The Effects of Incline Level on Optimized Lower-Limb Exoskeleton Assistance: A Case Series.

For exoskeletons to be successful in real-world settings, they will need to be effective across a variety of terrains, including on inclines. While some single-joint exoskeletons have assisted incline walking, recent successes in level-ground assistance suggest that greater improvements may be possible by optimizing assistance of the whole leg. To understand how exoskeleton assistance should change with incline, we used human-in-the-loop optimization to find whole-leg exoskeleton assistance torques that minimized metabolic cost on a range of grades. We optimized assistance for three non-disabled, expert participants on 5 degree, 10 degree, and 15 degree inclines using a hip-knee-ankle exoskeleton emulator. For all assisted conditions, the cost of transport was reduced by at least 50% relative to walking in the device with no assistance, which is a large improvement to walking comparable to the benefits of whole-leg assistance on level-ground (N = 3). Optimized extension torque magnitudes and exoskeleton power increased with incline. Hip extension, knee extension and ankle plantarflexion often grew as large as allowed by comfort-based limits. Applied powers on steep inclines were double the powers applied during level-ground walking, indicating that greater exoskeleton power may be optimal in scenarios where biological powers and costs are higher. Future exoskeleton devices could deliver large improvements in walking performance across a range of inclines if they have sufficient torque and power capabilities.

Optimized hip-knee-ankle exoskeleton assistance reduces the metabolic cost of walking with worn loads.

BACKGROUND: Load carriage is common in a wide range of professions, but prolonged load carriage is associated with increased fatigue and overuse injuries. Exoskeletons could improve the quality of life of these professionals by reducing metabolic cost to combat fatigue and reducing muscle activity to prevent injuries. Current exoskeletons have reduced the metabolic cost of loaded walking by up to 22% relative to walking in the device with no assistance when assisting one or two joints. Greater metabolic reductions may be possible with optimized assistance of the entire leg. METHODS: We used human-in the-loop optimization to optimize hip-knee-ankle exoskeleton assistance with no additional load, a light load (15% of body weight), and a heavy load (30% of body weight) for three participants. All loads were applied through a weight vest with an attached waist belt. We measured metabolic cost, exoskeleton assistance, kinematics, and muscle activity. We performed Friedman's tests to analyze trends across worn loads and paired t-tests to determine whether changes from the unassisted conditions to the assisted conditions were significant. RESULTS: Exoskeleton assistance reduced the metabolic cost of walking relative to walking in the device without assistance for all tested conditions. Exoskeleton assistance reduced the metabolic cost of walking by 48% with no load (p = 0.05), 41% with the light load (p = 0.01), and 43% with the heavy load (p = 0.04). The smaller metabolic reduction with the light load may be due to insufficient participant training or lack of optimizer convergence. The total applied positive power was similar for all tested conditions, and the positive knee power decreased slightly as load increased. Optimized torque timing parameters were consistent across participants and load conditions while optimized magnitude parameters varied. CONCLUSIONS: Whole-leg exoskeleton assistance can reduce the metabolic cost of walking while carrying a range of loads. The consistent optimized timing parameters across participants and conditions suggest that metabolic cost reductions are sensitive to torque timing. The variable torque magnitude parameters could imply that torque magnitude should be customized to the individual, or that there is a range of useful torque magnitudes. Future work should test whether applying the load to the exoskeleton rather than the person's torso results in larger benefits.

Optimized hip-knee-ankle exoskeleton assistance at a range of walking speeds.

BACKGROUND: Autonomous exoskeletons will need to be useful at a variety of walking speeds, but it is unclear how optimal hip-knee-ankle exoskeleton assistance should change with speed. Biological joint moments tend to increase with speed, and in some cases, optimized ankle exoskeleton torques follow a similar trend. Ideal hip-knee-ankle exoskeleton torque may also increase with speed. The purpose of this study was to characterize the relationship between walking speed, optimal hip-knee-ankle exoskeleton assistance, and the benefits to metabolic energy cost. METHODS: We optimized hip-knee-ankle exoskeleton assistance to reduce metabolic cost for three able-bodied participants walking at 1.0 m/s, 1.25 m/s and 1.5 m/s. We measured metabolic cost, muscle activity, exoskeleton assistance and kinematics. We performed Friedman's tests to analyze trends across walking speeds and paired t-tests to determine if changes from the unassisted conditions to the assisted conditions were significant. RESULTS: Exoskeleton assistance reduced the metabolic cost of walking compared to wearing the exoskeleton with no torque applied by 26%, 47% and 50% at 1.0, 1.25 and 1.5 m/s, respectively. For all three participants, optimized exoskeleton ankle torque was the smallest for slow walking, while hip and knee torque changed slightly with speed in ways that varied across participants. Total applied positive power increased with speed for all three participants, largely due to increased joint velocities, which consistently increased with speed. CONCLUSIONS: Exoskeleton assistance is effective at a range of speeds and is most effective at medium and fast walking speeds. Exoskeleton assistance was less effective for slow walking, which may explain the limited success in reducing metabolic cost for patient populations through exoskeleton assistance. Exoskeleton designers may have more success when targeting activities and groups with faster walking speeds. Speed-related changes in optimized exoskeleton assistance varied by participant, indicating either the benefit of participant-specific tuning or that a wide variety of torque profiles are similarly effective.

