Reducing the Energy Cost of Human Running Using an Unpowered Exoskeleton

Gearing Up the Human Ankle-Foot System to Reduce Energy Cost of Fast Walking

During locomotion, the human ankle-foot system dynamically alters its gearing, or leverage of the ankle joint on the ground. Shifting ankle-foot gearing regulates speed of plantarflexor (i.e., calf muscle) contraction, which influences economy of force production. Here, we tested the hypothesis that manipulating ankle-foot gearing via stiff-insoled shoes will change the force-velocity operation of plantarflexor muscles and influence whole-body energy cost differently across walking speeds. We used in vivo ultrasound imaging to analyze fascicle contraction mechanics and whole-body energy expenditure across three walking speeds (1.25, 1.75, and 2.0 m/s) and three levels of foot stiffness. Stiff insoles increased leverage of the foot upon the ground  (p < 0.001), and increased dorsiflexion range-of-motion (p < 0.001). Furthermore, stiff insoles resulted in a 15.9% increase in average force output (p < 0.001) and 19.3% slower fascicle contraction speed (p = 0.002) of the major plantarflexor (Soleus) muscle, indicating a shift in its force-velocity operating region. Metabolically, the stiffest insoles increased energy cost by 9.6% at a typical walking speed (1.25 m/s, p = 0.026), but reduced energy cost by 7.1% at a fast speed (2.0 m/s, p = 0.040). Stiff insoles appear to add an extra gear unavailable to the human foot, which can enhance muscular performance in a specific locomotion task.

A Hip Active Assisted Exoskeleton That Assists the Semi-Squat Lifting

(1) Background: In the case of quick picking and heavy lifting, the carrying action results in a much more active myoelectric signal in the lower back than in an upright stationary one, and there is a high risk of back muscle injury without proper handling skills and equipment. (2) Methods: To reduce the risk of LBP during manual handing tasks, a hip active exoskeleton is designed to assist human manual lifting. A power control method is introduced into the control loop in the process of assisting human transportation. The power curve imitates the semi-squat movement of the human body as the output power of the hip joint. (3) Results: According to the test, the torque can be output according to the wearer’s movement. During the semi-squat lifting process, the EMG (electromyogram) signal of the vertical spine at L5/S1 was reduced by 30–48% and the metabolic cost of energy was reduced by 18% compared the situation of without EXO. (4) Conclusion: The exoskeleton joint output torque can change in an adaptive manner according to the angular velocity of the wearer’s joint. The exoskeleton can assist the waist muscles and the hip joint in the case of the reciprocating semi-squat lifting movement.

A biarticular passive exosuit to support balance control can reduce metabolic cost of walking

Nowadays, the focus on the development of assistive devices just for people with mobility disorders has shifted towards enhancing physical abilities of able-bodied humans. As a result, the interest in the design of cheap and soft wearable exoskeletons (called exosuits) is distinctly growing. In this paper, a passive lower limb exosuit with two biarticular variable stiffness elements is introduced. These elements are in parallel to the hamstring muscles of the leg and controlled based on a new version of the FMCH (force modulated compliant hip) control framework in which the force feedback is replaced by the length feedback (called LMCH). The main insight to employ leg length feedback is to develop a passive exosuit. Fortunately, similar to FMCH, the LMCH method also predicts human-like balance control behaviours, such as the VPP (virtual pivot point) phenomenon, observed in human walking. Our simulation results, using a neuromuscular model of human walking, demonstrate that this method could reduce the metabolic cost of human walking by 10%. Furthermore, to validate the design and simulation results, a preliminary version of this exosuit comprised of springs with constant stiffness was built. An experiment with eight healthy subjects was performed. We made a comparison between the walking experiments while the exosuit is worn but the springs were slack and those when the appropriate springs were contributing. It shows that passive biarticular elasticity can result in a metabolic reduction of 14.7 [Formula: see text] 4.27%. More importantly, compared to unassisted walking (when exosuit is not worn), such a passive device can reduce walking metabolic cost by 4.68 [Formula: see text] 4.24%.

How to run 50% faster without external energy

Technological innovations may enable next-generation running shoes to provide unprecedented mobility. But how could a running shoe increase the speed of motion without providing external energy? We found that the top speed of running may be increased more than 50% using a catapult-like exoskeleton device, which does not provide external energy. Our finding uncovers the hidden potential of human performance augmentation via unpowered robotic exoskeletons. Our result may lead to a new-generation of augmentation devices developed for sports, rescue operations, and law enforcement, where humans could benefit from increased speed of motion.

The exoskeleton expansion: improving walking and running economy

Since the early 2000s, researchers have been trying to develop lower-limb exoskeletons that augment human mobility by reducing the metabolic cost of walking and running versus without a device. In 2013, researchers finally broke this 'metabolic cost barrier'. We analyzed the literature through December 2019, and identified 23 studies that demonstrate exoskeleton designs that improved human walking and running economy beyond capable without a device. Here, we reviewed these studies and highlighted key innovations and techniques that enabled these devices to surpass the metabolic cost barrier and steadily improve user walking and running economy from 2013 to nearly 2020. These studies include, physiologically-informed targeting of lower-limb joints; use of off-board actuators to rapidly prototype exoskeleton controllers; mechatronic designs of both active and passive systems; and a renewed focus on human-exoskeleton interface design. Lastly, we highlight emerging trends that we anticipate will further augment wearable-device performance and pose the next grand challenges facing exoskeleton technology for augmenting human mobility.

