Center of Mass Estimation for Impaired Gait Assessment Using Inertial Measurement Units.

Injury or disease often compromise walking dynamics and negatively impact quality of life and independence. Assessing methods to restore or improve pathological gait can be expedited by examining a global parameter that reflects overall musculoskeletal control. Center of mass (CoM) kinematics follow well-defined trajectories during unimpaired gait, and change predictably with various gait pathologies. We propose a method to estimate CoM trajectories from inertial measurement units (IMUs) using a bidirectional Long Short-Term Memory neural network to evaluate rehabilitation interventions and outcomes. Five non-disabled volunteers participated in a single session of various dynamic walking trials with IMUs mounted on various body segments. A neural network trained with data from four of the five volunteers through a leave-one-subject out cross validation estimated the CoM with average root mean square errors (RMSEs) of 1.44cm, 1.15cm, and 0.40cm in the mediolateral (ML), anteroposterior (AP), and inferior/superior (IS) directions respectively. The impact of number and location of IMUs on network prediction accuracy was determined via principal component analysis. Comparing across all configurations, three to five IMUs located on the legs and medial trunk were the most promising reduced sensor sets for achieving CoM estimates suitable for outcome assessment. Lastly, the networks were tested on data from an individual with hemiparesis with the greatest error increase in the ML direction, which could stem from asymmetric gait. These results provide a framework for assessing gait deviations after disease or injury and evaluating rehabilitation interventions intended to normalize gait pathologies.

Distally-referred surface electrical nerve stimulation (DR-SENS) for haptic feedback.

Objective.This study's objective is to understand distally-referred surface electrical nerve stimulation (DR-SENS) and evaluates the effects of electrode placement, polarity, and stimulation intensity on the location of elicited sensations in non-disabled individuals.Approach.A two-phased human experiment was used to characterize DR-SENS. In Experiment One, we explored 182 electrode combinations to identify a subset of electrode position combinations that would be most likely to elicit distally-referred sensations isolated to the index finger without discomfort. In Experiment Two, we further examined this subset of electrode combinations to determine the effect of stimulation intensity and electrode position on perceived sensation location. Stimulation thresholds were evaluated using parameter estimation by sequential testing and sensation locations were characterized using psychometric intensity tests.Main Results.We found that electrode positions distal to the wrist can consistently evoke distally referred sensations with no significant polarity dependency. The finger-palm combination had the most occurrences of distal sensations, and the different variations of this combination did not have a significant effect on sensation location. Increasing stimulation intensity significantly expanded the area of the sensation, moved the most distal sensation distally, and moved the vertical centroid proximally. Also, a large anodic-leading electrode at the elbow mitigated all sensation at the anodic-leading electrode site while using symmetric stimulation waveforms. Furthermore, this study showed that the most intense sensation for a given percept can be distally referred. Lastly, for each participant, at least one of the finger-palm combinations evaluated in this study worked at both perception threshold and maximum comfortable stimulation intensities.Significance.These findings show that a non-invasive surface electrical stimulation charge modulated haptic interface can be used to elicit distally-referred sensations on non-disabled users. Furthermore, these results inform the design of novel haptic interfaces and other applications of surface electrical stimulation based haptic feedback on electrodes positioned distally from the wrist.

Biologically Inspired Optimal Terminal Iterative Learning Control for the Swing Phase of Gait in a Hybrid Neuroprosthesis: A Modeling Study.

(1) Background: An iterative learning control (ILC) strategy was developed for a "Muscle First" Motor-Assisted Hybrid Neuroprosthesis (MAHNP). The MAHNP combines a backdrivable exoskeletal brace with neural stimulation technology to enable persons with paraplegia due to spinal cord injury (SCI) to execute ambulatory motions and walk upright. (2) Methods: The ILC strategy was developed to swing the legs in a biologically inspired ballistic fashion. It maximizes muscular recruitment and activates the motorized exoskeletal bracing to assist the motion as needed. The control algorithm was tested using an anatomically realistic three-dimensional musculoskeletal model of the lower leg and pelvis suitably modified to account for exoskeletal inertia. The model was developed and tested with the OpenSim biomechanical modeling suite. (3) Results: Preliminary data demonstrate the efficacy of the controller in swing-leg simulations and its ability to learn to balance muscular and motor contributions to improve performance and accomplish consistent stepping. In particular, the controller took 15 iterations to achieve the desired outcome with 0.3% error.

Walking after incomplete spinal cord injury with an implanted neuromuscular electrical stimulation system and a hinged knee replacement: a single-subject study.

