89 How to identify fallers and non-fallers in people with HSP vertebra (i.e., C7-marker) and the manubrium sterni (i.e., CLAV-marker). The pelvis was defined as the midpoint between the center of the left and right spina iliaca posterior superior and the left and right spina iliaca anterior superior (i.e., LPSI, RPSI, LASI and RASI-markers, respectively). The LDEs of the raw velocity of the spatially filtered trunk and pelvis trajectories were calculated over 65 strides per trial, as this was the maximum available number of strides across all participants. Further details on the calculation of the LDEs can be found in Supplementary Explanation 2. Statistical Analysis All analyses were conducted using IBM SPSS statistics (version 25) software. Data were first checked for normality using Shapiro-Wilk Test. Then, a series of independent samples t-tests (or Mann-Whitney U tests when appropriate) were used to assess differences between participants with HSP and healthy controls. To correct for multiple testing, level of significance was set at p < 0.01. Group differences were assessed for subjective balance confidence, objective clinical balance and gait capacity tests, and the biomechanical measures of gait stability derived from walking trials at comfortable gait speed. As most biomechanical measures of gait stability are known to be speed-dependent, 7, 28, 29 we repeated the analyses for both groups when walking at the same fixed treadmill speed. In these analyses, we only included participants with HSP who were able to perform the additional fixed-speed trials at either 0.6ms-1 or 0.8ms-1. We assessed the distribution of 0.6ms-1 and 0.8ms-1 among these participants with HSP, and pseudo-randomly selected a similar distribution of the 0.6ms-1 and 0.8ms-1 trials from the healthy controls. Lastly, differences between fallers and non-fallers among participants with HSP were assessed. First, a series of independent samples t-tests (or Mann-Whitney U tests when appropriate) were used to assess differences between fallers and non-fallers regarding the clinical capacity and gait stability measures extracted from the walking trials at comfortable gait speed. The ability of these measures to discriminate fallers from non-fallers was evaluated using the area under the curve (AUC) of the receiving operating characteristics curve (ROC). An AUC ≥0.9 was considered outstanding discrimination, ≥0.8 to <0.9 excellent discrimination, ≥0.7 to <0.8 acceptable discrimination, and <0.7 poor discrimination. 30 Results Participants A molecular diagnosis was available for 26 of the 33 participants with HSP (SPG4 (n=23); SPG5A (n=1); SPG8 (n=1); NEFL (n=1)). The mean SPRS score of the whole 6
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