Biomechanical effects of passive hip springs during walking

Passive spring-like structures can store and return energy during cyclic movements and thereby reduce the energetic cost of locomotion. That makes them important components of the human body and wearable assistive devices alike. This study investigates how springs placed anteriorly across the hip joint affect leg joint angles and powers, and leg muscle activities during level walking at 0.5 to 2.1 m/s. We hypothesized that the anterior hip springs (I) load hip extension, (II) support hip flexion and (III) affect ankle muscle activity and dynamics during walking. Effects at the ankle were expected because hip and ankle redistribute segmental power in concert to achieve forward progression. We observed that the participants' contribution to hip power did not increase during hip extension as the spring stored energy. Simultaneously, the activities of plantarflexor muscles that modulate energy storage in the Achilles tendon were reduced by 28% (gastrocnemius medialis) and 9% (soleus). As the spring returned energy with the onset of hip flexion, the participants' contribution to hip power was reduced by as much as 23%. Soleus activity before push-off increased by up to 9%. Instead of loading hip extension, anterior hip springs seem to store and return parts of the energy normally exchanged with the Achilles tendon. Thereby, the springs support hip flexion but may reduce elastic energy storage in and hence recoil from the Achilles tendon. This interaction should be considered during the design and simulation of wearable assistive devices as it might - depending on user characteristics - enhance or diminish their overall functionality.

Reducing the metabolic rate of walking and running with a versatile, portable exosuit

Walking and running have fundamentally different biomechanics, which makes developing devices that assist both gaits challenging. We show that a portable exosuit that assists hip extension can reduce the metabolic rate of treadmill walking at 1.5 meters per second by 9.3% and that of running at 2.5 meters per second by 4.0% compared with locomotion without the exosuit. These reduction magnitudes are comparable to the effects of taking off 7.4 and 5.7 kilograms during walking and running, respectively, and are in a range that has shown meaningful athletic performance changes. The exosuit automatically switches between actuation profiles for both gaits, on the basis of estimated potential energy fluctuations of the wearer's center of mass. Single-participant experiments show that it is possible to reduce metabolic rates of different running speeds and uphill walking, further demonstrating the exosuit's versatility.

Reducing the energy cost of walking in older adults using a passive hip flexion device

BACKGROUND

Elevated energy cost is a hallmark feature of gait in older adults. As such, older adults display a general avoidance of walking which contributes to declining health status and risk of morbidity. Exoskeletons offer a great potential for lowering the energy cost of walking, however their complexity and cost often limit their use. To overcome some of these issues, in the present work we propose a passive wearable assistive device, namely Exoband, that applies a torque to the hip flexors thus reducing the net metabolic power of wearers.

METHODS

Nine participants (age: 62.1 ± 5.6 yr; height: 1.71 ± 0.05 m; weight: 76.3 ± 11.9 kg) walked on a treadmill at a speed of 1.1 m/s with and without the Exoband. Metabolic power was measured by indirect calorimetry and spatio-temporal parameters measured using an optical measurement system. Heart rate and ratings of perceived exertion were recorded during data collection to monitor relative intensity of the walking trials.

RESULTS

The Exoband was able to provide a consistent torque (~ 0.03-0.05 Nm/kg of peak torque) to the wearers. When walking with the Exoband, participants displayed a lower net metabolic power with respect to free walking (- 3.3 ± 3.0%; p = 0.02). There were no differences in spatio-temporal parameters or relative intensities when walking with or without the Exoband.

CONCLUSIONS

This study demonstrated that it is possible to reduce metabolic power during walking in older adults with the assistance of a passive device that applies a torque to the hip joint. Wearable, lightweight and low-cost devices such as the Exoband have the potential to make walking less metabolically demanding for older individuals.

Humans falling in holes: adaptations in lower-limb joint mechanics in response to a rapid change in substrate height during human hopping

In getting from here to there, we continuously negotiate complex environments and unpredictable terrain. Our ability to stay upright in the face of obstacles, such as holes in the ground, is quite remarkable. However, we understand relatively little about how humans adjust limb mechanical behaviour to recover from unexpected perturbations. In this study, we determined how the joints of the lower-limb respond to recover from a rapid, unexpected drop in substrate height during human hopping. We recorded lower-limb kinematics and kinetics while subjects performed steady-state hopping at their preferred frequency on an elevated platform (5, 10 and 20 cm). At an unknown time, we elicited an unexpected perturbation (i.e. a hole in the ground) via the rapid removal of the platform. Based on previous research in bipedal birds, we hypothesized (i) that distal joints would play an increased role in fall recovery when compared to steady-state hopping, and (ii) that patterns of joint power redistribution would be more pronounced with increases in perturbation height. Our results suggest that humans successfully recover from falling in a hole by increasing the energy absorbed predominantly in distal lower-limb joints (i.e. the ankle) across perturbation heights ranging from 5 to 10 cm. However, with increased perturbation height (20 cm) humans increase their reliance on the more proximal lower-limb joints (i.e. the knee and the hip) to absorb mechanical energy and stabilize fall recovery. Further investigations into the muscle-tendon mechanics underlying these joint-level responses will likely provide additional insights into the neuromotor control strategies used to regain the stability following unexpected perturbations and provide biological inspiration for the future design of wearable devices capable of performing within unpredictable environments.

