Time-varying and speed-matched model for the evaluation of stroke-induced changes in ankle mechanics.

The ankle mechanics (stiffness and moment) are modulated continuously when interacting with the environment during human walking. However, it remains unclear how ankle mechanics vary with walking speeds, and how they are affected by stroke. This study aimed to determine time-varying ankle stiffness and moment in stroke participants during walking, comparing them with healthy participants at matched speeds. A motion capture system, surface electromyography (EMG) system and force plates were used to measure biomechanics of seven healthy participants walking at 5 controlled speeds and ten patients with stroke at self-selected speeds. The ankle moment and stiffness during the stance phase were calculated using an EMG-driven musculoskeletal model. Surface equations of ankle moment and stiffness in healthy participants, with walking speed and stance phase as variables, were proposed based on polynomial fitting. Results showed that as walking speed increased, there was an increase in the ankle stiffness and moment of healthy participants during 77 %-89 % and 63 %-91 % of stance phase, respectively. Patients with stroke had lower ankle stiffness and moment at self-selected walking speed than healthy participants at 1.04 m/s walking speed during 52 %-87 % and 52 %-91 % of stance phase, respectively. At matched walking speed, the peak values of ankle stiffness and moment in patients with stroke were significantly less than those in healthy participants (p = 0.007; p = 0.028, respectively). This study proposes a novel approach to evaluate the ankle mechanics of patients with stroke using the speed-matched model of healthy participants and may provide insights into understanding speed-dependent movement mechanisms of human walking.

The effect of existing and novel walker boot designs on offloading and gait mechanics.

BACKGROUND: Orthopedic walker boots are often used to treat foot ulcers and other wounds with the goal of offloading plantar pressure. However, poor ulcer healing outcomes and high recurrence rates show a need for additional solutions in the growing diabetes epidemic. We compared a novel spring-loaded walker boot to a traditional rigid ankle boot and a hinged ankle boot as well as a control shoe. Our aim was to better understand how boot design affects offloading mechanisms. We hypothesized that all boots would offload force from the foot to the shank, but that the hinged boot would have fewer gait alterations and the spring boot would further reduce pressure in early and late stance. METHODS: Ten healthy participants tested each of the four conditions in static stance and walking gait. Offloading was quantified by the difference between pressure insole and platform forces, while joint mechanics changes were calculated from instrumented gait analysis and inverse dynamics. RESULTS: Minimal offloading was found in the rigid and hinged boots compared to athletic shoes. In contrast, the spring boot offloaded nearly 50% of total load in static stance, with similarly large reductions in peak pressures during gait, particularly under the hindfoot during early stance. All boots resulted in some ankle joint mechanics compensations, with the rigid and spring boots showing similar restrictions in ankle motion and propulsive work. While the hinged boot resulted in ankle mechanics more like the shoe condition, it increased dorsiflexion and negative work, suggesting energetic inefficiency. CONCLUSIONS: The novel spring boot shows promise for more effective offloading that could lead to improved healing outcomes.

Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control.

BACKGROUND: Ankle exoskeletons offer a promising opportunity to offset mechanical deficits after stroke by applying the needed torque at the paretic ankle. Because joint torque is related to gait speed, it is important to consider the user's gait speed when determining the magnitude of assistive joint torque. We developed and tested a novel exoskeleton controller for delivering propulsive assistance which modulates exoskeleton torque magnitude based on both soleus muscle activity and walking speed. The purpose of this research is to assess the impact of the resulting exoskeleton assistance on post-stroke walking performance across a range of walking speeds. METHODS: Six participants with stroke walked with and without assistance applied to a powered ankle exoskeleton on the paretic limb. Walking speed started at 60% of their comfortable overground speed and was increased each minute (n00, n01, n02, etc.). We measured lower limb joint and limb powers, metabolic cost of transport, paretic and non-paretic limb propulsion, and trailing limb angle. RESULTS: Exoskeleton assistance increased with walking speed, verifying the speed-adaptive nature of the controller. Both paretic ankle joint power and total limb power increased significantly with exoskeleton assistance at six walking speeds (n00, n01, n02, n03, n04, n05). Despite these joint- and limb-level benefits associated with exoskeleton assistance, no subject averaged metabolic benefits were evident when compared to the unassisted condition. Both paretic trailing limb angle and integrated anterior paretic ground reaction forces were reduced with assistance applied as compared to no assistance at four speeds (n00, n01, n02, n03). CONCLUSIONS: Our results suggest that despite appropriate scaling of ankle assistance by the exoskeleton controller, suboptimal limb posture limited the conversion of exoskeleton assistance into forward propulsion. Future studies could include biofeedback or verbal cues to guide users into limb configurations that encourage the conversion of mechanical power at the ankle to forward propulsion. TRIAL REGISTRATION: N/A.

