115 Summary and general discussion As a consequence, an RCT may not be the best design to evaluate the effectiveness of novel interventions over a prolonged period during the life span in people with HSP. Moreover, recent qualitative research from our group has shown that the longterm perspective for people with HSP is not so much to expect improvement, but rather to maintain a particular level of functioning as long as possible. 37 These consideration impact on the selection of the most appropriate study design, mode of intervention, and type of outcomes. Single-case designs have recently been given increased attention in patient-centered healthcare research. 50 These so-called n-of-1 trials are prospective, crossover trials where each case is exposed to alternative interventions based on a predetermined order and time schedule. 51,52 The major drawback of this crossover design, however, is the risk of transfer effects from one intervention period to the other, unless transfer effects are impossible or mitigated by sufficiently long ‘wash-out periods’. This makes n-of-1 trials unfeasible for training interventions, where (retention of) learning effects is crucial. In order to evaluate whether a novel type of intervention can impose an effect in the long term in people with a slowly progressive condition, large-scale longitudinal cohort studies are needed, preferably multi-centered. Although this study design does not randomly assign people to specific interventions and relies more on the natural variation of physical training and activity in the target population, its strength is the long follow-up and the representativeness of the results for the entire population. Longitudinal cohort studies are well suited to detect differences in study parameters across time between subgroups due to differences in training, physical activity or lifestyle. Moreover, the influence of various patient characteristics on the study outcomes can be investigated. Future perspectives In the previous paragraphs, I have already given several suggestions for future research focused on the mobility problems in people with HSP. In summary, I have discussed the following. With regard to assessment, I have mentioned the relevance of research into ‘online’ gait adaptability testing to assess the capacity of patients to deal with unexpected disruptions of gait. The use of wearable sensors in the form of activity trackers would provide essential information about the real life performance of gait-related activities, while self-assessment of gait-related activities would add the important dimension of gait perception to the capacity and performance measures. Regarding people with HSP, research in all areas is still scarce, so there are many options to explore. Lastly, since falls are a major problem in this population, research into detecting the ‘tipping point’ where an individual shows increased fall risk would 7
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