Trunk postural control during unstable sitting among individuals with and without low back pain: A systematic review with an individual participant data meta-analysis

INTRODUCTION

Sitting on an unstable surface is a common paradigm to investigate trunk postural control among individuals with low back pain (LBP), by minimizing the influence lower extremities on balance control. Outcomes of many small studies are inconsistent (e.g., some find differences between groups while others do not), potentially due to confounding factors such as age, sex, body mass index [BMI], or clinical presentations. We conducted a systematic review with an individual participant data (IPD) meta-analysis to investigate whether trunk postural control differs between those with and without LBP, and whether the difference between groups is impacted by vision and potential confounding factors.

METHODS

We completed this review according to PRISMA-IPD guidelines. The literature was screened (up to 7th September 2023) from five electronic databases: MEDLINE, CINAHL, Embase, Scopus, and Web of Science Core Collection. Outcome measures were extracted that describe unstable seat movements, specifically centre of pressure or seat angle. Our main analyses included: 1) a two-stage IPD meta-analysis to assess the difference between groups and their interaction with age, sex, BMI, and vision on trunk postural control; 2) and a two-stage IPD meta-regression to determine the effects of LBP clinical features (pain intensity, disability, pain catastrophizing, and fear-avoidance beliefs) on trunk postural control.

RESULTS

Forty studies (1,821 participants) were included for the descriptive analysis and 24 studies (1,050 participants) were included for the IPD analysis. IPD meta-analyses revealed three main findings: (a) trunk postural control was worse (higher root mean square displacement [RMSdispl], range, and long-term diffusion; lower mean power frequency) among individuals with than without LBP; (b) trunk postural control deteriorated more (higher RMSdispl, short- and long-term diffusion) among individuals with than without LBP when vision was removed; and (c) older age and higher BMI had greater adverse impacts on trunk postural control (higher short-term diffusion; longer time and distance coordinates of the critical point) among individuals with than without LBP. IPD meta-regressions indicated no associations between the limited LBP clinical features that could be considered and trunk postural control.

CONCLUSION

Trunk postural control appears to be inferior among individuals with LBP, which was indicated by increased seat movements and some evidence of trunk stiffening. These findings are likely explained by delayed or less accurate corrective responses.

SYSTEMATIC REVIEW REGISTRATION

This review has been registered in PROSPERO (registration number: CRD42021124658).

Generating accurate 3D gaze vectors using synchronized eye tracking and motion capture

Assessing gaze behavior during real-world tasks is difficult; dynamic bodies moving through dynamic worlds make gaze analysis difficult. Current approaches involve laborious coding of pupil positions. In settings where motion capture and mobile eye tracking are used concurrently in naturalistic tasks, it is critical that data collection be simple, efficient, and systematic. One solution is to combine eye tracking with motion capture to generate 3D gaze vectors. When combined with tracked or known object locations, 3D gaze vector generation can be automated. Here we use combined eye and motion capture and explore how linear regression models generate accurate 3D gaze vectors. We compare spatial accuracy of models derived from four short calibration routines across three pupil data inputs: the efficacy of calibration routines was assessed, a validation task requiring short fixations on task-relevant locations, and a naturalistic object interaction task to bridge the gap between laboratory and "in the wild" studies. Further, we generated and compared models using spherical and Cartesian coordinate systems and monocular (left or right) or binocular data. All calibration routines performed similarly, with the best performance (i.e., sub-centimeter errors) coming from the naturalistic task trials when the participant is looking at an object in front of them. We found that spherical coordinate systems generate the most accurate gaze vectors with no differences in accuracy when using monocular or binocular data. Overall, we recommend 1-min calibration routines using binocular pupil data combined with a spherical world coordinate system to produce the highest-quality gaze vectors.

Combining transcutaneous spinal stimulation and functional electrical stimulation increases force generated by lower limbs: When more is more

Background

Transcutaneous Spinal Stimulation (TSS) has been shown to promote activation of the lower limb and trunk muscles and is being actively explored for improving the motor outcomes of people with neurological conditions. However, individual responses to TSS vary, and often the muscle responses are insufficient to produce enough force for self-supported standing. Functional electrical stimulation (FES) can activate individual muscles and assist in closing this functional gap, but it introduces questions regarding timing between modalities.

Methods

To assess the effects of TSS and FES on force generation, ten neurologically intact participants underwent (1) TSS only, (2) FES only, and (3) TSS + FES. TSS was delivered using four electrodes placed at T10-T11 through the L1-L2 intervertebral spaces simultaneously, while FES was delivered to the skin over the right knee extensors and plantarflexors. For all conditions, TSS and FES were delivered using three 0.5 ms biphasic square-wave pulses at 15 Hz. During the TSS + FES condition, timing between the two modalities was adjusted in increments of ¼ time between pulses (16.5 ms).

Results

When TSS preceded FES, a larger force production was observed. We also determined several changes in muscle activation amplitude at different relative stimulus intervals, which help characterize our finding and indicate the facilitating and inhibitory effects of the modalities.

Conclusions

Utilizing a delay ranging from 15 to 30 ms between stimuli resulted in higher mean force generation in both the knee and ankle joints, regardless of the selected FES location (Average; knee: 112.0%, ankle: 103.1%).

Clinical Static Balance Assessment: A Narrative Review of Traditional and IMU-Based Posturography in Older Adults and Individuals with Incomplete Spinal Cord Injury

Maintaining a stable upright posture is essential for performing activities of daily living, and impaired standing balance may impact an individual's quality of life. Therefore, accurate and sensitive methods for assessing static balance are crucial for identifying balance impairments, understanding the underlying mechanisms of the balance deficiencies, and developing targeted interventions to improve standing balance and prevent falls. This review paper first explores the methods to quantify standing balance. Then, it reviews traditional posturography and recent advancements in using wearable inertial measurement units (IMUs) to assess static balance in two populations: older adults and those with incomplete spinal cord injury (iSCI). The inclusion of these two groups is supported by their large representation among individuals with balance impairments. Also, each group exhibits distinct aspects in balance assessment due to diverse underlying causes associated with aging and neurological impairment. Given the high vulnerability of both demographics to balance impairments and falls, the significance of targeted interventions to improve standing balance and mitigate fall risk becomes apparent. Overall, this review highlights the importance of static balance assessment and the potential of emerging methods and technologies to improve our understanding of postural control in different populations.

Technology-based balance performance assessment can eliminate floor and ceiling effects

Many clinical measurement tools for balance have ceiling effects. Technology-based assessments using virtual reality systems such as the Computer-Assisted Rehabilitation Environment (CAREN) may provide a way to develop objective, quantitative measures that scale from low to high levels of difficulty. Our objective was to: (1) develop a performance assessment tool (PAT) for the CAREN; (2) quantify the reliability of the tool; (3) validate the scores against clinical balance measures; and (4) compare the scores from a population with balance impairments to those from able-bodied individuals in a cross-sectional validation study. Three games were developed on the CAREN and tested on 49 participants (36 able-bodied and 13 with impaired mobility). For each module, the corresponding measures were transformed into scores using a series of functions such that ceiling and flooring effects would be minimized. The results showed an association between scores and age, an overlap in scores from impaired high-performance individuals and able-bodied low performance individuals, and a correlation of PAT scores with other clinical tests. Several of the limitations of current clinical tools, including floor and ceiling effects, were overcome by the PAT, suggesting that the PAT can be used to monitor the effect of rehabilitation and training.

