This study provides a comprehensive overview of determinants of exercise intervention adherence and exercise maintenance after completion of an intervention in cancer survivors. Eighteen studies were evaluated using a socio-ecological model of determinants of health behaviors, taking into account demographic and clinical, psychological, physical, social and environmental factors. Most studies examined demographic and clinical, psychological and physical factors, whereas few studies investigated social and environmental factors. We found moderate evidence for a positive association between exercise history and exercise adherence. For most demographic and clinical factors, we found insufficient evidence of an association with exercise adherence or maintenance. For exercise adherence, inconsistent findings were found for gender, type of treatment, as well as for psychological factors including perceived behavioral control, stage of change, self-efficacy, extraversion, the physical factor cardiovascular fitness and the environmental factor location of the fitness center. For exercise maintenance, we found inconsistent findings for age, education, self-efficacy, fatigue, attitude, quality of life, intention, PA intervention adherence, body mass index, baseline PA and cardiovascular fitness.
Similar to the review of Szymlek-Gay and colleagues , lower age, lower body mass index, more advanced disease stage, higher degree of readiness to change PA behavior, higher self-efficacy, higher physical fitness, and higher baseline PA were identified as possible determinants of exercise adherence. However, according to our best evidence synthesis, the level of evidence was insufficient mainly due to inconsistent findings across studies. In contrast to our review, Husebø et al. found exercise stage of change, intention, perceived behavioral control, and subjective norm to be a significant determinant of exercise adherence in their meta-analysis . However, although statistically significant, the strength of the associations were low (<0.3). They extracted their results from univariate analysis instead of multivariate analysis which may have overestimated the strength of the associations.
Most demographic and clinical factors were not significantly associated with exercise adherence or maintenance. The lack of statistically significant associations may be related to small sample sizes and the relatively low variability of exercise adherence and maintenance. Most studies were conducted as efficacy trials, evaluating the effects of exercise in ideal circumstances, in which usually a more homogenous group of patients participated with a relatively high adherence . On the contrary, effectiveness trials evaluating intervention effects under “real-world” conditions, generally have lower adherence levels . More well-powered studies are needed on determinants of exercise adherence in real-world circumstances. Although, most demographic and clinical factors, such as age, gender and type of cancer, are unmodifiable, insight into these factors provide valuable information about which subgroups of patients that are more or less likely to adhere to exercise programs or maintain exercise behaviors.
From previous research, it is well known that social factors including social support, having an exercise partner or role model, may influence exercise behavior  or exercise behavior change . From studies in the general population it is also known that the physical or built environment improving the availability, accessibility, and attractiveness of exercise opportunities (e.g., sidewalks, bicycle lanes, safe road crossings, availability of green spaces and recreation facilities) are related to exercise behavior . Because cancer survivors may experience even more barriers than the general population, social support, as well as attractive and easily accessible exercise facilities may even be more important determinants for cancer survivors compared to the general population. However, only few studies have evaluated the association of social and environmental factors with exercise adherence and maintenance in cancer survivors. The few studies published to date suggest that feedback from trainers or nurses was positively associated with exercise adherence , whereas no significant association was found of social support, having an exercise partner or role model  and the location of the fitness center  with exercise maintenance. Future studies are needed to further build the evidence for the influence of social and environmental factors on exercise adherence and maintenance.
Overall, the methodological quality of the reviewed studies was low, with only one study of high quality . A major concern regarding the quality of most included studies was the high likelihood of selection bias and small sample sizes. The included studies conducted secondary data analysis of RCTs that were not designed to evaluate determinants of exercise adherence. Further, many studies did not report point estimates and measures of variability. Another frequent methodological shortcoming was the lack of valid and reliable measures of adherence and maintenance. We recommend to systematically report session attendance in a supervised exercise intervention and/or using accelerometers of pedometers to assess PA levels.
Strengths and limitations
Strengths of this systematic review include the extensive literature search in multiple relevant databases, the in-depth methodological quality assessment and best evidence synthesis, as well as the presentation of determinants within ecological framework categorizing demographic and clinical, psychological, physical, social and environmental factors. Another strength is the attempt to differentiate determinants of adherence to exercise interventions at different time points during cancer survivorship according to the PACC framework. However, due to the limited number of studies we were unable to study differences in determinants of exercise adherence before, during and after cancer treatment. The limited number of studies also hampered us to examine whether determinants of exercise adherence vary across cancer types and exercise modalities such as mode (e.g. aerobic versus resistance exercises), delivery (e.g. supervised versus home-based), intensity and frequency. Further work is necessary to determine the most important determinants of exercise adherence and maintenance, and to study differences across cancer types and exercise modalities. Another limitation is the variety of definitions of exercise adherence, with some studies exclusively focusing on adherence, whereas other studies also incorporated a measure on compliance, i.e. whether the PA was conducted at the prescribed intensity . As a result, we could not differentiate between determinants of exercise adherence and determinants of compliance. Therefore, future studies should more clearly distinguish exercise adherence and compliance. Finally, similar to other reviews and meta-analysis, publication bias cannot be ruled out.