Explore the vast range of titles available.

Spasticity attributable to exaggerated stretch reflex pathways, particularly affecting the ankle plantar flexors, often impairs overground walking in persons with incomplete spinal cord injury. Compelling evidence from rodent models underscores how exposure to acute intermittent hypoxia (AIH) can provide a unique medium to induce spinal plasticity in key inhibitory pathways mediating stretch reflex excitability and potentially affect spasticity. In this study, we quantify the effects of a single exposure to AIH on the stretch reflex in able‐bodied individuals. We hypothesized that a single sequence of AIH will increase the stretch reflex excitability of the soleus muscle during ramp‐and‐hold angular perturbations applied to the ankle joint while participants perform passive and volitionally matched contractions. Our results revealed that a single AIH exposure did not significantly change the stretch reflex excitability during both passive and active matching conditions. Furthermore, we found that able‐bodied individuals increased their stretch reflex response from passive to active matching conditions after both sham and AIH exposures. Together, these findings suggest that a single AIH exposure might not engage inhibitory pathways sufficiently to alter stretch reflex responses in able‐bodied persons. However, the generalizability of our present findings requires further examination during repetitive exposures to AIH along with potential reflex modulation during functional movements, such as overground walking. What is the central question of this study? Acute intermittent hypoxia (AIH) has been shown to enhance walking performance in persons with spinal cord injury but might also trigger changes affecting dysregulated stretch reflex pathways, leading to spasticity. We examined the effect of AIH on stretch reflex excitability in able‐bodied individuals. What is the main finding and its importance? We did not find significant changes in reflex responses after AIH nor changes in reflex scaling from passive to active conditions. This study provides a crucial insight that enhances our understanding of AIH engagement of key inhibitory pathways that affect reflex excitability.

[...]there is a tremendous need for novel treatments that may enhance efficacy of these residual pathways and subsequently provide controlled restoration of hand function after cervical SCI. Persons with cervical SCI often regain function through learned use of assistive devices,9 as well as alternative muscle coordination strategies of the upper extremity.10 In intact neural systems, the acquisition of motor skills depends on experience-dependent functional and structural reorganization of the spinal cord and motor cortex.11 Following SCI, skill acquisition is confronted with competing injury-induced and experience-driven neural changes that impact hand function. Because enduring benefits are not known, dosing then becomes clinically relevant because a balance between maximizing and retaining benefits is important, especially if there are potential safety risks. [...]the efficacy and safety of noninvasive stimulation treatment warrants investigations with appropriately normalized protocols that rigorously characterize stimulation location, timing, dose, frequency, and intensity with neural mechanisms of action.

Humans routinely modify their walking speed to adapt to functional goals and physical demands. However, damage to the central nervous system (CNS) often results in abnormal modulation of walking speed and increase risk of falls. There is considerable interest in pursuing therapies that incorporate augmented sensory feedback devices to compensate for less reliable signaling within the CNS after injury or disease. Augmented visual feedback uses a range of immersive environments to enhance walking ability after injury to the CNS. Fully immersive virtual reality technologies show benefits in boosting training-related gains in walking performance; however, they lack views of the real world that may limit functional carryover. Mixed reality provides partially immersive environments to extend the virtual reality benefits of interacting with virtual objects but within an unobstructed view of the real world. Despite this potential advantage, the feasibility of using a wearable MR head mount device (MR-HMD) for visual feedback to prompt goal-directed changes in overground walking speed remains unclear. Thus, we developed and evaluated a novel mixed reality application using the Microsoft HoloLens MR-HMD to augment visual feedback during overground walking in a cohort of able-bodied adults. This platform provided real-time visual cues related to each participant’s self-selected walking speed. Participants walked with MR-HMD visual feedback at 85%, 100%, and 115% of their baseline self-selected speed. We block-randomized the order of these feedback conditions. Walking performance corresponded to the accuracy and variability of three walking parameters (speed, stride length, and stride time) within and between the feedback conditions. Overall, participants matched their overground walking speed to the target speed of the MR-HMD visual feedback conditions (all p-values > 0.05). Walking speed variability did not differ between walking with and without visual feedback. However, stride length and time variability increased with visual feedback as compared to without feedback. The findings offer support for MR-based visual feedback as a method to provoke goal-specific changes in overground walking behavior. Further studies are necessary to determine the clinical safety and efficacy of this MR-HMD technology to provide extrinsic sensory feedback in combination with traditional treatments in rehabilitation.

