Department of Biomechanics and Center for Research in Human Movement Variability, University of Nebraska at Omaha, Omaha, Nebraska, United States of America.
Department of Movement and Sports Sciences, Ghent University, Ghent, Belgium.
PLoS One. 2018 Oct 24;13(10):e0205088. doi: 10.1371/journal.pone.0205088. eCollection 2018.
Exoskeletons can influence human gait. A healthy gait is characterized by a certain amount of variability compared to a non-healthy gait that has more inherent variability; however which exoskeleton assistance parameters are necessary to avoid increasing gait variability or to potentially lower gait variability below that of unassisted walking are unknown. This study investigated the interaction effects of exoskeleton timing and power on gait variability. Ten healthy participants walked on a treadmill with bilateral ankle-foot exoskeletons under ten conditions with different timing (varied from 36% to 54% of the stride) and power (varied from 0.2 to 0.5 W∙kg-1) combinations. We used the largest Lyapunov exponent (LyE) and maximum Floquet multiplier (FM) to evaluate the stride-to-stride fluctuations of the kinematic time series. We found the lowest LyE at the ankle and a significant reduction versus powered-off with exoskeleton power (summed for both legs) of 0.45 W∙kg-1 and actuation timing at 48% of the stride cycle. At the knee, a significant positive effect of power and a negative interaction effect of power and timing were found for LyE. We found significant positive interaction effects of the square of timing and power for LyE at the knee and hip joints. In contrast, the FM at the ankle increased with increasing power and later timing. We found a significant negative effect of power and a positive interaction effect of power and timing for FM at the knee and no significant effects of any of the exoskeleton parameters for FM at the hip. The ability of the exoskeleton to reduce the LyE at the ankle joint offers new possibilities in terms of altering gait variability, which could have applications for using exoskeletons as rehabilitation devices. Further efforts could examine if it is possible to simultaneously reduce the LyE and FM at one or more lower limb joints.
外骨骼可以影响人类的步态。与固有变异性较大的非健康步态相比，健康步态具有一定的可变性；然而，为了避免增加步态的可变性，或者为了使步态的可变性潜在地低于无辅助行走，需要哪些外骨骼辅助参数，目前还不得而知。本研究调查了外骨骼定时和功率对步态可变性的交互影响。10 名健康参与者在跑步机上用双侧踝足外骨骼行走，外骨骼的定时（从步幅的 36%到 54%变化）和功率（从 0.2 到 0.5 W∙kg-1 变化）组合各有 10 种。我们使用最大 Lyapunov 指数（LyE）和最大 Floquet 乘数（FM）来评估运动学时间序列的步间波动。我们发现踝关节处的 LyE 最低，与外骨骼功率（双腿之和）为 0.45 W∙kg-1 和定时在步幅周期的 48%时相比，LyE 显著降低。在膝关节处，我们发现 LyE 存在功率的显著正效应和功率与定时的负交互效应。我们发现膝关节和髋关节处 LyE 的定时平方和功率存在显著的正交互效应。相比之下，FM 随着功率的增加和定时的延迟而增加。我们发现 FM 在膝关节处存在功率的显著负效应和功率与定时的正交互效应，而在髋关节处不存在外骨骼参数的任何显著影响。外骨骼降低踝关节 LyE 的能力为改变步态可变性提供了新的可能性，这在外骨骼作为康复设备的应用中具有重要意义。进一步的研究可以探讨是否有可能同时降低一个或多个下肢关节的 LyE 和 FM。