STUDY DESIGN: Single-subject repeated measures study. OBJECTIVES: Neuromuscular electrical stimulation (NMES) can enhance walking for people with partial paralysis from incomplete spinal cord injury (iSCI). This single-subject study documents an individual's experience who both received an experimental implanted NMES system and underwent clinical bilateral hinged total knee arthroplasty (TKA). She walked in the community with knee pain prior to either intervention. Walking performance improved with an implanted NMES system. Knee pain and instability continued to worsen over time and eventually required TKA. This study evaluates the effects of these interventions. SETTING: Louis Stokes Cleveland Veterans Affairs Medical Center, Cleveland OH, USA. METHODS: The differential and combined effects of NMES and hinged knee replacement were assessed in terms of walking speed, toe clearance, knee angle, and participant perceptions with and without stimulation assistance both before and after TKA. RESULTS: The combined approach both reduced pain and restored walking ability to levels achieved prior to developing significant knee pain that prevented walking without NMES. There was an interaction effect between NMES and TKA on walking speed. Toe clearance consistently improved with stimulation assistance and TKA prevented significant knee hyperextension. The greatest impact was on endurance. Knee replacement re-enabled long distance walking with the addition of stimulation again more than doubling her maximum walking distance from 214 to 513 m. CONCLUSIONS: These data support further research of combined implantable interventions that may benefit people with iSCI. Furthermore, joint laxity and pain may not necessarily be contraindications to NMES if addressed with conventional clinical treatments.

Adding Contralaterally Controlled Electrical Stimulation of the Triceps to Contralaterally Controlled Functional Electrical Stimulation of the Finger Extensors Reduces Upper Limb Impairment and Improves Reachable Workspace but not Dexterity: A Randomized Controlled Trial.

OBJECTIVE: Different methods of neuromuscular electrical stimulation may be used for poststroke upper limb rehabilitation. This study evaluated the effects of contralaterally controlled functional electrical stimulation of the triceps and finger extensors. DESIGN: This is a randomized controlled trial of 67 participants who were less than 2 yrs poststroke and assigned to the following: (a) arm + hand contralaterally controlled functional electrical stimulation, (b) hand contralaterally controlled functional electrical stimulation, or (c) arm + hand cyclic neuromuscular electrical stimulation. Participants were prescribed 10 sessions/week of assigned electrical stimulation at home plus 24 sessions of functional task practice in the laboratory for 12 wks. The primary outcome measure was the Box and Blocks Test. Secondary measures included reachable workspace, Upper Extremity Fugl-Meyer, Stroke Upper Limb Capacity Scale, Arm Motor Abilities Test, and Motor Activity Log. RESULTS: There were no significant between-group differences on the Box and Blocks Test. At 6 mos after treatment, arm + hand contralaterally controlled functional electrical stimulation improved reachable workspace more than hand contralaterally controlled functional electrical stimulation, between-group difference of 264 (95% confidence interval = 28-500) cm and more than arm + hand cyclic neuromuscular electrical stimulation, between-group difference of 281 (95% confidence interval = 22-540) cm. Arm + hand contralaterally controlled functional electrical stimulation improved Upper Extremity Fugl-Meyer score more than hand contralaterally controlled functional electrical stimulation, between-group difference of 6.7 (95% confidence interval = 0.6-12.7). The between-group differences on the Stroke Upper Limb Capacity Scale and Arm Motor Abilities Test were not significant. CONCLUSIONS: Adding contralaterally controlled elbow extension to hand contralaterally controlled functional electrical stimulation does not improve on gains in hand dexterity, but it further reduces upper limb impairment and improves reachable workspace measured in the laboratory. However, these additional benefits may not be large enough to be perceived by stroke survivors when they are performing activities of daily living at home.

Oxygen Consumption While Walking With Multijoint Neuromuscular Electrical Stimulation After Stroke.

This case study evaluated the effect of implanted multijoint neuromuscular electrical stimulation gait assistance on oxygen consumption relative to walking without neuromuscular electrical stimulation after stroke. The participant walked slowly with an asymmetric gait pattern after stroke. He completed repeated 6-min walk tests at a self-selected walking speed with and without hip, knee, and ankle stimulation assistance. His walking speed with neuromuscular electrical stimulation more than doubled from 0.28 ± 0.01 m/sec to 0.58 ± 0.04 m/sec, whereas average step length and cadence increased by 0.12 m and 24 steps/min, respectively. As a result, energy cost of walking with neuromuscular electrical stimulation decreased by 0.19 ml O2/kg per meter as compared with walking without stimulation while oxygen consumption increased by 1.1 metabolic equivalent of tasks (3.9 ml O2/kg per minute). These metabolic demands are similar to those reported for stroke survivors capable of walking at equivalent speeds without stimulation, suggesting the increase in oxygen consumption and decreased energy cost result from improved efficiency of faster walking facilitated by neuromuscular electrical stimulation. Although the effect of neuromuscular electrical stimulation on gait economy has implications for community walking within the user's metabolic reserves, this case study's results should be interpreted with caution and the hypothesis that multijoint neuromuscular electrical stimulation improves metabolic efficiency should be tested in a wide population of stroke survivors with varied deficits.