Evaluating the Intracranial Pressure Biofidelity and Response Repeatability of a Physical Head-Brain Model in Frontal Impacts

Headforms are widely used in head injury research and headgear assessment. Common headforms are limited to replicating global head kinematics, although intracranial responses are crucial to understanding brain injuries. This study aimed to evaluate the biofidelity of intracranial pressure (ICP) and the repeatability of head kinematics and ICP of an advanced headform subjected to frontal impacts. Pendulum impacts were performed on the headform using various impact velocities (1-5 m/s) and impactor surfaces (vinyl nitrile 600 foam, PCM746 urethane, and steel) to simulate a previous cadaveric experiment. Head linear accelerations and angular rates in three axes, cerebrospinal fluid ICP (CSFP), and intraparenchymal ICP (IPP) at the front, side, and back of the head were measured. The head kinematics, CSFP, and IPP demonstrated acceptable repeatability with coefficients of variation generally being less than 10%. The BIPED front CSFP peaks and back negative peaks were within the range of the scaled cadaver data (between the minimum and maximum values reported by Nahum et al.), while side CSFPs were 30.9-92.1% greater than the cadaver data. CORrelation and Analysis (CORA) ratings evaluating the closeness of two time histories demonstrated good biofidelity of the front CSFP (0.68-0.72), while the ratings for the side (0.44-0.70) and back CSFP (0.27-0.66) showed a large variation. The BIPED CSFP at each side was linearly related to head linear accelerations with coefficients of determination greater than 0.96. The slopes for the BIPED front and back CSFP-acceleration linear trendlines were not significantly different from cadaver data, whereas the slope for the side CSFP was significantly greater than cadaver data. This study informs future applications and improvements of a novel head surrogate.

Differences in angular kinematics when using thigh, implant, or medial knee markers in osseointegrated transfemoral prosthetic gait

BACKGROUND

The Helen Hayes anatomical model is commonly used in clinical gait analysis with standard medial/lateral knee and thigh markers.

METHODS

To quantify soft-tissue artifacts associated with the thigh marker following osseointegration surgery, we added an "implant marker" on the implant extending from the femur, with the objective of identifying the differences in the angular kinematics when using the standard versus implant marker. One female adult with an osseointegrated transfemoral prosthesis walked overground for three trials, and common kinematic measures were calculated from motion capture data.

FINDINGS

The results indicated that, when using the thigh marker, a peak of knee varus occurred during the swing phase on the prosthetic side, which is unusual during gait and not feasible for hinge joint prostheses. When using the implant marker, knee varus/valgus was closer to normative. Using the thigh marker, the results showed an internal hip rotation at the start of stance and during the mid and terminal swing phases. In contrast, external hip rotation occurred in both stance and swing phases using the implant marker. Moreover, when selecting the medial knee marker instead of the thigh marker, the angular kinematics and range of motion of knee varus/valgus and hip rotation were comparable to those for the implant marker.

INTERPRETATION

This finding suggests that when studying osseointegration gait, using an implant marker will result in more accurate femoral and knee joint motion than using the thigh marker. Changing the selection of markers can reduce the errors of knee varus/valgus and hip kinematics in osseointegrated transfemoral prosthetic gait.

Influence of surrogate scalp material and thickness on head impact responses: Toward a biofidelic head-brain physical model

Advanced physical head models capable of replicating both global kinematics and intracranial mechanics of the human head are required for head injury research and safety gear assessment. These head surrogates require a complex design to accommodate realistic anatomical details. The scalp is a crucial head component, but its influence on the biomechanical response of such head surrogates remains unclear. This study aimed to evaluate the influence of surrogate scalp material and thickness on head accelerations and intraparenchymal pressures using an advanced physical head-brain model. Scalp pads made from four materials (Vytaflex20, Vytaflex40, Vytaflex50, PMC746) and each material with four thicknesses (2, 4, 6, and 8 mm) were evaluated. The head model attached to the scalp pad was dropped onto a rigid plate from two heights (5 and 19.5 cm) and at three head locations (front, right side, and back). While the selected materials' modulus exhibited a relatively minor effect on head accelerations and coup pressures, the effect of scalp thickness was shown to be major. Moreover, by decreasing the thickness of the head's original scalp by 2 mm and changing the original scalp material from Vytaflex 20 to Vytaflex 40 or Vytaflex 50, the head acceleration biofidelity ratings could improve by 30% and approached the considered rating (0.7) of good biofidelity. This study provides a potential direction for improving the biofidelity of a novel head model that might be a useful tool in head injury research and safety gear tests. This study also has implications for selecting appropriate surrogate scalps in the future design of physical and numerical head models.

Nonlinear neural control using feedback linearization explains task goals of central nervous system for trunk stability in sitting posture

OBJECTIVE

Characterizing the task goals of the neural control system for achieving seated stability has been a fundamental challenge in human motor control research. This study aimed to experimentally identify the task goals of the neural control system for seated stability.
Approach: Ten able-bodied young individuals participated in our experiments, which allowed us to measure their body motion and muscle activity during perturbed sitting. We used a nonlinear neuromechanical model of the seated human, along with a full-state feedback linearization approach and optimal control theory for identifying the neural control system and characterizing its task goals.
Main results: We demonstrated that the neural feedback for trunk stability during seated posture uses angular position, velocity, acceleration, and jerk in a linearized space. The mean squared error between the predicted and measured motor commands was less than 0.6% among all trials and participants, with a median correlation coefficient (r) of more than 0.9. Our identified optimal neural control primarily used trunk angular acceleration and near-minimum muscle activation to achieve seated stability while keeping the deviations of the trunk angular position and acceleration sufficiently small. 
Significance: Our proposed approach to neural control system identification relied on a performance criterion (e.g., cost function) explaining what the functional goal is and subsequently, finds the control law that leads to the best performance. Therefore, instead of assuming what control schemes the neural control might utilize (e.g., proportional-integral-derivative control), optimal control allows the motor task and the neuromechanical model to dictate a control law that best describes the physiological process. This approach allows for a mechanistic understanding of the neuromuscular mechanisms involved in seated stability and for inferring the task goals used by the neural control system to achieve the targeted motor behaviour. Such neural control characterization can contribute to the development of objective balance evaluation tools and of bio-inspired assistive neuromodulation technologies.&#xD.

Clinical utility of gait stability measures: Selection and preliminary evaluation of the margin of stability

Introduction: gait-related falls account for most falls in the elderly. The identification of individuals at risk of falling due to unstable gait requires clinically feasible measures that can detect impairments in gait. Our obiective was to : (1) assess the feasibility for clinical implementation of existing gait stability measures and selected measures to detect gait impairments. Methods: Nine measures were assessed for clinical feasibility, with only the margin of stability(MOS) being selected for evaluation. Ground reactions and motion of the lower body were recorded in fifteen non-disabled and three disabled participants during treadmil walking. Media-lateral MOS was calculated and compared between the non-disabled and disabled participants to evaluate its ability to detect gait impairments. Results: MOS values at heel strike(HS) and at the point between HS and contralateral toe-off with minimum MOS deviated between participants with lower limb impaiments and non-disabled participants. Conclusion: MOS at HS demonstrated the greatest potential to assess fall risk. Additional work is required before MOS can be recommanded for clinical use.

Development and Validation of a Closed-Loop Functional Electrical Stimulation-Based Controller for Gait Rehabilitation Using a Finite State Machine Model

Functional electrical stimulation (FES) can be used to initiate lower limb muscle contractions and has been widely applied in gait rehabilitation. Establishing the correct timing of FES activation during each phase of the gait (walking) cycle remains challenging as most FES systems rely on open-loop control, whereby the controller receives no feedback about joint kinematics and instead relies on predetermined/timed muscle stimulation. The objective of this study was to develop and validate a closed-loop FES-based control solution for gait rehabilitation using a finite state machine (FSM) model. A two-phased study approach was taken: (1) Experimentally-Informed Study: A neuromuscular-derived FSM model was developed to drive closed-loop FES-based control for gait rehabilitation. The finite states were determined using electromyography and joint kinematics data of 12 non-disabled adults, collected during treadmill walking. The gait cycles were divided into four states, namely: swing-to-stance, push off, pre-swing, and toe up. (2) Simulation Study: A closed-loop FES-based control solution that employed the resulting FSM model, was validated through comparisons of neuro-musculo-skeletal computer simulations of impaired versus healthy gait. This closed-loop controller yielded steadier simulated impaired gait, in comparison to an open-loop alternative. The simulation results confirmed that accurate timing of FES activation during the gait cycle, as informed by kinematics data, is important to natural gait retraining. The closed-loop FES-based solution, introduced in this study, contributes to the repository of gait rehabilitation control options and offers the advantage of being simplistic to implement. Furthermore, this control solution is expected to integrate well with powered exoskeleton technologies.

