Influence of Power Delivery Timing on the Energetics and Biomechanics of Humans Wearing a Hip Exoskeleton

Abstract

A broad goal in the field of powered lower limb exoskeletons is to reduce the metabolic cost of walking. Ankle exoskeletons have successfully achieved this goal by correctly timing a plantarflexor torque during late stance phase. Hip exoskeletons have the potential to assist with both flexion and extension during walking gait, but the optimal timing for maximally reducing metabolic cost is unknown. The focus of our study was to determine the best assistance timing for applying hip assistance through a pneumatic exoskeleton on human subjects. Ten non-impaired subjects walked with a powered hip exoskeleton, and both hip flexion and extension assistance were separately provided at different actuation timings using a simple burst controller. The largest average across-subject reduction in metabolic cost for hip extension was at 90% of the gait cycle (just prior to heel contact) and for hip flexion was at 50% of the gait cycle; this resulted in an 8.4 and 6.1% metabolic reduction, respectively, compared to walking with the unpowered exoskeleton. However, the ideal timing for both flexion and extension assistance varied across subjects. When selecting the assistance timing that maximally reduced metabolic cost for each subject, average metabolic cost for hip extension was 10.3% lower and hip flexion was 9.7% lower than the unpowered condition. When taking into account user preference, we found that subject preference did not correlate with metabolic cost. This indicated that user feedback was a poor method of determining the most metabolically efficient assistance power timing. The findings of this study are relevant to developers of exoskeletons that have a powered hip component to assist during human walking gait.

Keywords: biomechanics; hip exoskeleton; human walking; metabolic cost; powered orthosis; robotic control.

Figure 1

An experimental setup of one subject wearing our exoskeleton.

Figure 2

Metabolic cost of different powered conditions relative to the unpowered condition. The “optimized” condition for both hip flexion and extension was calculated by only using the best assistance timing (the timing that had the lowest metabolic cost) on a subject-by-subject basis. In all conditions, metabolic cost was lower on average compared to the unpowered condition. Stars indicate conditions that had significantly (p < 0.05) lower metabolic cost compared to the unpowered condition in the pairwise Bonferroni correction post hoc tests. Data are averaged across the 10 users, and error bars show ±1 SEM.

Figure 3

Box and whiskers plot showing user preference versus change in metabolic cost of walking between conditions. The black bars show the data median, and the diamonds show the data mean. The box indicates the range of the second and third data quartiles. The bars show the range of the first quartile (lower) and fourth quartile (upper). A negative change corresponded to a user selecting a condition that incurred an increased metabolic cost, while a positive change corresponded to a user selecting a condition that incurred a reduced metabolic cost. Distributions are shown for user preference comparisons (from left to right) between flexion conditions, between extension conditions, across flexion and extension conditions, and across powered and unpowered conditions. The compared conditions vary from subject to subject based on the randomized condition order, as user preference comparisons were made between consecutive conditions. From this figure, our results indicate that subject preference is a poor way to try to tune the assistance timing parameter for reducing metabolic cost.

Figure 4

Example of exoskeleton torque and power profiles for one subject across different timing conditions (top). Curves for torque (A) and power (B) are shown for only one subject (averaged across left and right side) normalized to body weight, but similar torque and power curves were observed across all subjects. The torque curve of the unpowered exoskeleton was subtracted from each of the powered curves such that the displayed graph shows only the difference between the powered and unpowered conditions. The timing for each condition is shown below the torque graph (bottom). Darker conditions correspond to hip extension and lighter to hip flexion. The duration of the power signal was always set to 25% of the gait cycle, with only the onset and offset timings varying between conditions.

Figure 5

Biomechanics of hip, knee, and ankle during powered hip extension conditions. Joint extension is always positive, and joint flexion is negative. It is important to note that the joint torques and powers presented are a combination of exoskeleton and human joint torque and power. The first column corresponds to the ankle, the second to the knee, and the third to the hip. The first row is joint angles, the second row is joint moments, and the third row is joint powers. Data are normalized to the gait cycle (0% indicates heel strike). Data were averaged across subjects, and shaded regions represent ±1 SD for the unpowered condition only.

Figure 6

Biomechanics of hip, knee, and ankle during powered hip flexion conditions. Joint extension is always positive, and joint flexion is negative. It is important to note that the joint torques and powers presented are a combination of exoskeleton and human joint torque and power. The first column corresponds to the ankle, the second to the knee, and the third to the hip. The first row is joint angles, the second row is joint moments, and the third row is joint powers. Data are normalized to the gait cycle (0% indicates heel strike). Data were averaged across subjects, and shaded regions represent ±1 SD for the unpowered condition only.