Improving Walking with an Implanted Neuroprosthesis for Hip, Knee, and Ankle Control After Stroke.

OBJECTIVE: The objective of this work was to quantify the effects of a fully implanted pulse generator to activate or augment actions of hip, knee, and ankle muscles after stroke. DESIGN: The subject was a 64-year-old man with left hemiparesis resulting from hemorrhagic stroke 21 months before participation. He received an 8-channel implanted pulse generator and intramuscular stimulating electrodes targeting unilateral hip, knee, and ankle muscles on the paretic side. After implantation, a stimulation pattern was customized to assist with hip, knee, and ankle movement during gait.The subject served as his own concurrent and longitudinal control with and without stimulation. Outcome measures included 10-m walk and 6-minute timed walk to assess gait speed, maximum walk time, and distance to measure endurance, and quantitative motion analysis to evaluate spatial-temporal characteristics. Assessments were repeated under 3 conditions: (1) volitional walking at baseline, (2) volitional walking after training, and (3) walking with stimulation after training. RESULTS: Volitional gait speed improved with training from 0.29 m/s to 0.35 m/s and further increased to 0.72 m/s with stimulation. Most spatial-temporal characteristics improved and represented more symmetrical and dynamic gait. CONCLUSIONS: These data suggest that a multijoint approach to implanted neuroprostheses can provide clinically relevant improvements in gait after stroke. TO CLAIM CME CREDITS: Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME CME OBJECTIVES:: Upon completion of this article, the reader should be able to do the following: (1) Describe the rationale for evaluating a multijoint implanted neuroprosthesis to improvewalkingafter stroke; (2)Understand the study design and conclusions that can be inferred as a result of the design; and (3) Discuss the statistical significance and clinical relevance of changes between (a) volitional walking at baseline, (b) volitional walking after training, and (c) walking with stimulation after training. LEVEL: Advanced ACCREDITATION:: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians. The Association of Academic Physiatrists designates this activity for a maximum of 1.5 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.

Accelerometer-based step initiation control for gait-assist neuroprostheses.

Electrical activation of paralyzed musculature can generate or augment joint movements required for walking after central nervous system trauma. Proper timing of stimulation relative to residual volitional control is critical to usefully affecting ambulation. This study evaluates three-dimensional accelerometers and customized algorithms to detect the intent to step from voluntary movements to trigger stimulation during walking in individuals with significantly different etiologies, mobility limitations, manual dexterities, and walking aids. Three individuals with poststroke hemiplegia or partial spinal cord injury exhibiting varying gait deficits were implanted with multichannel pulse generators to provide joint motions at the hip, knee, and ankle. An accelerometer integrated into the external control unit was used to detect heel strike or walker movement, and wireless accelerometers were used to detect crutch strike. Algorithms were developed for each sensor location to detect intent to step to progress through individualized stimulation patterns. Testing these algorithms produced detection accuracies of at least 90% on both level ground and uneven terrain. All participants use their accelerometer-triggered implanted gait systems in the community; the validation/system testing was completed in the hospital. The results demonstrated that safe, reliable, and convenient accelerometer-based step initiation can be achieved regardless of specific gait deficits, manual dexterities, and walking aids.

Control of robotic assistance using poststroke residual voluntary effort.

Poststroke hemiparesis limits the ability to reach, in part due to involuntary muscle co-activation (synergies). Robotic approaches are being developed for both therapeutic benefit and continuous assistance during activities of daily living. Robotic assistance may enable participants to exert less effort, thereby reducing expression of the abnormal co-activation patterns, which could allow participants to reach further. This study evaluated how well participants could perform a reaching task with robotic assistance that was either provided independent of effort in the vertical direction or in the sagittal plane in proportion to voluntary effort estimated from electromyograms (EMG) on the affected side. Participants who could not reach targets without assistance were enabled to reach further with assistance. Constant anti-gravity force assistance that was independent of voluntary effort did not reduce the quality of reach and enabled participants to exert less effort while maintaining different target locations. Force assistance that was proportional to voluntary effort on the affected side enabled participants to exert less effort and could be controlled to successfully reach targets, but participants had increased difficulty maintaining a stable position. These results suggest that residual effort on the affected side can produce an effective command signal for poststroke assistive devices.

