Improving Walking Economy With an Ankle Exoskeleton Prior to Human-in-the-Loop Optimization

Lower limb robotic exoskeletons have shown the capability to enhance human locomotion for healthy individuals or to assist motion rehabilitation and daily activities for patients. Recent advances in human-in-the-loop optimization that allowed for assistance customization have demonstrated great potential for performance improvement of exoskeletons. In the optimization process, subjects need to experience multiple types of assistance patterns, thus, leading to a long evaluation time. Besides, some patterns may be uncomfortable for the wearers, thereby resulting in unpleasant optimization experiences and inaccurate outcomes. In this study, we investigated the effectiveness of a series of ankle exoskeleton assistance patterns on improving walking economy prior to optimization. We conducted experiments to systematically evaluate the wearers' biomechanical and physiological responses to different assistance patterns on a lightweight cable-driven ankle exoskeleton during walking. We designed nine patterns in the optimization parameters range which varied peak torque magnitude and peak torque timing independently. Results showed that metabolic cost of walking was reduced by 17.1 ± 7.6% under one assistance pattern. Meanwhile, soleus (SOL) muscle activity was reduced by 40.9 ± 19.8% with that pattern. Exoskeleton assistance changed maximum ankle dorsiflexion and plantarflexion angle and reduced biological ankle moment. Assistance pattern with 48% peak torque timing and 0.75 N·m·kg−1 peak torque magnitude was effective in improving walking economy and can be selected as an initial pattern in the optimization procedure. Our results provided a preliminary understanding of how humans respond to different assistances and can be used to guide the initial assistance pattern selection in the optimization.

Real-world walking economy: can laboratory equations predict field energy expenditure?

We addressed a practical question that remains largely unanswered after more than a century of active investigation: can equations developed in the laboratory accurately predict the energy expended under free-walking conditions in the field? Seven subjects walked a field course of 6415 meters that varied in gradient (-3.0 to +5.0%) and terrain (asphalt, grass) under unloaded (body weight only, Wb) and balanced, torso-loaded (1.30 x Wb) conditions at self-selected speeds while wearing portable calorimeter and GPS units. Portable calorimeter measures were corrected for a consistent measurement-range offset (+13.8±1.8%, mean±sd) vs. a well-validated laboratory system (Parvomedics TrueOne). Predicted energy expenditure totals (mls O2/kg) from four literature equations: ACSM, Looney, Minimum Mechanics and Pandolf, were generated using the speeds and gradients measured throughout each trial in conjunction with empirically determined terrain/treadmill factors (asphalt=1.0, grass=1.08). The mean energy expenditure total measured for the unloaded field trials (981±91 mls O2/kg) was over-predicted by +4%, +13%, +17% and +20% by the Minimum Mechanics, ACSM, Pandolf, and Looney equations, respectively (corresponding predicted totals: 1018±19, 1108±26, 1145±37, and 1176±24 mls O2/kg). The measured loaded-trial total (1310±153 mls O2/kg) was slightly under-predicted by the Minimum Mechanics equation (-2%, 1289±22 mls O2/kg) and over-predicted by the Pandolf equation (+13%, 1463±32 mls O2/kg). Computational comparisons for hypothetical trials at different constant speeds (range: 0.6-1.8 m/s) on variable-gradient loop courses revealed between-equation prediction differences from 0 to 37%. We conclude that treadmill-based predictions of free-walking field energy expenditure are equation-dependent but can be highly accurate with rigorous implementation.

Factors influencing self-selected walking speed in fibrotic interstitial lung disease

This study aimed to investigate the walking economy and possible factors influencing self-selected walking speed (SSWS) in patients with fibrotic interstitial lung disease (ILD) compared to controls. In this study, 10 patients with ILD (mean age: 63.8 ± 9.2 years, forced expiratory volume in the first second: 56 ± 7% of predicted) and 10 healthy controls underwent resting pulmonary function tests, cardiopulmonary exercise, and submaximal treadmill walking tests at different speeds. The walking economy was assessed by calculating the cost-of-transport (CoT). Dynamic stability was assessed by stride-to-stride fluctuations using video recordings. Patients with ILD showed reduced peak oxygen uptake with a tachypneic breathing pattern and significant oxygen desaturation during exercise. The CoT did not differ between the groups (p = 0.680), but dyspnea and SpO2 were higher and lower, respectively, in patients with ILD at the same relative speeds. SSWS was reduced in ILD patients (2.6 ± 0.9 vs. 4.2 ± 0.4 km h−1p = 0.001) and did not correspond to the energetically optimal walking speed. Dynamic stability was significantly lower in patients with ILD than in healthy controls, mainly at lower speeds. Patients with ILD presented a similar cost of transport compared to healthy controls; however, they chose lower SSWS despite higher walking energy expenditure. Although walking stability and dyspnea were negatively affected, these factors were not associated with the slower walking speed chosen by individuals with ILD.

