This meta-analysis documented that subjects who receive interventions designed to increase their PA experience better QOL outcomes over their baseline scores and in comparison to control subjects. The magnitude of the ES is difficult to assess because too few studies used any single QOL measure in exactly the same way to allow us to convert the ES to an original metric. The ES magnitude, as calculated and as depicted by CLES scores, seems modest. It is unclear what ES would represent a clinically meaningful improvement in QOL among chronically ill adults. People with major chronic illnesses experience many reasons for declining QOL, including the disease itself and physical or psychosocial sequelae of the disease, and onerous or distressing treatments. Studies may recruit chronically ill study subjects from specialty medical practice settings where the subjects may already be receiving optimal medical care. Even a small change in QOL may be important since QOL is a complex phenomenon likely affected by diverse factors.
It is important to note that these findings were heterogeneous, as expected, though less so for two-group than pre-post comparisons and less so for QOL outcomes than for other health and PA outcomes reported in these primary studies (Conn, Hafdahl, Brown et al., 2008
; Conn, Hafdahl, Mehr et al., 2007
; Conn, Hafdhal, Minor et al., 2008
; Conn, Hafdahl, Moore et al. 2008
). Interventions varied dramatically from brief motivational sessions to extended supervised exercise programs. Diverse measures were used to assess QOL and PA. No gold standards exist for interventions or measures of QOL and PA. Other important factors that may affect validity of findings which are infrequently reported in primary studies could not be assessed (e.g. treatment fidelity). As more primary research accumulates, future meta-analyses may be able to determine if ES are related to research methods.
The explanation for QOL changes is unclear. These interventions were designed to change PA behavior, not to directly affect QOL. Mean differences in PA behavior were not associated with QOL mean differences in the moderator analyses. It is possible that people achieved small increases in PA that were not detected by the PA measures but that contributed to increased QOL; however, this is inconsistent with the magnitude of ESs on PA in a related study (Conn, Hafdahl, Brown et al., 2008
). Even small improvements in functional status from slight increases in PA may contribute to improved QOL. Previous meta-analyses of disease-specific outcomes (e.g. HbA1c, arthritis functional status) among common chronic illnesses documented improved health outcomes (Conn, Hafdahl, Mehr et al., 2007
; Conn, Hafdahl, Minor et al., 2008
; Conn, Hafdahl, Moore et al., 2008
; Nielsen et al., 2006
). These improvements may explain the improvements in QOL. It is also possible that subjects experienced enhanced perceived mastery over their chronic illnesses. Future primary PA research should include QOL measures and report the association between improvements in QOL and PA behavior changes to address this issue and avoid possible ecological fallacy in interpreting meta-analytic findings (Berlin, Santanna, Schmid, Szczech & Feldman, 2002
). Research syntheses focused on correlates of and explanatory models for QOL could address the association between PA and QOL more directly.
The exploratory moderator analyses documented some intriguing suggestive findings that future research should examine. The larger ES among unpublished and unfunded studies was somewhat surprising. These findings do not support the pattern of publication bias against studies with small ESs often reported in the literature (Cook et al., 1993
; Conn, Valentine, Cooper, & Rantz, 2003
). These unpublished and unfunded studies may include projects with extraordinary researcher effort to ensure successful projects, such as graduate student research. It is also possible investigators were more likely to provide study information about unpublished studies if the study reported large ESs. The finding of no association between ES and random assignment of subjects does not support the common assumption of bias toward positive effects in studies without random assignment.
Our finding that behavior target (PA behavior only vs. multiple health behavior) was not associated with ES differences contrasts with previous meta-analyses of PA behavior and health outcomes that have documented better outcomes among studies that targeted only PA behavior (Conn, Valentine, & Cooper, 2002
; Conn, Hafdahl, Mehr et al., 2007
; Conn, Hafdahl, Brown et al., 2008
). The meta-analyses that reported larger effects for interventions that focused exclusively on PA behavior have examined outcomes directly affected by PA behavior. The explanation for these differences may become clearer as more intervention trials include QOL outcomes as well as PA behavior and health outcomes.
The absence of moderator effects for age, gender, and minority distribution suggests that diverse samples may experience modest improvement from interventions to increase PA. People with chronic illnesses may avoid changing PA behavior because they fear further decline in their QOL. Many are dealing with demanding chronic illnesses that require continual self-management. Health care providers and health educators may use these exploratory findings to counter fears that increased PA will necessarily decrease QOL.
These findings suggest that interventions using only supervised center-based exercise may have more impact on QOL than interventions including educational-motivational content, regardless of whether it is accompanied by supervised exercise. These findings contrast with previous work documenting a lack of superiority of supervised exercise for PA behavior and health outcomes (Conn, Hafdahl, Mehr et al., 2007
). Although the exploratory moderator analyses are intriguing, they should be interpreted with caution. Relationships documented in the moderator analyses may be confounded by other sample- or study-level characteristics. Further primary studies testing differences within randomized controlled trials are needed.
QOL outcomes as measured by well-being, life satisfaction, and QOL measures were included. Studies that used mood, energy, or fatigue measures as QOL outcomes were excluded. Although findings have been mixed, some research has suggested a link between PA and mood (Conn, Hafdahl, Porock et al., 2006
; Rietberg et al., 2005
) and between PA and energy/fatigue (Puetz, Beasman, & O’Connor, 2006
). Few studies in this meta-analysis addressed mood outcomes; after more primary studies reporting mood outcomes have been conducted, a synthesis of mood outcomes would be valuable.
In conclusion, this meta-analysis documented modest improvements in QOL outcomes among adults with chronic illnesses following interventions to increase PA. These findings should encourage researchers and providers evaluating interventions designed to increase PA to include QOL outcome measures in their projects.