## 2 Materials and Methods  
### Study Design  
The study was conducted at a single center and followed a prospective observational design. The data were collected from children with CP between February to November 2024. All children in the study population participated in the rehabilitation program called 'Auf die Beine' ('on your feet') at the Center of Prevention and Rehabilitation (University of Cologne, Germany). Participation in the program was funded by the German public health system.  
The rehabilitation program 'Auf die Beine' includes therapy sessions using whole body vibration training (WBV), along with additional individualized physiotherapy. At the beginning of the program, all children are assessed for their motor abilities using the GMFM-66 to evaluate potential therapy outcomes. Data collection for the present study was conducted at the start of the rehabilitation program.  
The inclusion criteria for this study were a diagnosis of CP [14], a written consent form provided by the patient's caregivers, and an age between 3 to 18 years. At the start of their rehabilitation stay, children participating in the 'Auf die Beine' therapy program at UniReha GmbH Cologene underwent a GMFM-66 assessment conducted by rehabilitation professionals. The rGMFM-66 was administered 1 to 2 days later. This brief interval between tests was designed to provide sufficient recovery time while ensuring that any therapeutic interventions during the rehabilitation stay did not immediately influence the rGMFM-66 results. In addition to these objectives, the time required to complete both the GMFM-66 and rGMFM-66 assessments was recorded. This was done to facilitate a direct comparison of the time efficiency between the two testing protocols. Moreover, the study was partially blinded. The GMFM-66 assessments were conducted by multiple physiotherapists and sports scientists, all of whom received the same internal training to ensure consistency. These therapists were blinded to whether the individuals they were testing were study participants or not. In contrast, the rGMFM-66 was conducted by a single physiotherapist who also received the same internal training as those performing the GMFM-66 assessments. This blinding for the GMFM-66 was implemented to reduce potential bias and ensure that the results were not influenced by the therapists' knowledge of the study. The physiotherapist who carried out the rGMFM-66 could not be blinded due to the method. Therefore, there was partial blinding. The assessor was aware of the general purpose of the study, including the evaluation of time efficiency and score agreement. A possible limitation could be that this physiotherapist may make an effort to perform the rGMFM-66 more quickly.  
All assessments were conducted by therapists with a minimum of one year of professional experience in the treatment of children with cerebral palsy. Prior to participation in the study, all assessors underwent standardized internal training aligned with the official GMFM-66 manual. This training included joint assessment sessions, video-based practice tasks, and interrater reliability exercises to ensure consistency in scoring and standardized administration procedures.  
In the literature, a median sample size of n = 42 is specified for studies on the estimation of intraclass correlation coefficients (ICC) [15]. In addition, a sample size estimation was carried out using Bonett's formula in order to determine the ICC with a predefined accuracy [16]. This resulted in a required sample size of 61 participants (with \(\alpha\) = 0.05, two raters, an expected ICC value of 0.99 based on preliminary investigations, and a desired confidence interval width of 0.01 for the 95% CI). Taking this preliminary information and practical considerations into account, it was decided to include a total of n = 50 children in the study.