Energy cost of walking and functional aerobic capacity during moderate intensity exercise in adults with obstructive sleep apnea: a cross-sectional study

Background Autonomic dysregulation associated with obstructive sleep apnea (OSA) may limit cardiopulmonary responses to exercise, which, in turn, may impair functional aerobic capacity (FAC) and walking economy. We aimed to characterize walking economy and FAC in OSA patients compared with healthy adults (non-OSA) and examine their relationship with OSA severity (apnea-hypopnea index [AHI]). Methods A total of 26 adults (OSA, n = 13; non-OSA, n = 13) participated in this cross-sectional study. In this study, the participants with OSA were between the ages of 25 and 60 years, with a body mass index of 25 kg/m2 to 39 kg/m2, and who had undergone a recent third-party sleep study with an AHI of 5 or greater. Participants completed a maximal integrated cardiopulmonary exercise test, three separate exercise bouts of constant work rate (CWR) treadmill test at 85% of anaerobic threshold (AT), and a 10-min walk test (10MWT). Multiple linear regression analysis corrected for weight, age, and BMI were performed to examine the associations. Results There were significant differences between OSA and non-OSA participants in VO2peak (29.7 ± 5.6 mL/kg/min vs. 37.5 ± 6.5 mL/kg/min, p = 0.03) and Net VO2 during CWR (12.7 ± 5 vs.19 ± 6 mL/kg/min, p = 0.02). The 10MWT speed and distance were significantly lower in the OSA group (all p < 0.001). The energy cost of walking during submaximal exercise and 10-min walk test was higher among patients with OSA (all p < 0.001). The AHI scores were associated with 10MWT distance (R2 = 0.85, p < 0.001), energy cost of walking (R2 = 87, p < 0.001), and VO2 at anaerobic threshold (R2 = 0.92, p < 0.001). Conclusions The findings of this study show that patients with OSA have reduced FAC and a higher energy cost of walking. AHI explained 87% of variance in the energy cost of walking during the 10MWT. The results suggest that individuals with more severe obstructive sleep apnea experience greater impairment in functional performance.

Energy Cost of Walking and Functional Aerobic Capacity During Moderate Intensity Exercise in Adults with Obstructive Sleep Apnea: A Cross-Sectional Study

Background: Autonomic dysregulation associated with obstructive sleep apnea (OSA) may limit cardiopulmonary responses to exercise which, in turn, may impair functional aerobic capacity (FAC) and walking economy. We aimed to characterize walking economy and FAC in OSA patients compared with healthy adults (non-OSA) and examine their relationship with OSA severity (apnea-hypopnea index [AHI]). Participants: In this cross-sectional study, a total of 26 adults (OSA: n=13; non-OSA: n=13) participated in the study. In this study, the participants with OSA were adults between the ages of 25 and 60 with a body mass index between 25 kg/m2 and 39 kg/m2 who had undergone a recent third-party sleep study with an AHI of 5 or greater. Methods: Participant completed a maximal integrated cardiopulmonary exercise test, three separate exercise bouts of a constant work rate (CWR) treadmill test at 85% of anaerobic threshold (AT), and a 10-minute walk test (10MWT). Multiple linear regression corrected for weight, age, and BMI was conducted to examine the associations. Results: There were significant differences between OSA and Non-OSA participants in VO2peak (29.7±5.6mL/kg/min vs. 37.5±6.5mL/kg/min, p=0.03) and in Net VO2 during CWR (12.7±5 vs.19±6mL/kg/min, p=0.02). The 10MWT speed, distance, and energy expenditure were significantly lower in the OSA group (all p<0.001). The AHI scores associated with 10MWT distance (R2=0.85, p<0.001), energy cost of walking (R2=87, p<0.001), VO2 at anaerobic threshold (R2=0.92, p<0.001). Conclusions: The findings of this study show that patients with OSA have reduced FAC and have a higher energy cost of walking. AHI explained 87% of the variance in the energy cost of walking during the 10MWT. The results suggest that individuals with more severe obstructive sleep apnea experience greater impairment in functional performance.

Development of an Unpowered Energy Recycling Exoskeleton for Walking Assist

Background A reduction of energy used during walking could present a significant advantage for both healthy individuals and those with reduced mobility. Current unpowered exoskeletons have shown a net reduction in energy consumption for walking, but commonly with a clutch and assist on the heel. This paper presents the development of a lightweight energy recycling exoskeleton without a clutch and assists on the forefoot. Methods Eight healthy participants were tested during walking at 1.25 m/s on a treadmill wearing exoskeletons of four kinds of conditions. Electromyography (EMG) of the soleus muscle and gastrocnemius muscle were collected. Results Our results showed that the novel exoskeleton could make the peak and average EMG-values of soleus muscles drop about 13% and 8% respectively, and that of gastrocnemius muscles drop about 12% and 13% respectively. Conclusions The results of the present work demonstrate for the first time that the novel energy recycling exoskeleton can improve the walking economy, and this device could be feasibly worn for a broad range of individuals.