Original ArticlesThree independent factors predicted adherence in a randomized controlled trial of resistance exercise training among prostate cancer survivors
Introduction
Researchers have used randomized controlled trial (RCT) methodology to test the utility of exercise interventions in cancer survivors (e.g., [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]). A critical component to the success of these trials is achieving high levels of exercise adherence in the exercise group and low levels of exercise contamination in the control group [12], [13]. Exercise adherence may be defined as the extent to which the exercise group has performed the exercise prescription. Conversely, exercise contamination refers to the extent to which the control group has performed the exercise prescription. Theoretically, to provide an optimal test of the efficacy of an exercise intervention it is necessary to achieve 100% adherence in the exercise group and 0% contamination in the control group. Any deviation from these exercise rates may have profound implications for estimating the efficacy of the intervention. Unfortunately, research to date has reported significant problems with exercise adherence and contamination in RCTs of exercise in cancer survivors.
More specifically, exercise adherence rates in cancer survivors have typically ranged from 60 to 85% for both home-based (e.g., [2], [3], [7], [14], [15]) and supervised (e.g., [3], [4], [9], [11]) RCTs. These rates are comparable to those reported in exercise RCTs of older adults without cancer [16]. Exercise contamination rates, on the other hand, are rarely reported in RCTs, but have ranged from 20–50% in several home-based exercise programs in cancer survivors [2], [7], [15]. These data suggest that RCTs in cancer survivors have been hampered by less than optimal exercise adherence and contamination rates. Before any corrective action can be taken, however, it is necessary to have an understanding of the determinants of exercise adherence and contamination in RCTs of cancer survivors.
To date, only two studies have examined this issue in cancer survivors [12], [17]. Unfortunately, the studies were limited by small sample sizes, unsupervised home-based exercise programs, and reliance on self-reports of exercise. Moreover, given the heterogeneous nature of cancer survivor groups, cancer treatments, and exercise programs, the generalizability of the findings is unclear.
We have recently completed an RCT that examined the effects of a 12-week, supervised resistance exercise training (RET) program on fatigue, quality of life (QOL), and muscular strength in prostate cancer survivors receiving androgen deprivation therapy (ADT) [4]. The trial demonstrated clinically meaningful changes in these outcomes that favored the exercise group. We also reported a 78.3% adherence rate to the supervised RET program. The primary purpose of the present study was to examine predictors of adherence to the supervised RET program in the trial. A secondary purpose was to report the contamination rate. Such information should aid in the design and conduct of future trials in this population.
We adopted Ajzen's [18] Theory of Planned Behavior (TPB) to assess the social cognitive determinants of exercise. We also included traditional demographics (e.g., age, marital status), medical variables (e.g., cancer stage, treatments), behavioral/fitness indicators (e.g., past exercise, smoking, muscular strength), and QOL as additional predictors. Many of these variables have been associated with exercise behavior in the general population [19] and with exercise adherence in previous RCTs involving noncancer populations [16].
Our primary objective was to examine predictors of adherence to the supervised RET program in the exercise group. Based on previous research [12], [17], we expected exercise stage of change, employment status, treatment protocol, and perceived behavioral control (PBC) to be important predictors of adherence to the supervised RET program. Our secondary objective was to document the level of exercise contamination (i.e., exercise performed outside the formal supervised program) in both the exercise and control groups. Based on our supervised exercise protocol, we expected the contamination rate to be minimal [1].
Section snippets
Participants
The methods of the trial have been described elsewhere [4]. Briefly, the trial was conducted at the Ottawa Regional Cancer Center in Ottawa, Canada (coordinating center) and the Cross Cancer Institute in Edmonton, Canada. Approval was obtained from the ethics committees at both sites and all participants provided written informed consent. Participants were 155 prostate cancer survivors recruited between September 1999 and August 2001. Eligibility criteria for the study were: (1) histologically
Flow of participants through the study
Flow of participants through the trial is presented elsewhere [4]. Briefly, we randomized 155 of 507 (30.6%) eligible participants—82 to the exercise group and 73 to the control group. We had 8 drop outs in the exercise group (9.8%) and 12 (16.4%) in the control group (P = .790). Baseline data were available on all 155 participants and we had at least one measure of self-reported nonprogram exercise (i.e., the 6 or 12 week assessment) for 151 of 155 (97.4%) participants. Last observation carried
Discussion
Our adherence rate to the supervised RET program during the trial was 78.3%. In univariate analyses, eight different variables predicted exercise adherence: preprogram overall exercise stage, intention, age, QOL, fatigue, subjective norm, leg-press test, and PBC. In a multivariate analysis, the independent predictors of exercise adherence were preprogram overall exercise stage, age, and intention. We also found minimal contamination in the control group but a significant decline in nonprogram
Acknowledgements
This study was funded by the National Cancer Institute of Canada (NCIC) with funds from the Canadian Cancer Society (CCS). K.S. Courneya was supported by an Investigator Award from the Canadian Institutes of Health Research and a Research Team Grant from the NCIC with funds from the CCS and the CCS/NCIC Sociobehavioral Cancer Research Network. R.D. Reid was supported by a New Investigator Award from the Heart and Stroke Foundation of Canada. We thank Todd Bobick, MSc, and Neil Eves, MSc, for
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