37 Development of the Exercise Adherence Scale (EXAS) in patients with low back pain adherence of the study participants, possibly reducing the underestimation of treatment effects. A second strength of this study is the high number of 200 observations used for the reliability testing of the EXAS. Furthermore, video recordings were used to rate the quality of performance, which eliminated the impact of possible variations in the performance of exercises by patients or differences in the rater’s point of view between measurements. This increases confidence in the findings for intrarater and interrater reliability. The current study also has several limitations. The first limitation is that the participants recruited for the reliability study were healthy subjects. Healthy subjects perform better during functional movement tasks when compared to patients with chronic LBP (35). However, the reliability of the scoring system for the quality of exercise performance depends on the agreement between different ratings made by the same rater. The underlying causes for better or worse performance by the patient are not relevant. In daily practice, a physical therapist will select exercises for an HBE program and tailor the difficulty of these exercises to correspond with the patient’s level of ability, thereby eliminating any differences in performance with a healthy subject. The second limitation is the low number of only four different exercises used for the reliability study. As a physical therapist can select from a vast list of possible exercises when designing an HBE program, using all of these exercises would have been impractical. Therefore, an expert panel selected four exercises commonly recommended to patients with LBP to be used in the study. The third limitation is the potential patient bias when reporting adherence to frequency and intensity recommendations. However, the impact of overreporting of adherence by patients is most likely mitigated by adding the quality of performance score to the assessment of adherence. Quality of performance is likely to be low in nonadherent patients, resulting in a lower EXAS score and a more realistic approximation of actual adherence. However, more research is required to confirm this hypothesis. Another limitation is the lack of reliability testing of the frequency and intensity aspects of the EXAS. Although the validity of the EXAS and the reliability of the quality of performance assessment were investigated in the current study, additional research is needed to determine the clinimetric properties of the EXAS in patients with LBP. The last limitation is the relatively small sample size for the validity study of the EXAS. A sample size of 50 or greater is recommended by the COSMIN checklist for a “good” rating, but practical reasons prevented the achievement of this goal (27). Although 42 physical therapists agreed to recruit patients with LBP for the study, many of them did not manage to do so during the inclusion period of the study. Nevertheless, given the 2
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