Nonlinear Response of Human Trunk Musculature Explains Neuromuscular Stabilization Mechanisms in Sitting Posture

OBJECTIVE

Determining the roles of underlying mechanisms involved in stabilizing the human trunk during sitting is a fundamental challenge in human motor control. However, distinguishing their roles requires understanding their complex interrelations and describing them with physiologically meaningful neuromechanical parameters. The literature has shown that such mechanistic understanding contributes to diagnosing and improving impaired balance as well as developing assistive technologies for restoring trunk stability. This study aimed to provide a comprehensive characterization of the underlying neuromuscular stabilization mechanisms involved in human sitting.

APPROACH

This study characterized passive and active stabilization mechanisms involved in seated stability by identifying a nonlinear neuromechanical physiologically-meaningful model in ten able-bodied individuals during perturbed sitting via an adaptive unscented Kalman filter to account for the nonlinear time-varying process and measurement noises.

MAIN RESULTS

We observed that the passive mechanism provided instant resistance against gravitational disturbances, whereas the active mechanism provided delayed complementary phasic response against external disturbances by activating appropriate trunk muscles while showing non-isometric behavior. The model predicted the trunk sway behavior during perturbed sitting with high accuracy and correlation (average: 0.0007 [rad2] and 86.77%). This allows a better mechanistic understanding of the roles of passive and active stabilization mechanisms involved in sitting.

SIGNIFICANCE

Our characterization approach accounts for the inherently nonlinear behavior of the neuromuscular mechanisms and physiological uncertainties, while allowing for real-time tracking and correction of parameters' variations due to external disturbances and muscle fatigue. The outcome of our research, for the first time, (1) allows a better mechanistic understanding of the roles of passive and active stabilization mechanisms involved in sitting; (2) enables objective evaluation and targeted rehabilitative interventions for impaired balance; facilitate bio-inspired designs of assistive technologies, and (3) opens new horizons in mathematical identification of neuromechanical mechanisms employed in the stable control of human body postures and motions.

Instrumented Functional Test for Objective Outcome Evaluation of Balance Rehabilitation in Elderly Fallers: A Clinical Study

<b><i>Introduction:</i></b> Observational tests, e.g., the Berg Balance Scale (BBS) are widely used for balance evaluation in the elderly fallers. However, they do not allow objective outcome evaluation of rehabilitative interventions. This study aimed to investigate, in a clinical setting, the use of inertial measurement units (IMUs) integrated into the BBS test for objective outcome evaluation of balance rehabilitation in elderly fallers compared to conventional BBS scores. <b><i>Methods:</i></b> Thirty-six elderly fallers were recruited from the in-patient population of a geriatrics Clinic. Participants performed the BBS test while wearing 3 IMUs placed on the sternum, sacrum, and tibia of the dominant leg following admission to the clinic. Subsequently, they completed a rehabilitation program for 2–4 weeks. They performed a similar test before their discharge. The physical therapist recorded the BBS scores at both sessions, and the sensor data of the 2-min quiet standing task (BBS task 2) were extracted for objective balance evaluation. Moreover, eleven young adults were recruited to perform a 2-min quiet standing test while wearing the same IMUs. Center-of-pressure (COP) and segmental center-of-mass (COM) accelerations were calculated to estimate time-domain, frequency-domain, and intersegment coordination biomarkers of balance. <b><i>Results:</i></b> COP time- and frequency-domain measures, COM acceleration time-domain measures, and intersegment coordination measures could identify age-related changes in balance of seniors compared to young adults (<i>p</i> &#x3c; 0.05). Moreover, balance biomarkers of senior adults exhibited a reduced sway acceleration and jerkiness in the medial-lateral direction post-rehabilitation (<i>p</i> &#x3c; 0.05). Although the total BBS scores increased post-rehabilitation, sway displacement and velocity did not significantly improve. We observed a significant association between pelvis-leg coordination at high sway oscillations and the total BBS scores pre- and post-rehabilitation. <b><i>Conclusion:</i></b> IMUs enable not only the characterization of underlying causes of impaired balance but also the identification of improved and yet impaired aspects of balance post-rehabilitation. Hence, IMUs allow us to characterize risk factors post-rehabilitation in elderly fallers, whereas the BBS scores only show changes in overall balance. It is crucial to objectively evaluate the effectiveness of such interventions to reduce future falls and their adverse consequences. Therefore, instrumented balance assessment is recommended since it can provide quantitative and objective measures for clinical outcome evaluations.

Evacuation solutions for individuals with functional limitations in the built environment: a scoping review protocol

BACKGROUND

Whether due to aging, disability, injury, or other circumstances, an increasing number of Canadians experience functional limitations that reduce their ability to participate in activities of daily life. While the built environment has become increasingly accessible, existing Canadian evacuation guidelines lack comprehensive strategies for evacuating individuals with functional limitations from buildings during emergencies. To inform guideline revisions, a map of existing solutions for evacuating such individuals is required. Therefore, this scoping review aims to provide an account of solutions that have been reported to safely evacuate individuals with functional limitations from the built environment.

METHODS

We will conduct a scoping review using the Arksey and O'Malley methodological framework. To identify potentially relevant studies, comprehensive searches (from January 2002 onwards) of the CINAHL, Ei Compendex, Inspec, Embase, MEDLINE, KCI, RSCI, SciELO CI, Web of Science Collection, and Scopus databases will be performed. Using a set of inclusion and exclusion criteria, two reviewers will independently (1) classify identified studies as relevant, irrelevant, or maybe relevant by evaluating their titles and abstracts and (2) classify the relevant and maybe relevant studies as included or excluded by evaluating their full-text. From each included study, data on publication information, study purpose, methodological details, evacuation information, and outcomes will be extracted using a set of data extraction items. We will present a numerical summary of the key characteristics of the included studies. For each evacuation activity, reported evacuation solutions will be summarized, and citations provided for functional limitations that are targeted by a given evacuation solution. To inform Canadian evacuation guideline revisions, we will tabulate evacuation activities common to different types of buildings and emergencies.

DISCUSSION

To our knowledge, this will be the first scoping review to identify the state and use of solutions for evacuating individuals with functional limitations from the built environment. Identifying solutions that enable all individuals to safely evacuate from different types of buildings will allow us to inform recommendations for the revision of evacuation guidelines in Canada and other jurisdictions. The findings of this scoping review will be published in a peer-reviewed journal, presented at relevant conferences, and made publicly available on the internet.

SYSTEMATIC REVIEW REGISTRATION

Open Science Framework: osf.io/jefgy.

Evaluation of the Kinematic Biofidelity and Inter-Test Repeatability of Global Accelerations and Brain Parenchyma Pressure for a Head-Brain Physical Model

Head surrogates are widely used in biomechanical research and headgear assessment. They are designed to approximate the inertial and mechanical properties of the head and are instrumented to measure global head kinematics. Due to the recent interest in studying disruption to the brain, some head models include internal fluid layers and brain tissue, and instrumentation to measure head intracranial biomechanics. However, it is unknown whether such models exhibit realistic human responses. Therefore, this study aims to assess the biofidelity and repeatability of a head model, the Blast Injury Protection Evaluation Device (BIPED), that can measure both global head kinematics and intraparenchymal pressure (IPP) for application in blunt impact, a common loading scenario in civilian life. Drop tests were conducted with the BIPED and the widely used Hybrid III headform. BIPED measures were compared to the Hybrid III data and published cadaveric data, and the biofidelity level of the global linear acceleration was quantified using CORrelation and Analysis (CORA) ratings. The repeatability of the acceleration and IPP measurements in multiple impact scenarios was evaluated via the coefficient of variation (COV) of the magnitudes and pulse durations. BIPED acceleration peaks were generally not significantly different from cadaver and Hybrid III data. The CORA ratings for the BIPED and Hybrid III accelerations ranged from 0.50 to 0.61 and 0.51 to 0.77, respectively. The COVs of acceleration and IPP were generally below 10%. This study is an important step toward a biofidelic head surrogate measuring both global kinematics and IPP in blunt impact.

