Date Published: October 3, 2018
Publisher: Public Library of Science
Author(s): Chase G. Rock, Shane R. Wurdeman, Nicholas Stergiou, Kota Z. Takahashi, Yih-Kuen Jan.
Stride-to-stride fluctuations of joint kinematics during walking reflect a highly structured organization that is characteristic of healthy gait. The organization of stride-to-stride fluctuations is disturbed in lower-limb prosthesis users, yet the factors contributing to this difference are unclear. One potential contributor to the changes in stride-to-stride fluctuations is the altered push-off mechanics experienced by passive prosthesis users. The purpose of our study was to determine if changes in push-off mechanics affect stride-to-stride fluctuations in transtibial amputees. Twenty-two unilateral transtibial amputees were enrolled in the 6-week cross-over study, where High and Low Activity (based on the Medicare Functional Classification System) prostheses were worn for three weeks each. Data collection took place at the end of the third week. Participants walked on a treadmill in a motion capture laboratory to quantify stride-to-stride fluctuations of the lower extremity joint angle trajectories using the largest Lyapunov Exponent, and over floor-embedded force platforms to enable calculating push-off work from the prosthesis and the sound limb. Push-off work was 140% greater in the High Activity prosthesis compared to the Low Activity prosthesis (p < 0.001), however no significant change was observed in stride-to-stride fluctuations of the ankle between the two prosthesis types (p = 0.576). There was no significant correlation between changes in prosthesis push-off work and the largest Lyapunov exponent. Though differences in push-off work were observed between the two prosthesis types, stride-to-stride fluctuations remained similar, indicating that prosthesis propulsion mechanics may not be a strong determinant of stride-to-stride fluctuations in unpowered transtibial prosthesis users.
During human walking, subtle fluctuations occur from stride to stride, causing the walking pattern to vary over time [1–3]. These stride-to-stride fluctuations are the end behavior of a complex motor system, which includes the various neural, muscular, and mechanical interactions of the body . Such interacting subsystems produce stride-to-stride fluctuations that have a highly structured organization [5–7]. It has been speculated that the temporal organization of the stride-to-stride fluctuations in healthy populations exhibit an optimal state of movement variability which is indicative of high adaptability . Optimal movement variability is observed when a system is neither too restrained nor too disordered . In contrast, stride-to-stride fluctuations exhibit an altered organization in those with compromised neuromuscular systems [8,9]. For example, altered stride-to-stride fluctuations are observed in people who have experienced lower limb loss resulting in reliance on a prosthetic limb for locomotion .
Our aim was to investigate how prostheses with different energy return characteristics affect stride-to-stride fluctuations at the ankle in individuals with a unilateral transtibial amputation. By calculating the push-off work of Low Activity and High Activity prostheses in the same user, we were able to specify how the prosthesis types differed in function. In addition, we investigated the stride-to-stride fluctuations of the sound and prosthetic ankle angle, in order to detect how stride-to-stride fluctuations may be affected by different prostheses.
We aimed to study how the changes in push-off work from prostheses may affect changes in stride-to-stride fluctuations. Distinctly different push-off work was observed between High and Low Activity prostheses. Push-off work from the High Activity prosthesis equaled about 50% of the push-off work value from the sound limb, while the Low Activity prosthesis provided less than 25%. Despite these differences in push-off work between the prostheses in this study, there was no significant change in the stride-to-stride fluctuations of the ankle, knee, and hip flexion angles. Stride-to-stride fluctuations may not be sensitive to the difference in prosthesis push-off work achieved by different prostheses (i.e., High Activity vs Low Activity), implying that stride-to-stride fluctuations arise from other, perhaps non-mechanical, factors.