Spatiotemporal Parameters are not Substantially Influenced by Load Carriage or Inclination During Treadmill and Overground Walking.

Influences of load carriage and inclination on spatiotemporal parameters were examined during treadmill and overground walking. Ten soldiers walked on a treadmill and overground with three load conditions (00 kg, 20 kg, 40 kg) during level, uphill (6% grade) and downhill (-6% grade) inclinations at self-selected speed, which was constant across conditions. Mean values and standard deviations for double support percentage, stride length and a step rate were compared across conditions. Double support percentage increased with load and inclination change from uphill to level walking, with a 0.4% stance greater increase at the 20 kg condition compared to 00 kg. As inclination changed from uphill to downhill, the step rate increased more overground (4.3 ± 3.5 steps/min) than during treadmill walking (1.7 ± 2.3 steps/min). For the 40 kg condition, the standard deviations were larger than the 00 kg condition for both the step rate and double support percentage. There was no change between modes for step rate standard deviation. For overground compared to treadmill walking, standard deviation for stride length and double support percentage increased and decreased, respectively. Changes in the load of up to 40 kg, inclination of 6% grade away from the level (i.e., uphill or downhill) and mode (treadmill and overground) produced small, yet statistically significant changes in spatiotemporal parameters. Variability, as assessed by standard deviation, was not systematically lower during treadmill walking compared to overground walking. Due to the small magnitude of changes, treadmill walking appears to replicate the spatiotemporal parameters of overground walking.

Biomechanical Effects of Stiffness in Parallel With the Knee Joint During Walking.

The human knee behaves similarly to a linear torsional spring during the stance phase of walking with a stiffness referred to as the knee quasi-stiffness. The spring-like behavior of the knee joint led us to hypothesize that we might partially replace the knee joint contribution during stance by utilizing an external spring acting in parallel with the knee joint. We investigated the validity of this hypothesis using a pair of experimental robotic knee exoskeletons that provided an external stiffness in parallel with the knee joints in the stance phase. We conducted a series of experiments involving walking with the exoskeletons with four levels of stiffness, including 0%, 33%, 66%, and 100% of the estimated human knee quasi-stiffness, and a pair of joint-less replicas. The results indicated that the ankle and hip joints tend to retain relatively invariant moment and angle patterns under the effects of the exoskeleton mass, articulation, and stiffness. The results also showed that the knee joint responds in a way such that the moment and quasi-stiffness of the knee complex (knee joint and exoskeleton) remains mostly invariant. A careful analysis of the knee moment profile indicated that the knee moment could fully adapt to the assistive moment; whereas, the knee quasi-stiffness fully adapts to values of the assistive stiffness only up to ∼80%. Above this value, we found biarticular consequences emerge at the hip joint.

Design and evaluation of a quasi-passive knee exoskeleton for investigation of motor adaptation in lower extremity joints.

In this study, we describe the mechanical design and control scheme of a quasi-passive knee exoskeleton intended to investigate the biomechanical behavior of the knee joint during interaction with externally applied impedances. As the human knee behaves much like a linear spring during the stance phase of normal walking gait, the exoskeleton implements a spring across the knee in the weight acceptance (WA) phase of the gait while allowing free motion throughout the rest of the gait cycle, accomplished via an electromechanical clutch. The stiffness of the device is able to be varied by swapping springs, and the timing of engagement/disengagement changed to accommodate different loading profiles. After describing the design and control, we validate the mechanical performance and reliability of the exoskeleton through cyclic testing on a mechanical knee simulator. We then describe a preliminary experiment on three healthy adults to evaluate the functionality of the device on both left and right legs. The kinetic and kinematic analyses of these subjects show that the exoskeleton assistance can partially/fully replace the function of the knee joint and obtain nearly invariant moment and angle profiles for the hip and ankle joints, and the overall knee joint and exoskeleton complex under the applied moments of the exoskeleton versus the control condition, implying that the subjects undergo a considerable amount of motor adaptation in their lower extremities to the exoskeletal impedances, and encouraging more in-depth future experiments with the device.

Preliminary investigation of effects of a quasi-passive knee exoskeleton on gait energetics.

