Postural instability in Parkinson's disease (PD) is commonly assessed by the pull test. This clinical test may be biased by the variability of the pull force applied. Our objective was to study the postural responses elicited by reproducible pull forces in healthy subjects and PD patients at different stages of the disease. We performed a multimodal approach that included a systematic analysis of the pull force needed to reach the backward limit of stability (FBLoS) assessed by mechanically produced forces, the displacements of the center of pressure (CoP) recorded on a force platform, and the latencies and patterns of activation of the stabilizing muscles. Comparisons between groups were performed by univariate and multivariate statistical analyses. Sixty-four healthy subjects and 32 PD patients, 22 Hoehn-Yahr (H-Y) stages I-II and 10 H-Y stage III, were studied. In healthy subjects, FBLoS decreased with aging and was lower in females. Mean (SD) FBLoS was 98.1 (48.9) Newtons (N) in healthy subjects, 70.5 (39.8) N in PD patients H-Y stages I-II, and 37.7 (18.9) N in PD patients H-Y stage III. Compared to healthy subjects and when adjusted for age and gender, PD patients H-Y stages I-II exhibited the following: (a) a reduced FBLoS; (b) larger CoP displacements and higher velocities for the same applied force; and (c) combined ankle and hip strategies elicited by less intense pull forces. All of these abnormalities were more pronounced in H-Y stage III PD patients compared to H-Y stages I-II PD patients. In conclusion, patients in the early stages of PD already exhibit a degree of postural instability due to inefficient postural adjustments, and they can more easily be destabilized by small perturbations than healthy subjects. This balance impairment becomes more pronounced in more advanced PD. In the pull test, pull force to step back should be a variable to consider when testing balance in clinical practice.

中文翻译：

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帕金森病的早期姿势不稳定：拉力测试的生物力学分析。

帕金森病 (PD) 的姿势不稳定性通常通过拉力测试进行评估。该临床测试可能会因所施加的拉力的变化而产生偏差。我们的目的是研究健康受试者和帕金森病患者在疾病不同阶段由可重复的拉力引起的姿势反应。我们采用了多模式方法，包括系统分析达到由机械产生的力评估的向后稳定性极限 (FBLoS) 所需的拉力、力平台上记录的压力中心 (CoP) 位移以及延迟以及稳定肌肉的激活模式。通过单变量和多变量统计分析进行组间比较。研究对象包括 64 名健康受试者和 32 名 PD 患者，其中 22 名 Hoehn-Yahr (HY) I-II 期患者和 10 名 HY III 期患者进行了研究。在健康受试者中，FBLoS 随着年龄的增长而下降，并且女性较低。健康受试者的平均 (SD) FBLoS 为 98.1 (48.9) 牛顿 (N)，PD 患者 HY I-II 期为 70.5 (39.8) N，PD 患者 HY III 期为 37.7 (18.9) N。与健康受试者相比，在调整年龄和性别后，PD 患者 HY I-II 期表现出以下特征：(a) FBLoS 降低；(b) 对于相同的作用力，CoP 位移更大，速度更高；(c) 由不太强烈的拉力引起的踝关节和髋关节的联合策略。与 HY I-II 期 PD 患者相比，所有这些异常在 HY III 期 PD 患者中更为明显。总之，PD早期患者由于姿势调整效率低下，已经表现出一定程度的姿势不稳定，并且他们比健康受试者更容易因微小的扰动而不稳定。这种平衡障碍在更严重的帕金森病中变得更加明显。在拉力测试中，后退的拉力应该是临床实践中测试平衡时要考虑的一个变量。