Exercise Adherence Scale (EXAS) validity study data
The aim of the study was to evaluate the construct validity of the Exercise Adherence Scale (EXAS). Construct validity was tested using four theoretical hypotheses. For convergent validity associations between perceived barriers and adherence were expected to be moderate (r=0.30 – r=0.50). For divergent validity the correlations between the EXAS and pain, and disability were expected to be low (r=0.00 – r=0.30). Participants and Setting For the validity study, sixteen physical therapy practices with a total of 42 physiotherapists participated and agreed to recruit patients with LBP. The inclusion criteria were: first visit to a physiotherapist for the current episode of LBP as the primary complaint, aged 20-65 years and having sufficient command of the Dutch language to read and understand questionnaires and spoken or written instructions. Patients were excluded if they were previously diagnosed with lumbar radiculopathy, spinal osteoarthritis or other conditions as the cause of their LBP or were unable to perform HBEs due to physical or mental problems. Measurements Adherence to HBEs was measured with the EXAS. The exercise adherence score was calculated using the assumption of 20% effectiveness for each point on the quality of performance scale. Barriers were measured using a modified version of the barriers subscale used by Sluijs, Kok and van der Zee (1993). Time to exercise was reported on a five-point scale ranging from 5 (“always”) to 1 (“never”). Motivation to exercise at home as recommended by the physiotherapist was measured on a 4-point scale, ranging from 1 (“very motivated”) to 4 (“not motivated”). Pain was measured with the Numeric Rating Scale for pain (NRS Pain). Patients were asked to rate the intensity of their current pain, on an 11-point numeric scale. Disability was measured with the Dutch language version of the Quebec Back Pain Disability Scale (QBPDS). The QBPDS quantifies disability caused by LBP in daily activities. Personal and demographic characteristics of the participants were measured. These variables were: age (in years), gender, height (in centimetres), weight (in kilograms), level of education and duration of symptoms. Validity Validity was assessed using Spearman’s rho for the correlations, as all comparator data was collected using ordinal scales. As no golden standard exists for the measurement of adherence to HBEs, it was decided a priori that at least three of the four pre-determined hypotheses are to be accepted to confirm validity of the EXAS. Correlations for “Time to exercise” and “Motivation to exercise” were rho = 0.47 (p = 0.013) and rho = 0.48 (p = 0.011) respectively, confirming both hypotheses for convergent validity. For divergent validity, correlations of rho = 0.24 (p = 0.22) for “Pain” and rho = 0.005 (p = 0.98) for “Disability” were found. As both correlation estimates are smaller than 0.3, the hypotheses for divergent validity are confirmed also. All hypotheses are confirmed.