Transcutaneous spinal cord stimulation improves postural stability in individuals with multiple sclerosis

BACKGROUND

Widespread demyelination in the central nervous system can lead to progressive sensorimotor impairments following multiple sclerosis, with compromised postural stability during standing being a common consequence. As such, clinical strategies are needed to improve postural stability following multiple sclerosis. The objective of this study was therefore to investigate the effect of non-invasive transcutaneous spinal stimulation on postural stability during upright standing in individuals with multiple sclerosis.

METHODS

Center of pressure displacement and electromyograms from the soleus and tibialis anterior were recorded in seven individuals with multiple sclerosis during standing without and with transcutaneous spinal stimulation. Center of pressure and muscle activity measures were calculated and compared between no stimulation and transcutaneous spinal stimulation conditions. The relationship between the center of pressure displacement and electromyograms was quantified using cross-correlation analysis.

RESULTS

For transcutaneous spinal stimulation, postural stability was significantly improved during standing with eyes closed: the time- and frequency-domain measures obtained from the anterior-posterior center of pressure fluctuation decreased and increased, respectively, and the tibialis anterior activity was lower compared to no stimulation. Conversely, no differences were found between no stimulation and transcutaneous spinal stimulation when standing with eyes open.

CONCLUSION

Following multiple sclerosis, transcutaneous spinal stimulation improved postural stability during standing with eyes closed, presumably by catalyzing proprioceptive function. Future work should confirm underlying mechanisms and explore the clinical value of transcutaneous spinal stimulation for individuals with multiple sclerosis.

Myoelectric prosthesis users and non-disabled individuals wearing a simulated prosthesis exhibit similar compensatory movement strategies

Background

Research studies on upper limb prosthesis function often rely on the use of simulated myoelectric prostheses (attached to and operated by individuals with intact limbs), primarily to increase participant sample size. However, it is not known if these devices elicit the same movement strategies as myoelectric prostheses (operated by individuals with amputation). The objective of this study was to address the question of whether non-disabled individuals using simulated prostheses employ the same compensatory movements (measured by hand and upper body kinematics) as individuals who use actual myoelectric prostheses.

Methods

The upper limb movements of two participant groups were investigated: (1) twelve non-disabled individuals wearing a simulated prosthesis, and (2) three individuals with transradial amputation using their custom-fitted myoelectric devices. Motion capture was used for data collection while participants performed a standardized functional task. Performance metrics, hand movements, and upper body angular kinematics were calculated. For each participant group, these measures were compared to those from a normative baseline dataset. Each deviation from normative movement behaviour, by either participant group, indicated that compensatory movements were used during task performance.

Results

Results show that participants using either a simulated or actual myoelectric prosthesis exhibited similar deviations from normative behaviour in phase durations, hand velocities, hand trajectories, number of movement units, grip aperture plateaus, and trunk and shoulder ranges of motion.

Conclusions

This study suggests that the use of a simulated prosthetic device in upper limb research offers a reasonable approximation of compensatory movements employed by a low- to moderately-skilled transradial myoelectric prosthesis user.

A Cost-Effective Inertial Measurement System for Tracking Movement and Triggering Kinesthetic Feedback in Lower-Limb Prosthesis Users

Advances in lower-limb prosthetic technologies have facilitated the restoration of ambulation; however, users of such technologies still experience reduced balance control, also due to the absence of proprioceptive feedback. Recent efforts have demonstrated the ability to restore kinesthetic feedback in upper-limb prosthesis applications; however, technical solutions to trigger the required muscle vibration and provide automated feedback have not been explored for lower-limb prostheses. The study's first objective was therefore to develop a feedback system capable of tracking lower-limb movement and automatically triggering a muscle vibrator to induce the kinesthetic illusion. The second objective was to investigate the developed system's ability to provide kinesthetic feedback in a case participant. A low-cost, wireless feedback system, incorporating two inertial measurement units to trigger a muscle vibrator, was developed and tested in an individual with limb loss above the knee. Our system had a maximum communication delay of 50 ms and showed good tracking of Gaussian and sinusoidal movement profiles for velocities below 180 degrees per second (error < 8 degrees), mimicking stepping and walking, respectively. We demonstrated in the case participant that the developed feedback system can successfully elicit the kinesthetic illusion. Our work contributes to the integration of sensory feedback in lower-limb prostheses, to increase their use and functionality.

Use of the extended feasible stability region for assessing stability of perturbed walking

Walking stability has been assessed through gait variability or existing biomechanical measures. However, such measures are unable to quantify the instantaneous risk of loss-of-balance as a function of gait parameters, body sway, and physiological and perturbation conditions. This study aimed to introduce and evaluate novel biomechanical measures for loss-of-balance under various perturbed walking conditions. We introduced the concept of ‘Extended Feasible Stability Region (ExFSR)’ that characterizes walking stability for the duration of an entire step. We proposed novel stability measures based on the proximity of the body’s centre of mass (COM) position and velocity to the ExFSR limits. We quantified perturbed walking of fifteen non-disabled individuals and three individuals with a disability, and calculated our proposed ExFSR-based measures. 17.2% (32.5%) and 26.3% (34.0%) of the measured trajectories of the COM position and velocity during low (high) perturbations went outside the ExFSR limits, for non-disabled and disabled individuals, respectively. Besides, our proposed measures significantly correlated with measures previously suggested in the literature to assess gait stability, indicating a similar trend in gait stability revealed by them. The ExFSR-based measures facilitate our understanding on the biomechanical mechanisms of loss-of-balance and can contribute to the development of strategies for balance assessment.

Compensatory strategies of body-powered prosthesis users reveal primary reliance on trunk motion and relation to skill level

BACKGROUND

While body-powered prostheses are commonly used, the compensatory strategies required to operate body-powered devices are not well understood. Kinematic assessment in addition to standard clinical tests can give a comprehensive evaluation of prosthesis user function and skill. This study investigated the movement compensations of body-powered prosthesis users and determined whether a correlation is present between compensatory strategies and skill level, as measured by a standard clinical test.

METHODS

Five transradial body-powered prosthesis users completed two standardized upper limb tasks. A 12-camera motion capture system was used to obtain three-dimensional angular kinematics for eight degrees of freedom at the trunk, shoulder, and elbow. Range of motion was compared to a normative dataset. Pearson's correlation was used to assess the relationship between the Activities Measure for Upper Limb Amputees and range of motion for each degree of freedom.

FINDINGS

Participants displayed a statistically significant (P < .05) increase in range of motion at the trunk for both tasks. Shoulder flexion/extension range of motion was significantly reduced (P < .05) compared to normative values, but shoulder abduction/adduction range of motion did not show a consistent difference compared to norms. Skill level was correlated with range of motion for specific degrees of freedom at the trunk, shoulder, and elbow.

INTERPRETATION

Body-powered prosthesis users compensated with trunk movement and showed reduced motion for shoulder flexion/extension, with relatively normal shoulder abduction/adduction. Skill level was correlated with angular kinematic strategies, which may allow targeting of specific therapeutic interventions for reducing compensatory movements.

