A new study shows, that in test group of seven healthy wearers, they clearly saw that the more assistance provided to the ankle joints, the more energy the wearers could save with a maximum reduction of almost 23% compared to walking with the exosuit powered-off. This is the highest relative reduction in energy expenditure observed to date with a tethered exoskeleton or exosuit.
Wearers significantly adapted their gait with increasing levels of assistance. The changes were most significant at the ankle joint but also at the hip as the exosuit included straps coupling the assistance from the back of the lower legs to the front of the hip in a beneficial manner.
When defining requirements for any wearable robot for walking assistance, it is important to maximize the user’s metabolic benefit resulting from the exosuit assistance while limiting the metabolic penalty of carrying the system’s mass. Thus, the aim of this study was to isolate and characterize the relationship between assistance magnitude and the metabolic cost of walking while also examining changes to the wearer’s underlying gait mechanics. The study was performed with a tethered multiarticular soft exosuit during normal walking, where assistance was directly applied at the ankle joint and indirectly at the hip due to a textile architecture. The exosuit controller was designed such that the delivered torque profile at the ankle joint approximated that of the biological torque during normal walking. Seven participants walked on a treadmill at 1.5 meters per second under one unpowered and four powered conditions, where the peak moment applied at the ankle joint was varied from about 10 to 38% of biological ankle moment (equivalent to an applied force of 18.7 to 75.0% of body weight). Results showed that, with increasing exosuit assistance, net metabolic rate continually decreased within the tested range. When maximum assistance was applied, the metabolic rate of walking was reduced by 22.83 ± 3.17% relative to the powered-off condition (mean ± SEM).
Humans naturally walk in a manner that conserves energy; we optimize our cadence, step length, and arm swing to minimize metabolic energy consumption. It is a commonly held belief that deviations from this normal walking pattern increase energy expenditure. Certain diseases that affect gait, such as stroke, Parkinson’s disease, and cerebral palsy, increase the net energy expenditure of walking by as much as 70% compared with healthy individuals. In addition, the energy expenditure of healthy individuals increases under strenuous activities, such as walking uphill or carrying heavy loads. Such increases in metabolic consumption could lead to greater levels of fatigue and injury. For patient populations, the added effort could also decrease community involvement.
With increasing exosuit assistance, the net metabolic rate of walking continually decreased within the tested range. Under the MAX condition, the metabolic rate of walking was reduced by 22.83 ± 3.17% relative to the powered-off condition. At the time of this submission, this is the highest relative reduction reported with a tethered exoskeleton or exosuit.
The fact that we did not see diminishing returns of metabolic reduction with increasing assistance differs from other parameter sweep studies with ankle assistive devices (exoskeletons and prostheses), which found either a leveling off or increase in metabolic rate. Of these sweep studies, the most similar one was conducted by Jackson and Collins using a unilateral exoskeleton. In their study, metabolic rate decreased as net exoskeleton work rate at the ankle increased up to about 0.19 W kg−1 but remained similar after that. In our study, net exoskeleton work rate was increased up to 0.19 W kg−1 at both ankles, with higher values not being achievable because of limitations in the actuation system used. Thus, it is currently unknown how further increasing the exosuit assistance would result in further decreases in metabolic rate. Furthermore, it can be expected that results from such studies may differ when assistance is applied bilaterally versus unilaterally as well as with different hardware and control approaches, and thus, further exploration is required to understand the effects of increased assistance with the exosuit.
A possible insight into the large metabolic reduction found in our study may be the fact that we observed significant decreases in biological moments and powers at the target joints. Both the ankle and hip biological (total minus exosuit) moments significantly decreased in magnitude as exosuit assistance increased. Similarly, positive biological power generation decreased significantly at the hip as exosuit assistance increased and biological power generation at the ankle decreased, but not significantly.
The reduction in hip power is likely a combined result of (i) direct assistance by the exosuit due to its multiarticular nature and (ii) energy transfer between the hip and other joints as was reported in other studies (16, 17, 24). Unlike most other exoskeletons, which directly target only ankle plantar flexion, the multiarticular load path also transmits force to the hip joint, assisting hip flexion during late stance and early swing. Under the MAX condition, we calculated that 43.5% of peak hip flexion moment and 6.7% of hip positive work were assisted by the exosuit relative to the powered-off condition. Independent of the ankle assistance, this amount of hip assistance is already considerable compared with previous studies on hip exoskeletons showing metabolic reductions.
