All statistical analyses were performed in SPSS 28.0 (IBM, New York). First, a visual inspection of the data, using histograms and Q-Q plots, was performed. Next, outcome measures were assessed for normality and equal variance with the Saphiro-Wilk and Levene’s Tests. The results to these tests motivated the choice for parametric or non-parametric testing. Categorical data (i.e., sex, handedness, tested side based on body side or dominance) were compared between groups using a Chi-Square test. All the continuous data (see Table 1), except for the force data and Kinesia ONE scores, were compared between groups using an Independent Samples T-Test. Since the force data did not meet the assumption for normality (p-values for the Saphiro-Wilk Test < 0.05), we proceeded with non-parametric statistics. Specifically, for each task the Mann-Whitney U Test was used to assess group differences in the following measures: normalized force amplitude, normalized rate of force increase, normalized rate of force decrease, SD of the normalized force amplitude, constant force error, absolute force error, and trial duration. For the same force measures, we performed a Wilcoxon Signed-Rank Test to assess limb differences (hand vs. foot), separately for PD and controls. Significance for statistical tests was set an α = 0.05. Spearman’s Rank-Order correlation analyses were conducted in the PD group with the goal to assess the relation between force control deficits (as determined by the previous group analyses) and the severity of motor symptoms as assessed by the total MDS-UPDRS-III, MDS-UPDRS-III bradykinesia subscores corresponding to the tested hand and foot, and the Kinesia ONE scores for the tested hand and foot. Additionally, we conducted a Mann-Whitney U Test to assess differences in speed-related force measures between patients in the earlier vs. more advanced stages of the disease. For this analysis, PD were grouped as PD with a Hoehn and Yahr stage 1 (i.e., unilateral symptoms; n = 5) and PD with a Hoehn and Yahr stage ≥ 2 (bilateral symptoms; n = 15: 14 Hoehn and Yahr Stage 2 + 1 Hoehn and Yahr Stage 3). More information on the clinical characteristics of these two PD groups is available in Table 3.
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