The effects of total ankle replacement on ankle joint mechanics during walking.

BACKGROUND: End-stage ankle arthritis impairs joint function and patients' mobility. Total ankle replacement is a surgical procedure to treat severe ankle arthritis. Salto Talaris Anatomic AnkleTM (STAA) was designed to mimic the normal ankle anatomy and flexion/extension of the ankle movement. The purpose of this study was to examine the effect of an STAA ankle replacement on ankle joint function and mechanics during gait. METHODS: Five patients with end-stage unilateral ankle arthritis were recruited. Patients performed level walking in a laboratory setting on 2 occasions, prior to and 3 months after the STAA ankle surgeries. American Orthopedic Foot and Ankle Society (AOFAS) hindfoot score was obtained. A 12-camera motion capture system was used to perform walking analysis. Gait temporo-spatial parameters and ankle joint mechanics were evaluated. Paired Student's t tests and non-parametric Wilcoxon matched tests were performed to examine the differences in biomechanical variables between the pre- and post-surgery walking conditions. RESULTS: Compared to the pre-surgical condition, at 3 months of post-STAA surgery, patients experienced greater improvement in AOFAS hindfoot score (p = 0.0001); the STAA ankle demonstrated a 31% increase in ankle joint excursion (p = 0.045), a 22% increase in ankle plantarflexor moment (p = 0.075), a 60% increase in ankle power absorption (p = 0.023), and a 68% increase in ankle power production (p = 0.039). Patients also demonstrated a 26% increase in walking speed (p = 0.005), a 20% increase in stride length (p = 0.013), a 15% decrease in double support time (p = 0.043), and a 5% decrease in total stance time (p = 0.055). CONCLUSION: Three months after surgeries, the STAA patients experienced improvements in ankle function and gait parameters. The STAA ankle demonstrated improved ankle mechanics during daily activities such as walking.

Ankle mechanics during sidestep cutting implicates need for 2-degrees of freedom powered ankle-foot prostheses.

The ankle joint of currently available powered prostheses is capable of controlling one degree of freedom (DOF), focusing on improved mobility in the sagittal plane. To increase agility, the requirements of turning in prosthesis design need to be considered. Ankle kinematics and kinetics were studied during sidestep cutting and straight walking. There were no significant differences between the ankle sagittal plane mechanics when comparing sidestep cutting and straight walking; however, significant differences were observed in ankle frontal plane mechanics. During straight walking, the inversion-eversion (IE) angles were smaller than with sidestep cutting. The ankle that initiated the sidestep cutting showed progressively increasing inversion from 2 to 13 degrees while the following contralateral step showed progressively decreasing inversion from 8 to -4 degrees during normal walking speed. The changes in IE kinematics were the most significant during sidestep cutting compared with straight walking. The IE moments of the step that initiated the sidestep cutting were always in eversion, acting as a braking moment opposing the inverting motion. This suggests that an ankle-foot prosthesis with active DOFs in the sagittal and frontal planes will increase the agility of gait for patients with limb loss.