Skin Stretch Enhances Illusory Movement in Persons with Lower-Limb Amputation

Performance of lower limb prostheses is related not only to the mechanical design and the control scheme, but also to the feedback provided to the user. Proprioceptive feedback, which is the sense of position and movement of one's body parts, can improve the utility as well as facilitate the embodiment of the prosthetic device. Recent studies have shown that proprioceptive kinesthetic (movement) sense can be elicited when non-invasively vibrating a muscle tendon proximal to the targeted joint. However, consistency and quality of the elicited sensation depend on several parameters and muscle tendons after lower limb amputation may not always be accessible. In this study, we developed an experimental protocol to quantitatively and qualitatively assess the elicited proprioceptive kinesthetic illusion when non-invasively vibrating a muscle belly. Furthermore, we explored ways to improve consistency and strength of the illusion by integrating another non-invasive feedback method, namely cutaneous information manipulation via skin stretch. Our preliminary results from tests conducted with a person with transtibial (below knee) amputation show that stretching skin while vibrating a muscle belly on the residual limb provided a stronger and more consistent kinesthetic illusion (90%) than only vibrating the muscle (50%). In addition, we found that stretching skin enhances the range (1.5 times) and speed (3.5 times) of the illusory movement triggered by muscle vibration. These findings may enable the development of mechanisms for controlling feedback parameters in closing the control loop for various walking routines, which may improve performance of lower limb prostheses.

Gaze and Movement Assessment (GaMA): Inter-site validation of a visuomotor upper limb functional protocol

Background

Successful hand-object interactions require precise hand-eye coordination with continual movement adjustments. Quantitative measurement of this visuomotor behaviour could provide valuable insight into upper limb impairments. The Gaze and Movement Assessment (GaMA) was developed to provide protocols for simultaneous motion capture and eye tracking during the administration of two functional tasks, along with data analysis methods to generate standard measures of visuomotor behaviour. The objective of this study was to investigate the reproducibility of the GaMA protocol across two independent groups of non-disabled participants, with different raters using different motion capture and eye tracking technology.

Methods

Twenty non-disabled adults performed the Pasta Box Task and the Cup Transfer Task. Upper body and eye movements were recorded using motion capture and eye tracking, respectively. Measures of hand movement, angular joint kinematics, and eye gaze were compared to those from a different sample of twenty non-disabled adults who had previously performed the same protocol with different technology, rater and site.

Results

Participants took longer to perform the tasks versus those from the earlier study, although the relative time of each movement phase was similar. Measures that were dissimilar between the groups included hand distances travelled, hand trajectories, number of movement units, eye latencies, and peak angular velocities. Similarities included all hand velocity and grip aperture measures, eye fixations, and most peak joint angle and range of motion measures.

Discussion

The reproducibility of GaMA was confirmed by this study, despite a few differences introduced by learning effects, task demonstration variation, and limitations of the kinematic model. GaMA accurately quantifies the typical behaviours of a non-disabled population, producing precise quantitative measures of hand function, trunk and angular joint kinematics, and associated visuomotor behaviour. This work advances the consideration for use of GaMA in populations with upper limb sensorimotor impairment.

Quantitative Eye Gaze and Movement Differences in Visuomotor Adaptations to Varying Task Demands Among Upper-Extremity Prosthesis Users

IMPORTANCE

New treatments for upper-limb amputation aim to improve movement quality and reduce visual attention to the prosthesis. However, evaluation is limited by a lack of understanding of the essential features of human-prosthesis behavior and by an absence of consistent task protocols.

OBJECTIVE

To evaluate whether task selection is a factor in visuomotor adaptations by prosthesis users to accomplish 2 tasks easily performed by individuals with normal arm function.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study was conducted in a single research center at the University of Alberta, Edmonton, Alberta, Canada. Upper-extremity prosthesis users were recruited from January 1, 2016, through December 31, 2016, and individuals with normal arm function were recruited from October 1, 2015, through November 30, 2015. Eight prosthesis users and 16 participants with normal arm function were asked to perform 2 goal-directed tasks with synchronized motion capture and eye tracking. Data analysis was performed from December 3, 2018, to April 15, 2019.

MAIN OUTCOME AND MEASURES

Movement time, eye fixation, and range of motion of the upper body during 2 object transfer tasks (cup and box) were the main outcomes.

RESULTS

A convenience sample comprised 8 male prosthesis users with acquired amputation (mean [range] age, 45 [30-64] years), along with 16 participants with normal arm function (8 [50%] of whom were men; mean [range] age, 26 [18-43] years; mean [range] height, 172.3 [158.0-186.0] cm; all right handed). Prosthesis users spent a disproportionately prolonged mean (SD) time in grasp and release phases when handling the cups (grasp: 2.0 [2.3] seconds vs 0.9 [0.8] seconds; P < .001; release: 1.1 [0.6] seconds vs 0.7 [0.4] seconds; P < .001). Prosthesis users also had increased mean (SD) visual fixations on the hand for the cup compared with the box task during reach (10.2% [12.1%] vs 2.2% [2.8%]) and transport (37.1% [9.7%] vs 22.3% [7.6%]). Fixations on the hand for both tasks were significantly greater for prosthesis users compared with normative values. Prosthesis users had significantly more trunk flexion and extension for the box task compared with the cup task (mean [SD] trunk range of motion, 32.1 [10.7] degrees vs 21.2 [3.7] degrees; P = .01), with all trunk motions greater than normative values. The box task required greater shoulder movements compared with the cup task for prosthesis users (mean [SD] flexion and extension; 51.3 [12.6] degrees vs 41.0 [9.4] degrees, P = .01; abduction and adduction: 40.5 [7.2] degrees vs 32.3 [5.1] degrees, P = .02; rotation: 50.6 [15.7] degrees vs 35.5 [10.0] degrees, P = .02). However, other than shoulder abduction and adduction for the box task, these values were less than those seen for participants with normal arm function.

CONCLUSIONS AND RELEVANCE

This study suggests that prosthesis users have an inherently different way of adapting to varying task demands, therefore suggesting that task selection is crucial in evaluating visuomotor performance. The cup task required greater compensatory visual fixations and prolonged grasp and release movements, and the box task required specific kinematic compensatory strategies as well as increased visual fixation. This is the first study to date to examine visuomotor differences in prosthesis users across varying task demands, and the findings appear to highlight the advantages of quantitative assessment in understanding human-prosthesis interaction.

Predicted threshold against forward and backward loss of balance for perturbed walking

The biomechanical mechanisms of loss of balance have been studied before for slip condition but have not been investigated for arbitrary perturbation profiles under non-slip conditions in sagittal plane. This study aimed to determine the thresholds of center of mass (COM) velocity and position relative to the base of support (BOS) that predict forward and backward loss of balance during walking with a range of BOS perturbations. Perturbations were modeled as sinusoidal BOS motions in the vertical or anterior-posterior direction or as sagittal rotation. The human body was modeled using a seven-link model. Forward dynamics alongside with dynamic optimization were used to find the thresholds of initial COM velocity for each initial COM position that would predict forward or backward loss of balance. The effects of perturbation frequency and amplitude on these thresholds were modeled based on the simulation data. Experimental data were collected from 15 able-bodied individuals and three individuals with disability during perturbed walking. The simulation results showed similarity with the stability region reported for slip and non-slip conditions. The feasible stability region shrank when the perturbation frequency and amplitude increased, especially for larger initial COM velocities. 89.5% (70.9%) and 82.4% (68.2%) of the measured COM position and velocity combinations during low (high) perturbations were located inside the simulated limits of the stability region, for able-bodied and disabled individuals, respectively. The simulation results demonstrated the effects of different perturbation levels on the stability region. The obtained stability region can be used for developing rehabilitative programs in interactive environments.

Hand Function Kinematics when using a Simulated Myoelectric Prosthesis

Studies that investigate myoelectric prosthesis control commonly use non-disabled participants fitted with a simulated prosthetic device. This approach improves participant recruitment numbers but assumes that simulated movements represent those of actual prosthesis users. If this assumption is valid, then movement performance differences between simulated prosthesis users and normative populations should be similar to differences between actual prosthesis users and normative populations. As a first step in testing this assumption, the objective of this study was to quantify movement performance differences between simulated transradial myoelectric prosthesis hand function and normative hand function. Motion capture technology was used to obtain hand kinematics for 12 non-disabled simulated prosthesis participants who performed a functional object-manipulation task. Performance metrics, end effector movement, and grip aperture results were compared to 20 nondisabled participants who used their own hand during task execution. Simulated prosthesis users were expected to perform the functional task more slowly, with multiple peaks in end effector velocity profiles, and a plateau in grip aperture when reaching to pick up objects, when compared to non-disabled participants. This study confirmed these expectations and recommends that subsequent research be undertaken to quantify differences in actual myoelectric prosthesis hand function versus normative hand function.

