase exercise behavior in this population.