Enhancing performance during inclined loaded walking with a powered ankle-foot exoskeleton

A simple ankle-foot exoskeleton that assists plantarflexion during push-off can reduce the metabolic power during walking. This suggests that walking performance during a maximal incremental exercise could be improved with an exoskeleton if the exoskeleton is still efficient during maximal exercise intensities. Therefore, we quantified the walking performance during a maximal incremental exercise test with a powered and unpowered exoskeleton: uphill walking with progressively higher weights. Nine female subjects performed two incremental exercise tests with an exoskeleton: 1 day with (powered condition) and another day without (unpowered condition) plantarflexion assistance. Subjects walked on an inclined treadmill (15 %) at 5 km h(-1) and 5 % of body weight was added every 3 min until exhaustion. At volitional termination no significant differences were found between the powered and unpowered condition for blood lactate concentration (respectively, 7.93 +/- A 2.49; 8.14 +/- A 2.24 mmol L-1), heart rate (respectively, 190.00 +/- A 6.50; 191.78 +/- A 6.50 bpm), Borg score (respectively, 18.57 +/- A 0.79; 18.93 +/- A 0.73) and peak (respectively, 40.55 +/- A 2.78; 40.55 +/- A 3.05 ml min(-1) kg(-1)). Thus, subjects were able to reach the same (near) maximal effort in both conditions. However, subjects continued the exercise test longer in the powered condition and carried 7.07 +/- A 3.34 kg more weight because of the assistance of the exoskeleton. Our results show that plantarflexion assistance during push-off can increase walking performance during a maximal exercise test as subjects were able to carry more weight. This emphasizes the importance of acting on the ankle joint in assistive devices and the potential of simple ankle-foot exoskeletons for reducing metabolic power and increasing weight carrying capability, even during maximal intensities

Initial foot contact patterns during steady state shod running

C

The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity. Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m.s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work. Results: When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work. Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing

Analysis of walking with unilateral exoskeleton assistance compared to bilateral assistance with matched work

The finding of the highest negative metabolic rate versus mechanical work ratio in the Bilateral Matched Total Work condition means that if a constrained amount of mechanical work is available (e.g. from a battery) it is more advanta- geous to distribute this work evenly over both legs. The EMG reductions in the unassisted leg also suggest that if the goal is to maximize assistance to one (impaired) leg it might still be advantageous to use a bilateral exoskeleton, perhaps with a different actuation pattern for each leg that is specifically optimized such that each exoskeleton side assists specific phases in the impaired leg

Walking Easier by Attaching a Spring-Mass to the Body: A Preliminary Simulation

When carrying or pushing a load, more force is needed to accelerate and decelerate the additional mass, increasing Ground Reaction Force (GRF). However, this can be reduced by synchronizing the object\u27s movement with the individual\u27s Center of Mass (COM) using antiphase acceleration and deceleration. Past studies have shown this can decrease muscle work and metabolic costs. This study aimed to determine optimal spring parameters to minimize horizontal GRF (GRFh) when pushing a cart by connecting the human to the cart with a spring and damper system. Three walking conditions were simulated in MATLAB: 1) Normal walking; 2) Walking with a heavy cart (45 kg) attached to the person\u27s waist by a rigid bar, causing the person and cart to accelerate and decelerate in phase; and 3) Walking with the cart attached to the person\u27s waist by a spring with varying stiffness and damping, allowing for antiphase acceleration. The simulation results indicated that the greatest decrease in GRFh happens when the stiffness constant of the spring is 4360 N/m and the damping constant is 33 Ns/m. The most important finding of this simulation is that any spring with a constant below 5106 N/m leads to a reduction in GRFh, predicting that reducing the GRFh in human experiments will be feasible by starting with a soft spring and replacing it with higher stiffnesses if the constants stay below the optimum. Damping constants of up to 23230 Ns/m still allow reducing GRFh. These constants fall well within the range of existing springs and dampers, supporting the feasibility of lowering GRF with an actual prototype. High spring stiffnesses (\u3e 5106 N/m) rapidly increase the GRFh above the level of normal walking and even above the level experienced if the person were connected to a rigid bar. In this range of stiffness, even a low acceleration of the person leads to a high amplitude of cart acceleration. Additionally, High damping causes a delay in cart acceleration leading to unsynchronized acceleration of the person and cart. Low damping increases the cart\u27s acceleration fluctuation and results in inconsistent acceleration. The simulation results show that reducing GRFh while pushing an object is possible by attaching it to the waist with a soft spring. This could lead to designing a device to lower the energy cost of pushing an object while walking