40 Chapter 3 precision. Participants perform the test four times; first twice with one foot per target and, thereafter, twice with both feet in one target. A time penalty of 0.5s is added each time a participant makes a foot placement mistake. Spatiotemporal gait parameters Spatiotemporal gait parameters are collected with a 3D full body gait analysis using Vicon (Vicon© Motion systems Ltd.) at the movement laboratory of the Radboudumc, Nijmegen. Retroreflective markers are placed on anatomical landmarks according to the standard Plug-in Gait marker model for upper and lower body. In addition, participants will wear accelerometers on their lateral heels, as the higher measuring frequency will enable a more accurate gait event detection. Participants will walk two bouts of three minutes over an eight-meter walkway. The following spatiotemporal parameters will be retrieved: step length (cm), step width (cm), step time (s), walking speed (m/s), stride time (s), stride length (cm) and cadence (steps/min). Assessment of therapy adherence and co-interventions To support adherence to the protocol, participants will be in direct contact with the primary investigator (LV) by telephone every other week. This enables the investigator to verbally confirm assessments and training dates, check adherence to the fall calendar, and quickly address and resolve questions and possible problems that may interfere with continuation of the protocol. In addition, participants are offered flexible time slots for the assessments and training sessions. Assessment of adherence to the C-Mill protocol is possible as therapists will log the performed C-Mill trainings. In case of an unexpected cancellation, the reason will be registered, and the missed training can be compensated in the next week. When multiple consecutive training sessions cannot proceed, a pragmatic solution is sought so that the participant is able to complete the protocol. In addition, assessment of co-intervention will take place during the assessments. Participants are asked to self-report in a survey what type of co-intervention they received (e.g. physical therapy, occupation therapy). Sample size Sample size calculation is based on previous studies assessing effectiveness of C-Mill training on the obstacle subtask of the E-FAP scores in neurological populations.14, 15 A total of 32 participants is sufficient to demonstrate an improvement on the E-FAP score of 1.75 seconds (SD = 2.0s, α = 0.05, β = 0.2). Considering a 10% attrition rate, 36 participants will be included. Statistical analysis The effect of gait adaptability training on primary and secondary outcomes will be tested using ANCOVA. Post-intervention measurements will be used as dependent variables with pre-intervention measurements as the covariate. Group (C-Mill versus
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