The human motor system is highly redundant, having more kinematic degrees of freedom than necessary to complete a given task. Understanding how kinematic redundancies are utilized in different tasks remains a fundamental question in motor control. One possibility is that they can be used to tune the mechanical properties of a limb to the specific requirements of a task. For example, many tasks such as tool usage compromise arm stability along specific directions. These tasks only can be completed if the nervous system adapts the mechanical properties of the arm such that the arm, coupled to the tool, remains stable. The purpose of this study was to determine if posture selection is a critical component of endpoint stiffness regulation during unconstrained tasks. Three-dimensional (3D) estimates of endpoint stiffness were used to quantify limb mechanics. Most previous studies examining endpoint stiffness adaptation were completed in 2D using constrained postures to maintain a non-redundant mapping between joint angles and hand location. Our hypothesis was that during unconstrained conditions, subjects would select arm postures that matched endpoint stiffness to the functional requirements of the task. The hypothesis was tested during endpoint tracking tasks in which subjects interacted with unstable haptic environments, simulated using a 3D robotic manipulator. We found that arm posture had a significant effect on endpoint tracking accuracy and that subjects selected postures that improved tracking performance. For environments in which arm posture had a large effect on tracking accuracy, the self-selected postures oriented the direction of maximal endpoint stiffness towards the direction of the unstable haptic environment. These results demonstrate how changes in arm posture can have a dramatic effect on task performance and suggest that postural selection is a fundamental mechanism by which kinematic redundancies can be exploited to regulate arm stiffness in unconstrained tasks.

Incomplete spinal cord injury (iSCI) often leads to partial disruption of spinal pathways that are important for motor control of walking. Persons with iSCI present with deficits in walking ability in part because of inconsistent leg kinematics during stepping. Although kinematic variability is important for normal walking, growing evidence indicates that excessive variability may limit walking ability and increase reliance on assistive devices (AD) after iSCI. The purpose of this study was to assess the effects of iSCI-induced impairments on kinematic variability during overground walking. We hypothesized that iSCI results in greater variability of foot and joint displacement during overground walking compared with controls. We further hypothesized that variability is larger in persons with limited walking speed and greater reliance on ADs. To test these hypotheses, iSCI and control subjects walked overground. Kinematic variability was quantified as step-to-step foot placement variability (end-point), and variability in hip-knee, hip-ankle, and knee-ankle joint space (angular coefficient of correspondence [ACC]). We characterized sensitivity of kinematic variability to cadence, auditory cue, and AD. Supporting our hypothesis, persons with iSCI exhibited greater kinematic variability than controls, which scaled with deficits in overground walking speed (p < 0.01). Significant correlation between ACC and end-point variability, and with walking speed, indicates that both are markers of walking performance. Moreover, hip-knee and hip-ankle ACC discriminated AD use, indicating that ACC may capture AD-specific control strategies. We conclude that increased variability of foot and joint displacement are indicative of motor impairment severity and may serve as therapeutic targets to restore walking after iSCI.

Purpose of Review The reacquisition and preservation of walking ability are highly valued goals in spinal cord injury (SCI) rehabilitation. Recurrent episodes of breathing low oxygen (i.e., acute intermittent hypoxia, AIH) are a potential therapy to promote walking recovery after incomplete SCI via endogenous mechanisms of neuroplasticity. Here, we report on the progress of AIH, alone or paired with other treatments, on walking recovery in persons with incomplete SCI. We evaluate the evidence of AIH as a therapy ready for clinical and home use and the real and perceived challenges that may interfere with this possibility. Recent Findings Repetitive AIH is a safe and an efficacious treatment to enhance strength, walking speed, and endurance, as well as dynamic balance in persons with chronic, incomplete SCI. Summary The potential for AIH as a treatment for SCI remains high, but further research is necessary to understand treatment targets and effectiveness in a large cohort of persons with SCI.

