Consistent with other reports of behavioral treatments , adherence to yoga and exercise interventions in this clinical trial was significantly correlated with baseline variables including depression, fatigue, and physical aspects of quality of life. Demographic variables were not reliable predictors of adherence. It has been previously noted that factors that can be changed, such as mood and social support, more strongly influence adherence than factors that cannot, such as age and gender . While adherence in the present study was relatively low compared to what might be expected in a drug trial, adherence has been observed to be generally lower in behavioral interventions and also with more objective adherence measures compared to self-report . For this reason, we relied on objective class attendance as our primary adherence measure although the two self-reported measures were highly correlated with the objective measure.
The underlying mechanisms associated with greater adherence are not well understood. Because lower adherence, even in the placebo arm of a double blind trial, is associated with worse outcomes including greater mortality [2–4], it has been postulated that those with greater adherence may engage in many other health promoting behaviors. Thus, adherence may be a marker for a personality or related trait related to motivation or goal-directed behaviors. Self-efficacy, which may relate to motivation, is the perceived confidence in one's ability to accomplish a specific task . While self-efficacy was not assessed in this study, it has been shown to be an important correlate of adherence . Within the conceptual framework of self-efficacy, adherence is promoted by the belief that an intervention will be effective (the outcome expectancy) as well as the belief that the individual is capable of following the requirements of the intervention (the efficacy expectancy). When these expectations of success contribute to high self-efficacy, high adherence can result, and moreover, there is reciprocity; subjects who are highly adherent to an intervention may be strengthening their outcome and efficacy expectancies . Expectancy of outcome, besides contributing to adherence, is also a major component of the placebo effect [22, 23]. Thus, investigations of factors that predict adherence (or, for that matter, all clinical investigations) are likely to benefit from a good measure of expectancy, which was absent in the present study. The relationship between adherence and health outcomes may be due to mechanisms underlying mind-body interactions, which makes this an area of special interest for researchers conducting mind-body interventions.
Many complementary treatments take a patient-centered approach and utilize outcome and efficacy expectancies, but also require greater effort than simple pill-taking, and this unique set of qualities recommends controlling for factors that impact adherence in mind-body interventions. A discussion of adherence to Mindfulness-Based Stress Reduction programs  has identified elements of the intervention itself such as active participation, personal follow-up, accommodation of individual preferences, and emphasis on process instead of outcome, which are believed to effectively discourage attrition and relapse. These are common elements to other mind-body therapies, including both physically active interventions in the present study, which may contribute to differences with conventional clinical trials in adherence and in outcomes.
Improving adherence in a mind-body intervention has the potential to enhance the treatment effect, by increasing the dose of the intervention received by all subjects, especially those who (because of lower baseline functioning) might sustain the greatest benefit and are at greater risk for low adherence. These same subjects who may be most in need of intervention may fail to even meet inclusion criteria for some clinical trials where high adherence during a screen-in period is required. In these cases, results of the intervention can be difficult to generalize and potential magnitude of the treatment effect may be obscured by the relatively high baseline health status of subjects who do pass the adherence run-in phase . In the present study, subjects with a profile of self-reported baseline scores that was low on physical functioning and general health and high on fatigue were more likely to drop out, and less likely to maintain adherence if they did complete the study. This pattern of results accords with other published reports of factors that predict adherence to treatment, implying that at least some commonly identified determinants are equally relevant for mind-body interventions.
Reviews of efforts to improve adherence to therapeutic regimens have thus far been inconclusive, and suggest that no single approach is consistently effective for all subjects in all interventions [25, 26]. Strategies to promote adherence include making instructions to subjects simpler and less demanding, addressing cognitive-motivational factors such as self-efficacy and health beliefs, offering social support and reinforcement, and providing reminders, with results suggesting that highest success rates are achieved by a combination of such approaches. Specific strategies may be particularly important for groups of subjects that may have more difficulty with adherence to behavioral interventions because of a variety of issues such as getting to a class, physical limitations, parental or caregiver responsibilities, or, as already discussed, depression. Concerns such as these may be especially relevant for elderly populations , like those tested in the present study, who stand to gain greatly from improvements in health. Continued attention to maximizing adherence is important for enhancing treatment benefits, as well as controlling the costs of clinical trials and increasing statistical power to determine the effectiveness of interventions .
This analysis examined study completion or attrition as well as adherence to the terms of a physically active intervention, and it may be that unique factors differentiate correlates of these related measures. As previously mentioned, lower adherence, even in the placebo arm of a double blind trial, is associated with worse outcomes [2–4], although the underlying mechanisms linking adherence to health outcomes are still unknown. Future trials may elucidate this relationship, which could be related to mind-body interactions.