Walking with a powered ankle-foot orthosis: the effects of actuation timing and stiffness level on healthy users

Abstract

Background: In the last decades, several powered ankle-foot orthoses have been developed to assist the ankle joint of their users during walking. Recent studies have shown that the effects of the assistance provided by powered ankle-foot orthoses depend on the assistive profile. In compliant actuators, the stiffness level influences the actuator's performance. However, the effects of this parameter on the users has not been yet evaluated. The goal of this study is to assess the effects of the assistance provided by a variable stiffness ankle actuator on healthy young users. More specifically, the effect of different onset times of the push-off torque and different actuator's stiffness levels has been investigated. Methods: Eight healthy subjects walked with a unilateral powered ankle-foot orthosis in several assisted walking trials. The powered orthosis was actuated in the sagittal plane by a variable stiffness actuator. During the assisted walking trials, three different onset times of the push-off assistance and three different actuator's stiffness levels were used. The metabolic cost of walking, lower limb muscles activation, joint kinematics, and gait parameters measured during different assisted walking trials were compared to the ones measured during normal walking and walking with the powered orthosis not providing assistance. Results: This study found trends for more compliant settings of the ankle actuator resulting in bigger reductions of the metabolic cost of walking and soleus muscle activation in the stance phase during assisted walking as compared to the unassisted walking trial. In addition to this, the study found that, among the tested onset times, the earlier ones showed a trend for bigger reductions of the activation of the soleus muscle during stance, while the later ones led to a bigger reduction in the metabolic cost of walking in the assisted walking trials as compared to the unassisted condition. Conclusions: This study presents a first attempt to show that, together with the assistive torque profile, also the stiffness level of a compliant ankle actuator can influence the assistive performance of a powered ankle-foot orthosis.

Personalizing exoskeleton assistance while walking in the real world

“…Consequently, fall time was effectively constrained to its lower bound of 10% stride. The large model weight on peak time is consistent with previous observations that the timing of ankle exoskeleton assistance is important 51 , and that later onset of torque assistance can correspond to larger improvements in metabolic rate 52 . The data-driven model also favoured larger peak torque magnitudes, with the associated term contributing about 4% to the total classification.…”

Harnessing Energy of a Treadmill for Push-Off Assistance During Walking: In-Silico Feasibility Study

“…In our simulations, the timing interval of positive ankle power encompasses only 13% of stride (from 43% to 56%). For synergistic activity of the ankle exoskeleton and the plantarflexor muscle-tendon complex, proper timing of the assistance is paramount ( Malcolm et al, 2013 ; Galle et al, 2017 ; Moltedo et al, 2018 ; Moltedo et al, 2020 ). Temporal parameters contributing to AN-EXTRA-Push assistance timing are brake engagement and disengagement timing.…”

“…In practice, this may also cause a change in kinematics, where instead of opposing the exoskeleton torque, the person would plantarflex the ankle further, which may lead to problems with toe clearance during swing. In previous studies on ankle exoskeleton assistance timing ( Malcolm et al, 2013 ; Galle et al, 2017 ; Moltedo et al, 2020 ), optimal timing of assistance termination was not investigated. Our results suggest that timing of assistance termination is key in ankle exoskeleton control and even small deviations from the proper assistance termination timing may be detrimental.…”

“…The main parameters shaping the ankle torque assistance profile provided by AN-EXTRA-Push are elastic tendon stiffness and brake engagement and disengagement timings. Previous research has shown the importance of optimal assistance power and timing in ankle exoskeleton-based rehabilitation ( Malcolm et al, 2013 ; Galle et al, 2017 ; Moltedo et al, 2018 ; Moltedo et al, 2020 ). Experimentally testing the entire possible parameter space would be a prohibitively time-consuming task.…”

Application of Wearable Sensors in Actuation and Control of Powered Ankle Exoskeletons: A Comprehensive Review

“…Classical proportional–integral–derivative (PID) feedback controllers [ 39 , 53 , 54 , 55 , 56 , 67 , 68 , 76 , 77 , 78 , 79 , 82 , 86 , 101 , 102 , 108 , 109 , 110 , 114 , 119 , 120 , 121 , 136 , 138 , 139 , 140 , 154 , 155 ] were the most frequently used control algorithms in the reviewed PAEs due to the simplicity of real-time implementation [ 239 ]. Integral, proportional, and derivative feedback is based on the past (I), present (P) and future (D) state of the system Three types of controllers have been devised based on the integral, proportional, and derivative feedback controllers that are widely used in PAEs: P [ 100 , 106 , 107 , 113 , 146 , 172 , 178 , 185 , 186 , 187 , 204 ], PI [ 85 , 100 , 105 , 178 , 185 , 186 , 187 ], and PD [ 10 ,…”