Customized passive-dynamic ankle-foot orthoses can improve walking economy and speed for many individuals post-stroke.

BACKGROUND: Passive-dynamic ankle-foot orthoses (PD-AFOs) are often prescribed to address plantar flexor weakness during gait, which is commonly observed after stroke. However, limited evidence is available to inform the prescription guidelines of PD-AFO bending stiffness. This study assessed the extent to which PD-AFOs customized to match an individual's level of plantar flexor weakness influence walking function, as compared to No AFO and their standard of care (SOC) AFO. METHODS: Mechanical cost-of-transport, self-selected walking speed, and key biomechanical variables were measured while individuals greater than six months post-stroke walked with No AFO, with their SOC AFO, and with a stiffness-customized PD-AFO. Outcomes were compared across these conditions using a repeated measures ANOVA or Friedman test (depending on normality) for group-level analysis and simulation modeling analysis for individual-level analysis. RESULTS: Twenty participants completed study activities. Mechanical cost-of-transport and self-selected walking speed improved with the stiffness-customized PD-AFOs compared to No AFO and SOC AFO. However, this did not result in a consistent improvement in other biomechanical variables toward typical values. In line with the heterogeneous nature of the post-stroke population, the response to the PD-AFO was highly variable. CONCLUSIONS: Stiffness-customized PD-AFOs can improve the mechanical cost-of-transport and self-selected walking speed in many individuals post-stroke, as compared to No AFO and participants' standard of care AFO. This work provides initial efficacy data for stiffness-customized PD-AFOs in individuals post-stroke and lays the foundation for future studies to enable consistently effective prescription of PD-AFOs for patients post-stroke in clinical practice. TRIAL REGISTRATION: NCT04619043.

Metatarsophalangeal Joint Dynamic Stiffness During Toe Rocker Changes With Walking Speed.

Dynamic joint stiffness (or simply "stiffness") is a customization criteria used to tune mechanical properties of orthotic and prosthetic devices. This study examines metatarsophalangeal (MTP) joint stiffness during the toe-rocker phase of barefoot walking and establishes baseline characteristics of MTP joint stiffness. Ten healthy individuals walked at 4 speeds (0.4, 0.6, 0.8, and 1.0 statures·s-1) over level ground. MTP sagittal plane joint angles and moments were calculated during the toe-rocker phase of stance. Least-squares linear regressions were conducted on the MTP moment versus angle curve to determine joint stiffness during early toe rocker and late toe rocker. Multilevel linear models were used to test for statistically significant differences between conditions. Early toe rocker stiffness was positive, while late toe rocker was negative. Both early toe rocker and late toe rocker stiffness increased in magnitude significantly with speed. This study establishes baseline characteristics of MTP joint stiffness in healthy walking, which previously had not been examined through a range of controlled walking speeds. This information can be used in the future as design criteria for orthotic and prosthetic ankle and ankle-foot devices that can imitate, support, and facilitate natural human foot motion during walking better than existing devices.

Comparison of Existing Methods for Characterizing Bi-Linear Natural Ankle Quasi-Stiffness.

Natural ankle quasi-stiffness (NAS) is a mechanical property of the ankle joint during dynamic motion. NAS has been historically calculated as the average slope (linear regression) of the net ankle moment versus ankle angle during discrete phases of stance. However, recent work has shown that NAS is nonlinear during the stance phase. Specifically, during the loading phase of stance (∼10 to 60% of total stance), plantarflexion moment increases at an accelerating rate compared to dorsiflexion angle. Updated models have been developed to better capture this inherent nonlinearity. One type of model called bi-linear NAS (BL-NAS) divides the loading phase of stance into two subphases, called early loading (EL) and late loading (LL) NAS. Two papers, written by Crenna and Frigo (2011, "Dynamics of the Ankle Joint Analyzed Through Moment-Angle Loops During Human Walking: Gender and Age Effects," Hum. Mov. Sci., 30(6), pp. 1185-1198) and Shamaei et al. (2013, "Estimation of Quasi-Stiffness and Propulsive Work of the Human Ankle in the Stance Phase of Walking," PLoS One, 8(3), p. e59935), outline different BL-NAS models. Both models fit measured data better (lower root-mean-squared error (RMSE)) than standard single linear NAS (SL-NAS) models but have not been widely adopted, possibly because of methodological discrepancies and lack of applicability to physical devices at the time. This paper compares and contrasts these existing BL-NAS models and translates those findings to possible orthotic device designs. Results showed that both BL-NAS models had lower RMSE than SL-NAS, EL-NAS was not significantly different across walking speeds, and LL-NAS increased significantly at faster walking speeds. These improved models of NAS much better approximate natural human movement than commonly used SL-NAS models, and thus provide a basis to design ankle-foot devices with multiple stiffness properties to emulate and facilitate natural human motion.

Nonlinear net ankle quasi-stiffness reduces error and changes with speed but not load carried.

BACKGROUND: Natural ankle quasi-stiffness (NAS) is a key metric used to personalize orthotic and prosthetic ankle-foot devices. NAS has traditionally been defined as the average slope (i.e. linear regression) of the net ankle moment vs. ankle angle curve during stance. However, NAS appears to have nonlinear characteristics. Characterizing nonlinear NAS across a wide range of tasks will enable us to incorporate these attributes into future orthotic and prosthetic ankle-foot device designs. RESEARCH QUESTION: Does nonlinear NAS change across multiple intensities of walking, running, and load carriage tasks? METHODS: This observational study examined 22 young, healthy individuals as they walked, ran, and walked while carrying a load at three intensities (speed or load). Linear, quadratic, and cubic regressions were done on the net ankle moment vs. ankle angle curve over three phases of stance: impact, loading, and push-off. RMSE between regressions and measured data were computed to determine regression accuracy, and multilevel linear models (MLMs) were used to determine significant differences between coefficients across intensities. RESULTS: Quadratic and cubic regressions of NAS had significantly lower RMSE than linear NAS for all phases of stance. Because of diminishing reductions in RMSE between quadratic and cubic regressions, only quadratic regression coefficients were further analyzed. Most first (linear) and second (nonlinear) order coefficients of quadratic regressions exhibited clear trends with respect to changes in walking or running speed, but not to increases in load. SIGNIFICANCE: This was the first study to our knowledge to thoroughly characterize nonlinear NAS across multiple gait tasks and intensities. This study provides an advanced understanding of the characteristics of nonlinear NAS for the design of future prosthetic and orthotic ankle-foot devices.