139 Trajectories of Adherence to HBE recommendations among People With Low Back Pain leading to higher adherence numbers. Furthermore, the longer time period during which measurements were taken in the study with patients with osteoarthritis of the hip and/ or knee allows for more time for adherence to decline, resulting in a higher likelihood of decreasing adherence over time. Another explanation is the difference in measurement instruments used to measure adherence between the studies. The EXAS used in the current study provides a more accurate measurement of adherence than the recall over several weeks used in the other study. In patients with osteoarthritis of the knee and/or hip, differences between the identified groups were found for pain, function, and self-efficacy. This is in line with studies investigating factors associated with adherence but is in stark contrast to the findings from the current study (8). Despite the fact that baseline characteristics were selected for the comparison based on existing literature (8,10), none of the baseline characteristics measured were significantly different between the trajectory groups in our study. There are several possible explanations for the differences between factors related to adherence found in the literature and the findings of the current study. The first and most straightforward explanation is that patient adherence to HBE recommendations during treatment is determined by patient characteristics that were not measured and therefore no differences between groups could be found. However, the baseline characteristics chosen for baseline comparison between groups were carefully selected based on existing literature and have consistently been shown to be related to adherence. This makes it unlikely that a single patient characteristic explaining the different trajectories was missed and left out of the analysis. Another explanation is that adherence to HBE recommendations during treatment is mainly determined by factors outside of the patient, such as environmental factors, social factors, intervention-related factors, or therapist-related fac- tors. Indeed, a number of the factors related to adherence reported in the literature are external factors not directly related to the patient (8,10). For instance, a recent pilot study showed very high adherence when patients received external support in the form of telemonitoring and regular check-ups from their physical therapist (40). However, this would mean that external factors are far more important than patient factors for patient adherence during treatment. Although possible, it seems unlikely that patients have little influence on their own adherence to HBE recommendations during physical therapist treatment. A more plausible explanation is that adherence to HBE recommendations is not determined by baseline patient characteristics alone, but also by the change in these characteristics over time as treatment progresses and interactions between patients, their environments, and their physical therapists. For example, a physical therapist can incorporate strategies to support or increase self-efficacy in patients with low self-efficacy at the start of treatment in an attempt to increase adherence during treatment. For future research, 6
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