Feedback From Mono-Articular Muscles is Sufficient for Exoskeleton Torque Adaptation

In this paper, by using a biomechanical model of the human body, we prove that (1) due to the existence of bi-articular muscles and compliant-elements, blind full-torque-compensation at joint level leads to muscles' activity amplification and consequently online adaptation methods are required for exoskeleton torque optimization. Moreover, (2) we state a new hypothesis that "reducing the net torque of two antagonistic mono-articular muscles is sufficient for involved muscles' total effort reduction" and analytically discuss its validity condition. Using this hypothesis, (3) we develop an adaptation rule which optimizes the exoskeleton torque using EMG signals of only two antagonistic mono-articular muscles. Furthermore, (4) the stability, convergence, optimality, and robustness of our adaptation method are proved in the presence of electromyography's intrinsic noisy behavior. Finally, (5) we experimentally validate our EMG-based adaptation method on six healthy subjects. We show that adaptation of the elbow compliance in a 2-DOF semi-active assistive arm in a cyclic task results in significant muscles activity reduction in all our subjects.

Connecting the legs with a spring improves human running economy

Human running is inefficient. For every 10 calories burned, less than 1 is needed to maintain a constant forward velocity - the remaining energy is, in a sense, wasted. The majority of this wasted energy is expended to support the bodyweight and redirect the center of mass during the stance phase of gait. An order of magnitude less energy is expended to brake and accelerate the swinging leg. Accordingly, most devices designed to increase running efficiency have targeted the costlier stance phase of gait. An alternative approach is seen in nature: spring-like tissues in some animals and humans are believed to assist leg swing. While it has been assumed that such a spring simply offloads the muscles that swing the legs, thus saving energy, this mechanism has not been experimentally investigated. Here, we show that a spring, or 'exotendon', connecting the legs of a human reduces the energy required for running by 6.4±2.8%, and does so through a complex mechanism that produces savings beyond those associated with leg swing. The exotendon applies assistive forces to the swinging legs, increasing the energy optimal stride frequency. Runners then adopt this frequency, taking faster and shorter strides, and reduce the joint mechanical work to redirect their center of mass. Our study shows how a simple spring improves running economy through a complex interaction between the changing dynamics of the body and the adaptive strategies of the runner, highlighting the importance of considering each when designing systems that couple human and machine.

Hopping with degressive spring stiffness in a full-leg exoskeleton lowers metabolic cost compared with progressive spring stiffness and hopping without assistance

When humans hop with a passive-elastic exoskeleton with springs in parallel with both legs, net metabolic power (P_met) decreases compared with normal hopping (NH). Furthermore, humans retain near-constant total vertical stiffness (k_tot) when hopping with such an exoskeleton. To determine how spring stiffness profile affects P_met and biomechanics, 10 subjects hopped on both legs normally and with three full-leg exoskeletons that each used a different spring stiffness profile at 2.4, 2.6, 2.8, and 3.0 Hz. Each subject hopped with an exoskeleton that had a degressive spring stiffness (DG_exo), where stiffness, the slope of force vs. displacement, is initially high but decreases with greater displacement, linear spring stiffness (LN_exo), where stiffness is constant, or progressive spring stiffness (PG_exo), where stiffness is initially low but increases with greater displacement. Compared with NH, use of the DG_exo, LN_exo, and PG_exo numerically resulted in 13-24% lower, 4-12% lower, and 0-8% higher P_met, respectively, at 2.4-3.0 Hz. Hopping with the DG_exo reduced P_met compared with NH at 2.4-2.6 Hz (P ≤ 0.0457) and reduced P_met compared with the PG_exo at 2.4-2.8 Hz (P < 0.001). k_tot while hopping with each exoskeleton was not different compared with NH, suggesting that humans adjust leg stiffness to maintain overall stiffness regardless of the spring stiffness profile in an exoskeleton. Furthermore, the DG_exo provided the greatest elastic energy return, followed by LN_exo and PG_exo (P ≤ 0.001). Future full-leg, passive-elastic exoskeleton designs for hopping, and presumably running, should use a DG_exo rather than an LN_exo or a PG_exo to minimize metabolic demand.NEW & NOTEWORTHY When humans hop at 2.4-3.0 Hz normally and with an exoskeleton with different spring stiffness profiles in parallel to the legs, net metabolic power is lowest when hopping with an exoskeleton with degressive spring stiffness. Total vertical stiffness is constant when using an exoskeleton with linear or nonlinear spring stiffness compared with normal hopping. In-parallel spring stiffness influences net metabolic power and biomechanics and should be considered when designing passive-elastic exoskeletons for hopping and running.