Comparison of the dynamics of exoskeletal-assisted and unassisted locomotion in an FDA-approved lower extremity device: Controlled experiments and development of a subject-specific virtual simulator.

Robotic exoskeletons have considerable, but largely untapped, potential to restore mobility in individuals with neurological disorders, and other conditions that result in partial or complete immobilization. The growing demand for these devices necessitates the development of technology to characterize the human-robot system during exoskeletal-assisted locomotion (EAL) and accelerate robot design refinements. The goal of this study was to combine controlled experiments with computational modeling to build a virtual simulator of EAL. The first objective was to acquire a minimum empirical dataset comprising human-robot kinematics, ground reaction forces, and electromyography during exoskeletal-assisted and unassisted locomotion from an able-bodied participant. The second objective was to quantify the dynamics of the human-robot system using a subject-specific virtual simulator reproducing EAL compared to the dynamics of normal gait. We trained an able-bodied participant to ambulate independently in a Food and Drug Administration-approved exoskeleton, the ReWalk P6.0 (ReWalk Robotics, Yoknaem, Israel). We analyzed the motion of the participant during exoskeletal-assisted and unassisted walking, sit-to-stand, and stand-to-sit maneuvers, with simultaneous measurements of (i) three-dimensional marker trajectories, (ii) ground reaction forces, (iii) electromyography, and (iv) exoskeleton encoder data. We created a virtual simulator in OpenSim, comprising a whole-body musculoskeletal model and a full-scale exoskeleton model, to determine the joint kinematics and moments during exoskeletal-assisted and unassisted maneuvers. Mean peak knee flexion angles of the human subject during exoskeletal-assisted walking were 50.1° ± 0.6° (left) and 52.6° ± 0.7° (right), compared to 68.6° ± 0.3° (left) and 70.7° ± 1.1° (right) during unassisted walking. Mean peak knee extension moments during exoskeletal-assisted walking were 0.10 ± 0.10 Nm/kg (left) and 0.22 ± 0.11 Nm/kg (right), compared to 0.64 ± 0.07 Nm/kg (left) and 0.73 ± 0.10 Nm/kg (right) during unassisted walking. This work provides a foundation for parametric studies to characterize the effects of human and robot design variables, and predictive modeling to optimize human-robot interaction during EAL.

Dual Wavelength Photoplethysmography Framework for Heart Rate Calculation.

The quality of heart rate (HR) measurements extracted from human photoplethysmography (PPG) signals are known to deteriorate under appreciable human motion. Auxiliary signals, such as accelerometer readings, are usually employed to detect and suppress motion artifacts. A 2019 study by Yifan Zhang and his coinvestigatorsused the noise components extracted from an infrared PPG signal to denoise a green PPG signal from which HR was extracted. Until now, this approach was only tested on "micro-motion" such as finger tapping. In this study, we extend this technique to allow accurate calculation of HR under high-intensity full-body repetitive "macro-motion". Our Dual Wavelength (DWL) framework was tested on PPG data collected from 14 human participants while running on a treadmill. The DWL method showed the following attributes: (1) it performed well under high-intensity full-body repetitive "macro-motion", exhibiting high accuracy in the presence of motion artifacts (as compared to the leading accelerometer-dependent HR calculation techniques TROIKA and JOSS); (2) it used only PPG signals; auxiliary signals such as accelerometer signals were not needed; and (3) it was computationally efficient, hence implementable in wearable devices. DWL yielded a Mean Absolute Error (MAE) of 1.22|0.57 BPM, Mean Absolute Error Percentage (MAEP) of 0.95|0.38%, and performance index (PI) (which is the frequency, in percent, of obtaining an HR estimate that is within ±5 BPM of the HR ground truth) of 95.88|4.9%. Moreover, DWL yielded a short computation period of 3.0|0.3 s to process a 360-second-long run.

Muscle co-contractions are greater in older adults during walking at self-selected speeds over uneven compared to even surfaces.