Contributions to muscle force and EMG by combined neural excitation and electrical stimulation.

OBJECTIVE: Stimulation of muscle for research or clinical interventions is often superimposed on ongoing physiological activity without a quantitative understanding of the impact of the stimulation on the net muscle activity and the physiological response. Experimental studies show that total force during stimulation is less than the sum of the isolated voluntary and stimulated forces, but the occlusion mechanism is not understood. APPROACH: We develop a model of efferent motor activity elicited by superimposing stimulation during a physiologically activated contraction. The model combines action potential interactions due to collision block, source resetting, and refractory periods with previously published models of physiological motor unit recruitment, rate modulation, force production, and EMG generation in human first dorsal interosseous muscle to investigate the mechanisms and effectiveness of stimulation on the net muscle force and EMG. MAIN RESULTS: Stimulation during a physiological contraction demonstrates partial occlusion of force and the neural component of the EMG, due to action potential interactions in motor units activated by both sources. Depending on neural and stimulation firing rates as well as on force-frequency properties, individual motor unit forces can be greater, smaller, or unchanged by the stimulation. In contrast, voluntary motor unit EMG potentials in simultaneously stimulated motor units show progressive occlusion with increasing stimulus rate. The simulations predict that occlusion would be decreased by a reverse stimulation recruitment order. SIGNIFICANCE: The results are consistent with and provide a mechanistic interpretation of previously published experimental evidence of force occlusion. The models also predict two effects that have not been reported previously--voluntary EMG occlusion and the advantages of a proximal stimulation site. This study provides a basis for the rational design of both future experiments and clinical neuroprosthetic interventions involving either motor or sensory stimulation.

Alteration of neural action potential patterns by axonal stimulation: the importance of stimulus location.

OBJECTIVE: Stimulation of peripheral nerves is often superimposed on ongoing motor and sensory activity in the same axons, without a quantitative model of the net action potential train at the axon endpoint. APPROACH: We develop a model of action potential patterns elicited by superimposing constant frequency axonal stimulation on the action potentials arriving from a physiologically activated neural source. The model includes interactions due to collision block, resetting of the neural impulse generator, and the refractory period of the axon at the point of stimulation. MAIN RESULTS: Both the mean endpoint firing rate and the probability distribution of the action potential firing periods depend strongly on the relative firing rates of the two sources and the intersite conduction time between them. When the stimulus rate exceeds the neural rate, neural action potentials do not reach the endpoint and the rate of endpoint action potentials is the same as the stimulus rate, regardless of the intersite conduction time. However, when the stimulus rate is less than the neural rate, and the intersite conduction time is short, the two rates partially sum. Increases in stimulus rate produce non-monotonic increases in endpoint rate and continuously increasing block of neurally generated action potentials. Rate summation is reduced and more neural action potentials are blocked as the intersite conduction time increases. At long intersite conduction times, the endpoint rate simplifies to being the maximum of either the neural or the stimulus rate. SIGNIFICANCE: This study highlights the potential of increasing the endpoint action potential rate and preserving neural information transmission by low rate stimulation with short intersite conduction times. Intersite conduction times can be decreased with proximal stimulation sites for muscles and distal stimulation sites for sensory endings. The model provides a basis for optimizing experiments and designing neuroprosthetic interventions involving motor or sensory stimulation.

Contralaterally controlled functional electrical stimulation for recovery of elbow extension and hand opening after stroke: a pilot case series study.

OBJECTIVE: The aims of this study were to determine whether patients with moderate-to-severe upper limb hemiplegia could use contralaterally controlled functional electrical stimulation at the arm and hand (Arm+Hand CCFES) at home and to evaluate the feasibility of Arm+Hand CCFES to reduce arm and hand motor impairment. DESIGN: With Arm+Hand CCFES, the paretic elbow and hand extensors were stimulated with intensities proportional to the degree of elbow extension and hand opening, respectively, of the contralateral unimpaired side. For 12 wks, four participants with chronic (≥6 mos) upper limb hemiplegia received ∼7 hrs per week of self-administered home-based stimulation-mediated elbow extension and hand opening exercise plus ∼2.5 hrs per week of therapist-supervised laboratory-based stimulation-assisted functional task practice. Assessments of upper limb impairment were made at pretreatment, posttreatment, and 1 mo after treatment. RESULTS: All four participants were able to use the Arm+Hand CCFES system at home either independently or with very minimal assistance from a caregiver. All four participants had increases in the Fugl-Meyer score (1-9 points) and the Wolf Motor Function Test (0.2-0.8 points) and varying degrees of improvement in maximum hand opening, maximum elbow extension, and simultaneous elbow extension and hand opening. CONCLUSIONS: Arm+Hand CCFES can be successfully administered in stroke patients with moderate-to-severe impairment and can reduce various aspects of upper limb impairment. A larger efficacy study is warranted.