A method to estimate inertial properties and force plate inertial components for instrumented platforms

Kinetic data acquired from force plates embedded in moving platforms naturally contain artifacts due to platform acceleration, called force plate inertial components. While they can be estimated and removed from the measured signals, the system's inertial properties need to be known. Our objective was to: (1) develop a method for estimating the inertial properties and force plate inertial components for any instrumented platform; (2) estimate the inertial properties specifically for the Computer-Assisted Rehabilitation Environment (CAREN); and (3) validate the estimates with new experimental data. Unloaded ramp-and-hold perturbations (for estimation) and unloaded random perturbations (for validation) were executed to obtain the force, moment, and motion of the CAREN platform. Inertial properties were estimated by minimizing the error between the measured and computed inertial forces and moments. Obtained estimates were validated by calculating the coefficient of determination (R²) between the measured and computed forces or moments when keeping the inertial properties fixed. The estimates of the CAREN's inertial properties exhibited low variability across trials, and R² for the validation trials was 0.90 ± 0.08 (mean ± standard deviation). The developed method can be used for removing inertial components from force plate signals, yielding reliable estimates of ground reactions in dynamic biomechanical research and clinical assessments.

Characterization of normative angular joint kinematics during two functional upper limb tasks

BACKGROUND

Optical motion capture is a powerful tool for assessing upper body kinematics, including compensatory movements, in different populations. However, the lack of a standardized protocol with clear functional relevance hinders its clinical acceptance.

RESEARCH QUESTION

The objective of this study was to use motion capture to: (1) characterize angular joint kinematics in a normative population performing two complex, yet standardized upper limb tasks with clear functional relevance; and (2) assess the protocol's intra-rater reliability.

METHODS

Twenty non-disabled adults performed the previously developed Pasta Box Task and Cup Transfer Task. The kinematics of the upper body were captured using an optoelectronic motion capture system and rigid plates with reflective markers. Angular joint trajectories, peak angle, range of motion (RoM), and peak angular velocity were extracted for the trunk, shoulder, elbow, forearm, and wrist. Intra-class correlation was used to assess the intra-rater reliability of the kinematic measures.

RESULTS

Both tasks required minimal trunk motion. Cross-body movements required greater RoM at the trunk, shoulder, and elbow joints compared to movements in front of the body. Reaches to objects further away from the body required greater trunk and elbow joint RoM compared to reaches to objects closer to the body. Transporting the box of pasta required the wrist to maintain an extended position. The two different grasp patterns in the Cup Transfer Task forced the wrist into a flexed and ulnar-deviated position for the near cup, and an extended and radial-deviated position for the far cup. For both tasks, the majority of measures displayed intra-class correlation values above 0.75, indicating good reliability.

SIGNIFICANCE

Our protocol and functional tasks elicit a degree of movement sensitivity that is not available in current clinical assessments. Our study also provides a comprehensive dataset that can serve as a normative benchmark for quantifying movement compensations following impairment.

A Vibrotactile Feedback Device for Seated Balance Assessment and Training

Postural perturbations, motion tracking, and sensory feedback are modern techniques used to challenge, assess, and train upright sitting, respectively. The goal of the developed protocol is to construct and operate a sitting platform that can be passively destabilized while an inertial measurement unit quantifies its motion and vibrating elements deliver tactile feedback to the user. Interchangeable seat attachments alter the stability level of the device to safely challenge sitting balance. A built-in microcontroller allows fine-tuning of the feedback parameters to augment sensory function. Posturographic measures, typical of balance assessment protocols, summarize the motion signals acquired during timed balance trials. No dynamic sitting protocol to date provides variable challenge, quantification, and sensory feedback free of laboratory constraints. Our results demonstrate that non-disabled users of the device exhibit significant changes in posturographic measures when balance difficulty is altered or vibrational feedback provided. The portable, versatile device has potential applications in rehabilitation (following skeletal, muscular, or neurological injury), training (for sports or spatial awareness), entertainment (via virtual or augmented reality), and research (of sitting-related disorders).

The utility of normative foot floor angle data in assessing toe-walking

Initial heel contact is an important attribute of gait, and failure to complete the heel rocker reduces gait stability. One common goal in treating toe-walking is to restore heel strike and prevent or reduce early heel rise. Foot floor angle (FFA) is a measure of toe-walking that is valuable for quantifying foot orientation at initial contact when using ankle dorsiflexion angle alone is misleading. However, no age-standardized FFA norms exist for clinical evaluation. Our objectives were to: (1) obtain normative FFA in typically developing children; and (2) examine its utility in the example of toe-walking secondary to unilateral cerebral palsy. Gait kinematics were acquired and FFA trajectories computed for 80 typically developing children (4-18 years). They were also obtained retrospectively from 11 children with toe-walking secondary to unilateral cerebral palsy (4-10 years), before and after operative intervention, and compared to 40 age-matched, typically developing children. FFA at initial contact was significantly different (P<.001) between pre-surgery toe-walking (-14.7±9.7°; mean±standard deviation) and typical gait (18.7±2.8°). Following operative lengthening of the gastrocnemius-soleus complex on the affected side, FFA at initial contact (-0.9±5.3°) was significantly improved (P<.001). Furthermore, several cases were identified for which the sole use of ankle dorsiflexion angle to capture toe-walking is misleading. The assessment of FFA is a simple method for providing valuable quantitative information to clinicians regarding foot orientation during gait. The demonstrated limitations of using ankle dorsiflexion angle alone to estimate foot orientation further emphasize the utility of FFA in assessing toe-walking.

Trunk Stability Enabled by Noninvasive Spinal Electrical Stimulation after Spinal Cord Injury

Electrical neuromodulation of spinal networks improves the control of movement of the paralyzed limbs after spinal cord injury (SCI). However, the potential of noninvasive spinal stimulation to facilitate postural trunk control during sitting in humans with SCI has not been investigated. We hypothesized that transcutaneous electrical stimulation of the lumbosacral enlargement can improve trunk posture. Eight participants with non-progressive SCI at C3-T9, American Spinal Injury Association Impairment Scale (AIS) A or C, performed different motor tasks during sitting. Electromyography of the trunk muscles, three-dimensional kinematics, and force plate data were acquired. Spinal stimulation improved trunk control during sitting in all tested individuals. Stimulation resulted in elevated activity of the erector spinae, rectus abdominis, and external obliques, contributing to improved trunk control, more natural anterior pelvic tilt and lordotic curve, and greater multi-directional seated stability. During spinal stimulation, the center of pressure (COP) displacements decreased to 1.36 ± 0.98 mm compared with 4.74 ± 5.41 mm without stimulation (p = 0.0156) in quiet sitting, and the limits of stable displacement increased by 46.92 ± 35.66% (p = 0.0156), 36.92 ± 30.48% (p = 0.0156), 54.67 ± 77.99% (p = 0.0234), and 22.70 ± 26.09% (p = 0.0391) in the forward, backward, right, and left directions, respectively. During self-initiated perturbations, the correlation between anteroposterior arm velocity and the COP displacement decreased from r = 0.5821 (p = 0.0007) without to r = 0.5115 (p = 0.0039) with stimulation, indicating improved trunk stability. These data demonstrate that the spinal networks can be modulated transcutaneously with tonic electrical spinal stimulation to physiological states sufficient to generate a more stable, erect sitting posture after chronic paralysis.

Characterization of normative hand movements during two functional upper limb tasks

Background

Dexterous hand function is crucial for completing activities of daily living (ADLs), which typically require precise hand-object interactions. Kinematic analyses of hand trajectory, hand velocity, and grip aperture provide valuable mechanistic insights into task performance, but there is a need for standardized tasks representative of ADLs that are amenable to motion capture and show consistent performance in non-disabled individuals. Our objective was to develop two standardized functional upper limb tasks and to quantitatively characterize the kinematics of normative hand movement.