In this paper, we explain that the human knee behavior in the weight acceptance phase of gait (first ~40% of gait cycle) resembles that of a linear torsional spring. This led us to study the effects of the assistance provided by a pair of quasi-passive knee exoskeletons, which implement springs in parallel with the knee joints in the weight acceptance phase. Using the exoskeletons in a series of experiments on seven participants, we found that the exoskeleton mildly but non-significantly reduces the metabolic power of walking. We also found that the metabolic power of walking is significantly correlated with both the positive rate of moment generation and positive mechanical power of the lower extremity joints. This suggests that augmenting exoskeletons can aim to reduce both the muscle force and work generation to reduce the metabolic cost of walking.

Effects of a lower-body exoskeleton device on metabolic cost and gait biomechanics during load carriage.

This study investigated the effects on metabolic cost and gait biomechanics of using a prototype lower-body exoskeleton (EXO) to carry loads. Nine US Army participants walked at 1.34 m/s on a 0% grade for 8 min carrying military loads of 20 kg, 40 kg and 55 kg with and without the EXO. Mean oxygen consumption (VO(2)) scaled to body mass and scaled to total mass were significantly higher, by 60% and 41% respectively, when the EXO was worn, compared with the control condition. Mean VO(2) and mean VO(2) scaled to body mass significantly increased with load. The kinematic and kinetic data revealed significant differences between EXO and control conditions, such as walking with a more flexed posture and braking with higher ground reaction force at heel strike when wearing the EXO. Study findings demonstrate that the EXO increased users' metabolic cost while carrying various loads and altered their gait biomechanics compared with conventional load carriage. STATEMENT OF RELEVANCE: An EXO designed to assist in load bearing was found to raise energy expenditure substantially when tested by soldiers carrying military loads. EXO weight, weight distribution and design elements that altered users' walking biomechanics contributed to the high energy cost. To realise the potential of EXOs, focus on the user must accompany engineering advances.

The effects of a lower body exoskeleton load carriage assistive device on limits of stability and postural sway.

The study investigated the effects of using a lower body prototype exoskeleton (EXO) on static limits of stability and postural sway. Measurements were taken with participants, 10 US Army enlisted men, standing on a force platform. The men were tested with and without the EXO (15 kg) while carrying military loads of 20, 40 and 55 kg. Body lean to the left and right was significantly less and postural sway excursions and maximal range of movement were significantly reduced when the EXO was used. Hurst values indicated that body sway was less random over short-term time intervals and more random over long-term intervals with the EXO than without it. Feedback to the user's balance control mechanisms most likely was changed with the EXO. The reduced sway and relatively small changes in sway with increasing load weights suggest that the EXO structure may have functioned to provide a bracing effect on the body.

Discrete bandwidth visual feedback increases structure of output as compared to continuous visual feedback in isometric force control tasks.

BACKGROUND: Performance variability measures provide a partial picture of force control ability. Nonlinear analyses can reveal important information related to the randomness and complexity of the data, providing a more complete picture of the physiological process. METHODS: We investigated the effects of visual feedback on the structure and performance of the force output from isometric force control tasks. Twelve young volunteers completed isometric force control tasks using two types of visual feedback: discrete bandwidth (+/-4% maximal voluntary contraction) and continuous line matching. We determined force signal variability (standard deviation), self-similarity (fractal dimension), and complexity (approximate entropy). Analyses of variance (feedback x muscle group x force level) were conducted and P values less than 0.05 were considered significant. FINDINGS: The force signal in discrete bandwidth feedback, compared to continuous line matching, had significantly a higher standard deviation (P=.000): 2.18 Nm (SD 1.98) vs. 0.99 Nm (SD 0.91); lower fractal dimension (P=.000): 1.07 (SD 0.04) vs. 1.16 (SD 0.04); and lower approximate entropy (P=.000): 0.12 (SD 0.07) vs. 0.26 (SD 0.09). INTERPRETATION: The greater self-similarity (lower fractal dimension) and greater regularity (lower approximate entropy) of the discrete bandwidth, compared to the continuous line matching, may indicate a process that required more kinesthetic (intrinsic) feedback to modulate force. Clinicians may choose to employ visual feedback paradigms that target the use of intrinsic feedback during rehabilitation. Discrete bandwidth feedback may be useful for delineating impairments in motor skill and measuring outcomes of intervention programs.

Effects of carried weight on random motion and traditional measures of postural sway.

The purpose of the study was to investigate the effects of load weight carried by soldiers upon postural sway. Fourteen US Army enlisted men participated. Postural sway and muscle activity were measured while participants stood on a force plate. The load weight conditions, comprised of Army clothing and load-carriage equipment were 6, 16, and 40 kg. With an increase in load weight, stabilogram-diffusion analysis revealed that random movement of postural sway decreased. Also, with an increase in load weight, center of pressure excursions increased linearly but muscle activity changed minimally. In short, increasing load weight challenged the load carriers' stability, reduced the randomness of postural sway and required the load carriers to exert greater control of the load in order to maintain balance.