Development and validation of an exercise adherence prediction model 105 Measures Exercise adherence (the outcome) was assessed by the Dutch version of the Rehabilitation Adherence Measure for Athletic Training (RAdMAT-NL). The RAdMAT-NL has good psychometric properties with an internal consistency reliability of a = 0.91 [24]. The RAdMAT-NL is a 16-item questionnaire that uses a 4-point rating scale (never = 1, occasionally = 2, often = 3, always = 4) to evaluate clinic-based adherence [24]. The RAdMAT-NL consists of 2 subscales: Participation (items 1-5 and 9-16), range 13-52 points), and Communication (items 6-8, range 312 points). The total scale range is 16-64, a higher score indicates a higher degree of adherence. According to the American College of Sports Medicine guidelines, a score of at least 85% must be achieved to be adherent to the rehabilitation program [25]. This means, a minimum total score of 54 or higher must be achieved on the RAdMAT-NL to be adherent. The RAdMAT-NL was completed at 12 months by the physiotherapist, independent of the patient and not in their presence. TPB constructs were assessed at baseline. The constructs were measured according to the questions from Ajzen [22]. The four intention items focused on goals and plans for exercise and uses a 5-point rating scale (totally disagree = 1, disagree = 2, do not disagree/do not agree = 3, agree = 4, totally agree = 5). The total scale range is 4-20, a higher score indicates a higher degree of intention. Attitude was measured using seven bipolar adjective scales (5-point rating scales) that asked about both instrumental (e.g., useful–useless, bad–good) and affective (e.g., enjoyable–unenjoyable, boring– interesting) attitude. The total scale range is 7-35, a higher score indicates a more positive attitude. PBC was measured by three questions that asked about aspects of controllability and ease/difficulty and uses a 5-point rating scale. The total scale range is 3-15, a higher score indicates better PBC. Subjective norm was measured by three items that asked about approval and support for exercise and uses a 5-point rating scale (totally disagree = 1, disagree = 2, do not disagree/do not agree = 3, agree = 4, totally agree = 5). The total scale range is 3-15, a higher score indicates more social pressure. Alliance was assessed after three months by the Working Alliance Inventory (WAI) [21]. The patient-provider relationship was measured with 12 questions rated on a 5-point scale (range 12-60) and a higher score indicated a higher level of alliance. Depression was assessed at baseline by four depression questions of the FourDimensional Symptom Questionnaire (4DSQ) [26]. The 4DSQ is a self-report questionnaire that has been developed in primary care to distinguish non-specific general distress from depression, anxiety and somatization. All items were rated on a 4-point scale (range 4-16) and a higher score indicated a higher level of depression.
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