Methods

Twenty non-disabled participants were recruited to perform two tasks: the Pasta Box Task and Cup Transfer Task. A 12-camera motion capture system was used to collect kinematic data from which hand movement and grip aperture measures were calculated. Measures reported for reach-grasp and transport-release segments were hand distance travelled, hand trajectory variability, movement time, peak and percent-to-peak hand velocity, number of movement units, peak and percent-to-peak grip aperture, and percent-to-peak hand deceleration. A between-session repeatability analysis was conducted on 10 participants.

Results

Movement times were longer for transport-release compared to reach-grasp for every movement. Hand and grip aperture measures had low variability, with 55 out of 63 measures showing good repeatability (ICC > 0.75). Cross-body movements in the Pasta Box Task had longer movement times and reduced percent-to-peak hand velocity values. The Cup Transfer Task showed decoupling of peak grip aperture and peak hand deceleration for all movements. Movements requiring the clearing of an obstacle while transporting an object displayed a double velocity peak and typically a longer deceleration phase.

Discussion

Normative hand kinematics for two standardized functional tasks challenging various aspects of hand-object interactions important for ADLs showed excellent repeatability. The consistency in normative task performance across a variety of task demands shows promise as a potential outcome assessment for populations with upper limb impairment.

Optimal Estimation of Anthropometric Parameters for Quantifying Multisegment Trunk Kinetics

Kinetics assessment of the human head-arms-trunk (HAT) complex via a multisegment model is a useful tool for objective clinical evaluation of several pathological conditions. Inaccuracies in body segment parameters (BSPs) are a major source of uncertainty in the estimation of the joint moments associated with the multisegment HAT. Given the large intersubject variability, there is currently no comprehensive database for the estimation of BSPs for the HAT. We propose a nonlinear, multistep, optimization-based, noninvasive method for estimating individual-specific BSPs and calculating joint moments in a multisegment HAT model. Eleven nondisabled individuals participated in a trunk-bending experiment and their body motion was recorded using cameras and a force plate. A seven-segment model of the HAT was reconstructed for each participant. An initial guess of the BSPs was obtained by individual-specific scaling of the BSPs calculated from the male visible human (MVH) images. The intersegmental moments were calculated using both bottom-up and top-down inverse dynamics approaches. Our proposed method adjusted the scaled BSPs and center of pressure (COP) offsets to estimate optimal individual-specific BSPs that minimize the difference between the moments obtained by top-down and bottom-up inverse dynamics approaches. Our results indicate that the proposed method reduced the error in the net joint moment estimation (defined as the difference between the net joint moment calculated via bottom-up and top-down approaches) by 79.3% (median among participants). Our proposed method enables an optimized estimation of individual-specific BSPs and, consequently, a less erroneous assessment of the three-dimensional (3D) kinetics of a multisegment HAT model.

Kinematics-based prediction of trunk muscle activity in response to multi-directional perturbations during sitting

Recent work suggests that functional electrical stimulation can be used to enhance dynamic trunk stability following spinal cord injury. In this context, knowledge of the relation between trunk kinematics and muscle activation in non-disabled individuals may assist in developing kinematics-based neuroprostheses. Our objective was therefore to predict the activation profiles of the major trunk muscles from trunk kinematics following multi-directional perturbations during sitting. Trunk motion and electromyograms (EMG) from ten major trunk muscles were acquired in twelve non-disabled, seated individuals who experienced a force of approximately 200 N applied to the trunk in eight horizontal directions. A linear, time-invariant model with feedback gains on angular trunk displacement, velocity, and acceleration was optimized to predict the EMG from trunk kinematics. For each muscle, only the three directions that produced the largest EMG response were considered. Our results indicate that the time course of the processed EMG was similar across muscles and directions and that the model accounted for 68-92% of the EMG variance. A combination of neural and biomechanical mechanisms associated with trunk control can explain the obtained model parameters. Future work will apply the gained insights in the design of movement-controlled neuroprostheses for facilitating trunk stability following spinal cord injury.

Kinematic error magnitude in the single-mass inverted pendulum model of human standing posture

BACKGROUND

Many postural control studies employ a single-mass inverted pendulum model (IPM) to represent the body during standing. However, it is not known to what degree and for what conditions the model's kinematic assumptions are valid.

RESEARCH QUESTION

Our first objective was to quantify the IPM error, corresponding to a distance change between the ankle joint and center of mass (COM) during unrestricted, natural, unperturbed standing. A second objective was to quantify the error of having the ankle joint angle represent the COM angle.

METHODS

Eleven young participants completed five standing conditions: quiet standing with eyes open (EO) and closed (EC), voluntarily swaying forward (VSf) and backward (VSb), and freely moving (FR). The modified Helen-Hayes marker model was used to capture the body kinematics.

RESULTS

The COM distance changed <0.1% during EO and EC, <0.25% during VSf and VSb, and <1.5% during FR. The ankle angle moderately and positively correlated with the COM angle for EO, EC, and VSf, indicating that temporal features of the ankle angle moderately represent those of the COM angle. However, a considerable offset between the two existed, which needs to be considered when estimating the COM angle using the ankle angle. For VSb and FR, the correlation coefficients were low and/or negative, suggesting that a large error would result from using the ankle angle as an estimate of the COM angle.

SIGNIFICANCE

Insights from this study will be critical for deciding when to use the IPM in postural control research and for interpreting associated results.

Design and Evaluation of an Instrumented Wobble Board for Assessing and Training Dynamic Seated Balance

Methods that effectively assess and train dynamic seated balance are critical for enhancing functional independence and reducing risk of secondary health complications in the elderly and individuals with neuromuscular impairments. The objective of this research was to devise and validate a portable tool for assessing and training dynamic seated balance. An instrumented wobble board was designed and constructed that (1) elicits multidirectional perturbations in seated individuals, (2) quantifies seated balance proficiency, and (3) provides real-time, kinematics-based vibrotactile feedback. After performing a technical validation study to compare kinematic wobble board measurements against a gold-standard motion capture system, 15 nondisabled participants performed a dynamic sitting task using the wobble board. Our results demonstrate that the tilt angle measurements were highly accurate throughout the range of wobble board dynamics. Furthermore, the posturographic analyses for the dynamic sitting task revealed that the wobble board can effectively discriminate between the different conditions of perturbed balance, demonstrating its potential to serve as a clinical tool for the assessment and training of seated balance. Vibrotactile feedback decreased the variance of wobble board tilt, demonstrating its potential for use as a balance training tool. Unlike similar instrumented tools, the wobble board is portable, requires no laboratory equipment, and can be adjusted to meet the user's balance abilities. While future work is warranted, obtained findings will aid in effective translation of assessment and training techniques to a clinical setting, which has the potential to enhance the diagnosis and prognosis for individuals with seated balance impairments.

Functional electrical stimulation and spinal cord injury

Spinal cord injuries (SCI) can disrupt communications between the brain and the body, resulting in loss of control over otherwise intact neuromuscular systems. Functional electrical stimulation (FES) of the central and peripheral nervous system can use these intact neuromuscular systems to provide therapeutic exercise options to allow functional restoration and to manage medical complications following SCI. The use of FES for the restoration of muscular and organ functions may significantly decrease the morbidity and mortality following SCI. Many FES devices are commercially available and should be considered as part of the lifelong rehabilitation care plan for all eligible persons with SCI.

Center of pressure velocity reflects body acceleration rather than body velocity during quiet standing

The purpose of this study was to test the hypothesis that the center of pressure (COP) velocity reflects the center of mass (COM) acceleration due to a large derivative gain in the neural control system during quiet standing. Twenty-seven young (27.2±4.5 years) and twenty-three elderly (66.2±5.0 years) subjects participated in this study. Each subject was requested to stand quietly on a force plate for five trials, each 90 s long. The COP and COM displacements, the COP and COM velocities, and the COM acceleration were acquired via a force plate and a laser displacement sensor. The amount of fluctuation of each variable was quantified using the root mean square. Following the experimental study, a simulation study was executed to investigate the experimental findings. The experimental results revealed that the COP velocity was correlated with the COM velocity, but more highly correlated with the COM acceleration. The equation of motion of the inverted pendulum model, however, accounts only for the correlation between the COP and COM velocities. These experimental results can be meaningfully explained by the simulation study, which indicated that the neural motor command presumably contains a significant portion that is proportional to body velocity. In conclusion, the COP velocity fluctuation reflects the COM acceleration fluctuation rather than the COM velocity fluctuation, implying that the neural motor command controlling quiet standing posture contains a significant portion that is proportional to body velocity.