Functional electrical stimulation to augment poststroke reach and hand opening in the presence of voluntary effort: a pilot study.

BACKGROUND: Hemiparesis after stroke can severely limit an individual's ability to perform activities of daily living. Functional electrical stimulation (FES) has the potential to generate functional arm and hand movements. We have observed that FES can produce functional hand opening when a stroke patient is relaxed, but the FES-produced hand opening is often overpowered by finger flexor coactivation in response to patient attempts to reach and open the hand. OBJECTIVE: To determine if stimulating both reaching muscles and hand opening muscles makes it possible to achieve useful amounts of simultaneous reach and hand opening even in the presence of submaximal reaching effort. METHODS: We measured reach and hand opening during a reach-then-open the hand task under different combinations of voluntary effort and FES for both reach and hand opening. RESULTS: As effort was reduced and stimulation generated more movement, a greater amount of reach and hand opening was achieved. For the first time, this study quantified the effect of voluntary effort for reach and hand opening on stimulated hand opening. It also showed variability in the interaction of voluntary effort and stimulation between participants. Additionally, when participants were instructed to reach with partial effort during simultaneous FES, they achieved greater reach and hand opening. CONCLUSIONS: Simultaneous reaching and FES hand opening is improved by including FES for reach and reducing voluntary effort. In the future, an upper extremity neuroprosthesis that uses a combination of voluntary effort and FES assistance may enable users to perform activities of daily living.

Muscle response to simultaneous stimulated and physiological action potential trains--a simulation study.

The objective of this study was to assess the mechanisms responsible for the experimentally observed nonlinear addition of forces produced by voluntary contractions during superimposed electrical stimulation of the same muscle. A model of action potential interaction predicts increased motor unit firing rates during superimposed stimulation. The resulting effects on force production reproduce experimental results, confirming that motor unit force saturation contributes to nonlinear force addition. The model further predicts that the voluntary EMG will be reduced by stimulation, due to collision block and phase resetting of motor unit action potentials. Both effects have implications for the design of FES neuroprosthesis systems.

Contralaterally controlled functional electrical stimulation for stroke rehabilitation.

Contralaterally controlled functional electrical stimulation (CCFES) is an innovative method of delivering neuromuscular electrical stimulation for rehabilitation of paretic limbs after stroke. It is being studied to evaluate its efficacy in improving recovery of arm and hand function and ankle dorsiflexion in chronic and subacute stroke patients. The initial studies provide preliminary evidence supporting the efficacy of CCFES.

Variations in neuromuscular electrical stimulation's ability to increase reach and hand opening during voluntary effort after stroke.

Functional Electrical Stimulation (FES) has shown potential as a mechanism to augment functional arm and hand movement after stroke. However, neuroprostheses that combine voluntary effort and FES must account for co-activation patterns (synergies) that limit movement. The goal of this study is to explore the conditions under which voluntary effort and FES can be combined to achieve useful reach and hand opening in different subjects. Subjects performed a reach and hand opening task where different levels of voluntary effort and FES were applied to produce reach and hand opening while measuring the resulting hand opening and distance from a target. Initial results indicate that there are significant variations between participants and how much effort can be exerted while still eliciting effective reach and hand opening.

Neuromuscular electrical stimulation to augment reach and hand opening after stroke.

Functional Electrical Stimulation (FES) may be able to augment functional arm and hand movement after stroke. However, neuroprostheses that combine voluntary effort and FES must take into account the co-contraction patterns (synergies) that are common across multiple joints. The goal of this study is to determine the principles under which voluntary effort and FES can be combined to achieve useful reach and hand opening. A reach and hand opening task is performed where different levels of voluntary effort and FES are applied to produce reach while measuring the level of hand opening that FES can produce at the hand. Initial results indicate that low levels of voluntary effort allow both greater reach and the largest hand opening response to FES.