Falls in the aging population are a major public health concern. Outdoor falls in community-dwelling older adults are often triggered by uneven pedestrian walkways. Our understanding of the motor control adaptations to walk over an uneven surface, and the effects of aging on these adaptations is sparse. Here, we study changes in muscle co-contraction, a clinically accepted measure of motor control, due to changes in walking surfaces typically encountered in the outdoor built environment. We address the following research questions: 1) are there walking surface and sex-based differences in muscle co-contractions between young and older adults? and 2) is muscle co-contraction associated with age? We calculated muscle co-contractions from 13 young and 17 older adults during walking at self-selected speeds over even and uneven brick walkways. Muscle co-contraction at the ankle joint was determined from the tibialis anterior and lateral gastrocnemius muscle pair, and at the knee joint from the rectus femoris and semitendinosus muscle pair. Older adults displayed 8-13% greater ankle muscle co-contractions during walking over uneven compared to even surfaces. We found 55-61% (entire gait) and 73-75% (stance phase) greater ankle muscle co-contractions in older females compared to older males during walking over even and uneven surfaces. We found 31-43% greater knee muscle co-contractions in older females compared to older males during the swing phase of walking over even and uneven surfaces. This study underscores the need for determining muscle co-contractions from even and uneven surfaces for quantifying motor control deficits due to aging or neuromuscular disorders.

Tibiofemoral forces during FES rowing in individuals with spinal cord injury.

The purpose of this study is to determine the tibiofemoral forces during functional electrical stimulation (FES) rowing in individuals with spinal cord injury (SCI). We analysed the motion of five participants with SCI during FES rowing, with simultaneous measurements of (i) three-dimensional marker trajectories, (ii) foot reaction forces (FRFs), (iii) ergometer handle forces, and (iv) timestamps for electrical stimulation of the quadriceps and hamstrings muscles. We created full-body musculoskeletal models in OpenSim to determine subject-specific tibiofemoral forces during FES rowing. The peak magnitudes of tibiofemoral forces averaged over five participants with SCI were 2.43 ± 0.39 BW and 2.25 ± 0.71 BW for the left and right legs, respectively. The peak magnitudes of FRFs were 0.19 ± 0.04 BW in each leg. The peak magnitude of handle forces was 0.47 ± 0.19 BW. Peak tibiofemoral force was associated with peak FRF (magnitudes, R2 = 0.56, p = 0.013) and peak handle force (magnitudes, R2 = 0.54, p = 0.016). The ratios of peak magnitude of tibiofemoral force to peak magnitude of FRF were 12.9 ± 1.9 (left) and 11.6 ± 2.4 (right), and to peak magnitude of handle force were 5.7 ± 2.3 (left) and 4.9 ± 0.9 (right). This work lays the foundation for developing a direct exercise intensity metric for bone mechanical stimulus at the knee during rehabilitation exercises. Clinical Significance: Knowledge of tibiofemoral forces from exercises such as FES rowing may provide clinicians the ability to personalize rehabilitation protocols to ensure that an SCI patient is receiving the minimum dose of mechanical stimulus necessary to maintain bone health.

Knee muscle co-contractions are greater in old compared to young adults during walking and stair use.

BACKGROUND: Muscle co-contraction is an accepted clinical measure to quantify the effects of aging on neuromuscular control and movement efficiency. However, evidence of increased muscle co-contraction in old compared to young adults remains inconclusive. RESEARCH QUESTION: Are there differences in lower-limb agonist/antagonist muscle co-contractions in young and old adults, and males and females, during walking and stair use? METHODS: In a retrospective study, we analyzed data from 20 healthy young and 19 healthy old adults during walking, stair ascent, and stair descent at self-selected speeds, including marker trajectories, ground reaction force, and electromyography activity. We calculated muscle co-contraction at the knee (vastus lateralis vs. biceps femoris) and ankle (tibialis anterior vs. medial gastrocnemius) using the ratio of the common area under a muscle pairs' filtered and normalized electromyography curves to the sum of the areas under each muscle in that pair. RESULTS: Old compared to young adults displayed 18%-22% greater knee muscle co-contractions during the entire cycle of stair use activities. We found greater (17%-29%) knee muscle co-contractions in old compared to young adults during the swing phase of walking and stair use. We found no difference in ankle muscle co-contractions between the two age groups during all three activities. We found no difference in muscle co-contraction between males and females at the knee and ankle joints for all three activities. SIGNIFICANCE: Based on our findings, we recommend clinical evaluation to quantify the effects of aging through muscle co-contraction to include the knee joint during dynamic activities like walking and stair use, and independent evaluation of the stance and swing phases.