Speed of processing in the primary motor cortex: a continuous theta burst stimulation study

'Temporally urgent' reactions are extremely rapid, spatially precise movements that are evoked following discrete stimuli. The involvement of primary motor cortex (M1) and its relationship to stimulus intensity in such reactions is not well understood. Continuous theta burst stimulation (cTBS) suppresses focal regions of the cortex and can assess the involvement of motor cortex in speed of processing. The primary objective of this study was to explore the involvement of M1 in speed of processing with respect to stimulus intensity. Thirteen healthy young adults participated in this experiment. Behavioral testing consisted of a simple button press using the index finger following median nerve stimulation of the opposite limb, at either high or low stimulus intensity. Reaction time was measured by the onset of electromyographic activity from the first dorsal interosseous (FDI) muscle of each limb. Participants completed a 30 min bout of behavioral testing prior to, and 15 min following, the delivery of cTBS to the motor cortical representation of the right FDI. The effect of cTBS on motor cortex was measured by recording the average of 30 motor evoked potentials (MEPs) just prior to, and 5 min following, cTBS. Paired t-tests revealed that, of thirteen participants, five demonstrated a significant attenuation, three demonstrated a significant facilitation and five demonstrated no significant change in MEP amplitude following cTBS. Of the group that demonstrated attenuated MEPs, there was a biologically significant interaction between stimulus intensity and effect of cTBS on reaction time and amplitude of muscle activation. This study demonstrates the variability of potential outcomes associated with the use of cTBS and further study on the mechanisms that underscore the methodology is required. Importantly, changes in motor cortical excitability may be an important determinant of speed of processing following high intensity stimulation.

What triggers the continuous muscle activity during upright standing?

The ankle extensors play a dominant role in controlling the equilibrium during bipedal quiet standing. Their primary role is to resist the gravity toppling torque that pulls the body forward. The purpose of this study was to investigate whether the continuous muscle activity of the anti-gravity muscles during standing is triggered by the joint torque requirement for opposing the gravity toppling torque, rather than by the vertical load on the lower limbs. Healthy adults subjects stood on a force plate. The ankle torque, ankle angle, and electromyograms from the right lower leg muscles were measured. A ground-fixed support device was used to support the subject at his/her knees, without changing the posture from the free standing one. During the supported condition, which eliminates the ankle torque requirement while maintaining both the vertical load on the lower limbs and the natural upright standing posture, the plantarflexor activity was attenuated to the resting level. Also, this attenuated plantarflexor activity was found only in one side when the ipsilateral leg was supported. Our results suggest that the vertical load on the lower limb is not determinant for inducing the continuous muscle activity in the anti-gravity muscles, but that it depends on the required joint torque to oppose the gravity toppling torque.

Closed-loop control of functional electrical stimulation-assisted arm-free standing in individuals with spinal cord injury: a feasibility study

Objectives.  The purpose of the present study was to show that the design of a neuroprosthesis for unsupported (arm-free) standing is feasible. We review findings suggesting that a closed-loop controlled functional electrical stimulation (FES) system should be able to facilitate arm-free quiet standing in individuals with spinal cord injury (SCI). Particularly, this manuscript identifies: 1) a control strategy that accurately mimics the strategy healthy individuals apply to regulate the ankle joint position during quiet standing and 2) the degrees of freedom (DOF) of the redundant, closed-chain dynamic system of bipedal stance that have to be regulated to facilitate stable standing. Methods and Results.  First, we utilized a single DOF model of quiet standing (inverted pendulum) to analytically identify a proportional and derivative (PD) feedback controller that regulates the ankle torque in a physiologic manner despite a long sensory-motor time delay. Second, these theoretic results were experimentally validated by implementing the proposed PD controller to stabilize an individual with SCI during quiet standing. Third, a realistic, three-dimensional dynamic model of quiet standing with 12 DOF was used to determine the optimal combination of the minimum number of DOF that need to be regulated with the PD controller to ensure stability during quiet standing. Finally, perturbation simulations confirmed that the kinematics of this system are similar to those of healthy individuals during perturbed standing. Conclusions.  The presented results suggest that stable standing can be achieved in individuals with SCI by controlling only six DOF in the lower limbs using FES, and that a PD controller actuating these DOF can stabilize the system despite a long sensory-motor time delay. Our finding that not all DOF in the lower limbs need to be regulated is particularly relevant for individuals with complete SCI, because some of their muscles may be denervated or difficult to access.

Electrophysiological correlates of changes in reaction time based on stimulus intensity

BACKGROUND

Although reaction time is commonly used as an indicator of central nervous system integrity, little is currently understood about the mechanisms that determine processing time. In the current study, we are interested in determining the differences in electrophysiological events associated with significant changes in reaction time that could be elicited by changes in stimulus intensity. The primary objective is to assess the effect of increasing stimulus intensity on the latency and amplitude of afferent inputs to the somatosensory cortex, and their relation to reaction time.

METHODS

Median nerve stimulation was applied to the non-dominant hand of 12 healthy young adults at two different stimulus intensities (HIGH & LOW). Participants were asked to either press a button as fast as possible with their dominant hand or remain quiet following the stimulus. Electroencephalography was used to measure somatosensory evoked potentials (SEPs) and event related potentials (ERPs). Electromyography from the flexor digitorum superficialis of the button-pressing hand was used to assess reaction time. Response time was the time of button press.

RESULTS

Reaction time and response time were significantly shorter following the HIGH intensity stimulus compared to the LOW intensity stimulus. There were no differences in SEP (N20 & P24) peak latencies and peak-to-peak amplitude for the two stimulus intensities. ERPs, locked to response time, demonstrated a significantly larger pre-movement negativity to positivity following the HIGH intensity stimulus over the Cz electrode.

DISCUSSION

This work demonstrates that rapid reaction times are not attributable to the latency of afferent processing from the stimulated site to the somatosensory cortex, and those latency reductions occur further along the sensorimotor transformation pathway. Evidence from ERPs indicates that frontal planning areas such as the supplementary motor area may play a role in transforming the elevated sensory volley from the somatosensory cortex into a more rapid motor response.

Effects of balance training with visual feedback during mechanically unperturbed standing on postural corrective responses

Evidence of a non-specific effect of balance training on postural control mechanisms suggests that balance training during mechanically unperturbed standing may improve postural corrective responses following external perturbations. The purpose of the present study was to examine kinematics of the trunk as well as muscular activity of the lower leg and paraspinal muscles during postural responses to support-surface rotations after short-term balance training. Experiments were performed in control (n=10) and experimental (n=11) groups. The experimental group participated in the 3-day balance training program. During the training, participants stood on a force platform and were instructed to voluntarily shift their center of pressure in indicated directions as represented by a cursor on a monitor. Postural perturbation tests were executed before and after the training period: the slow and fast 10° dorsiflexions were induced at angular velocities of approximately 50°s(-1) and 200°s(-1), respectively. In the experimental group, the amplitude of the trunk displacements during slow and fast perturbations was up to 33.4% and 26.7% lower, respectively, following the training. The magnitude of the muscular activity was reduced in both the early and late components of the response. The kinematic parameters and muscular responses did not change in the control group. The results suggest that balance training during unperturbed standing has the potential to improve postural corrective responses to unexpected balance perturbation through (1) improved neuromuscular coordination of the involved muscles and (2) adaptive neural modifications on the spinal and cortical levels facilitated by voluntary activity.