In addition, several studies support point (ii), the hypothesis that energy is transferred between the hip and the ankle, reducing the moment and power requirements of the hip. Koller et al. (16) and Mooney and Herr (17) both found significant decreases in biological hip moment and power while assisting with ankle-only exoskeletons. In addition, another study by Lewis and Ferris (24) that did not involve an exoskeleton found a decrease in hip moment and power while participants intentionally walked with increased ankle push-off. These results suggest that increased plantar flexion power at the ankle (either voluntary or from external assistance) during push-off can be transferred through the lower limb linkage and thus may reduce hip flexion power requirements. The current embodiment of the suit, including coupling of the hip and ankle via the multiarticular straps, may further facilitate the energy transfer between joints, potentially making locomotion more efficient. This echoes recommendations from a simulation study on exotendons, suggesting that having tendons span multiple joints and crossing from the posterior to the anterior side of the leg may be energetically beneficial for human walking (26). We thus hypothesize that a combined effect of (i) and (ii) is likely and that the multiarticular nature of the exosuit may have added to the effects of energy transfer with the ankle.
However, the high metabolic reduction found in this study and an increase in the duration of positive power phase do not necessarily imply that such ankle joint behavior is energetically more efficient; rather, this may imply that, when provided with external assistance from a wearable robot, people alter their gait to maximize the benefit they receive, perhaps in an attempt to optimize their walking energetics. For example, in unassisted walking, the removal of the negative power absorption phase during mid-stance may be energetically detrimental because it would prevent the Achilles tendon from storing energy during stance phase to be released during push-off. However, with external plantar flexion assistance, it may be more optimal to have an increased positive power phase at the ankle where a wearer can acquire more positive work directly from the exosuit. Further studies are required to understand such a trade-off.
Although this study demonstrates a high metabolic reduction when comparing an exosuit powered versus unpowered, we acknowledge that there are a number of limitations to this work. First, the precise mechanism for this high metabolic reduction remains somewhat unclear. For example, it is currently unknown whether the assistance of the hip or ankle contributed more to the metabolic reduction and how the coupling of the two via the multiarticular straps contributed to the reduction. As a result, subsequent studies focused on providing assistance separately to the hip and ankle as well as with and without multiarticular straps could help better separate the impact of both joints as well as the impact of direct assistance and energy transfer between joints. In addition, subsequent studies that focused on better understanding the underlying muscle-tendon dynamics could provide further insight into contributions to metabolic reduction. Potential studies include exosuit experiments with electromyographic measurements or muscle-level imaging techniques, as well as musculoskeletal modeling and simulation work based on experimental data.
Furthermore, our ability to identify the complete relationship between exosuit assistance and metabolic rate was limited by the actuation system used. With increasing assistance, the metabolic rate continually decreased without diminishing returns within the tested range, but without further increasing assistance, the maximum potential metabolic reduction achievable with this exosuit architecture and control approach is currently unknown. Additional studies that sweep to higher magnitudes of assistance are required to determine whether the descending trend in metabolic rate continues.
Another limitation of the present study is that the baseline condition for comparison was a powered-off condition and not a no-suit condition. We chose to use a powered-off condition as opposed to a no-suit condition to reduce the length of testing sessions and to avoid repositioning the markers used for kinematic analysis, which could have led to increased variability in the kinematic and kinetic results. For the version of the exosuit used in this study, we have estimated an increase in metabolic cost in the range of 2.5 to 6.5% due to the weight of the textile, sensor, and attachment components compared with walking without the exosuit. Details are included in the Supplementary Materials.
Last, because the ultimate goal with such a system is to reduce the metabolic cost of walking, further studies are required with autonomous, body-worn systems. On the basis of the actuation parameters used in this study under the MAX condition (i.e., motor torque and speed), and knowing the mass of the suit components used in this study, a conservative total system mass estimate for an autonomous system is about 6 kg. This estimate is made up of ~4.9 kg for actuation and batteries worn around the waist, and suit components (textile, sensors, and attachments) distributed on the trunk, both shanks, and both feet of 0.443, 0.356, and 0.364 kg, respectively.