Background Restoring community walking remains a highly valued goal for persons recovering from traumatic incomplete spinal cord injury (SCI). Recently, studies report that brief episodes of low-oxygen breathing (acute intermittent hypoxia, AIH) may serve as an effective plasticity-inducing primer that enhances the effects of walking therapy in persons with chronic (> 1 year) SCI. More persistent walking recovery may occur following repetitive (weeks) AIH treatment involving persons with more acute SCI, but this possibility remains unknown. Here we present our clinical trial protocol, designed to examine the distinct influences of repetitive AIH, with and without walking practice, on walking recovery in persons with sub-acute SCI (< 12 months) SCI. Our overarching hypothesis is that daily exposure (10 sessions, 2 weeks) to AIH will enhance walking recovery in ambulatory and non-ambulatory persons with subacute (< 12 months) SCI, presumably by harnessing endogenous mechanisms of plasticity that occur soon after injury. Methods To test our hypothesis, we are conducting a randomized, placebo-controlled clinical trial on 85 study participants who we stratify into two groups according to walking ability; those unable to walk (non-ambulatory group) and those able to walk (ambulatory group). The non-ambulatory group receives either daily AIH (15, 90s episodes at 10.0% O 2 with 60s intervals at 20.9% O 2 ) or daily SHAM (15, 90s episodes at 20.9% O 2 with 60s intervals at 20.9% O 2 ) intervention. The ambulatory group receives either 60-min walking practice (WALK), daily AIH + WALK, or daily SHAM+WALK intervention. Our primary outcome measures assess overground walking speed (10-Meter Walk Test), endurance (6-Minute Walk Test), and balance (Timed Up & Go Test). For safety, we also measure levels of pain, spasticity, systemic hypertension, and autonomic dysreflexia. We record outcome measures at baseline, days 5 and 10, and follow-ups at 1 week, 1 month, 6 months, and 12 months post-treatment. Discussion The goal of this clinical trial is to reveal the extent to which daily AIH, alone or in combination with task-specific walking practice, safely promotes persistent recovery of walking in persons with traumatic, subacute SCI. Outcomes from this study may provide new insight into ways to enhance walking recovery in persons with SCI. Trial registration ClinicalTrials.gov, NCT02632422 . Registered 16 December 2015,

Persons with incomplete spinal cord injury (iSCI) face ongoing struggles with walking, including reduced speed and increased reliance on assistive devices (ADs). The forces underlying body weight support and gait, as measured by ground reaction forces (GRFs), are likely altered after iSCI because of weakness and AD dependence but have not been studied. The purpose of this study was to examine GRF production during overground walking after iSCI, because greater insight into GRF constraints is important for refining therapeutic interventions. Because of reduced and discoordinated motor output after iSCI, we hypothesized that persons with iSCI would exert smaller GRFs and altered GRF modifications to increased cadence compared with able-bodied (AB) persons, especially when using an AD. Fifteen persons with chronic iSCI, stratified into no AD (n = 7) and AD (n = 8) groups, walked across an instrumented walkway at self-selected and fast (115% self-selected) cadences. Fifteen age-matched AB controls walked at their own cadences and iSCI-matched conditions (cadence and AD). Results showed fore-aft GRFs are reduced in persons with iSCI compared with AB controls, with reductions greatest in persons dependent on an AD. When controlling for cadence and AD, propulsive forces were still lower in persons with iSCI. Compared with AB controls, persons with iSCI demonstrated altered GRF modifications to increased cadence. Persons with iSCI exhibit different stance-phase forces compared with AB controls, which are impacted further by AD use and slower walking speed. Minimizing AD use and/or providing propulsive biofeedback during walking could enhance GRF production after iSCI.

The intact neuromotor system prepares for object grasp by first opening the hand to an aperture that is scaled according to object size and then closing the hand around the object. After cervical spinal cord injury (SCI), hand function is significantly impaired, but the degree to which object-specific hand aperture scaling is affected remains unknown. Here, we hypothesized that persons with incomplete cervical SCI have a reduced maximum hand opening capacity but exhibit novel neuromuscular coordination strategies that permit object-specific hand aperture scaling during reaching. To test this hypothesis, we measured hand kinematics and surface electromyography from seven muscles of the hand and wrist during attempts at maximum hand opening as well as reaching for four balls of different diameters. Our results showed that persons with SCI exhibited significantly reduced maximum hand aperture compared to able-bodied (AB) controls. However, persons with SCI preserved the ability to scale peak hand aperture with ball size during reaching. Persons with SCI also used distinct muscle coordination patterns that included increased co-activity of flexors and extensors at the wrist and hand compared to AB controls. These results suggest that motor planning for aperture modulation is preserved even though execution is limited by constraints on hand opening capacity and altered muscle co-activity. Thus, persons with incomplete cervical SCI may benefit from rehabilitation aimed at increasing hand opening capacity and reducing flexor–extensor co-activity at the wrist and hand.

Simultaneous contraction of agonist and antagonist muscles acting about a joint influences joint stiffness and stability. Although several studies have shown that reflexes in the muscle lengthened by a joint perturbation are modulated during co-contraction, little attention has been given to reflex regulation in the antagonist (shortened) muscle. The goal of the present study was to determine whether co-contraction gives rise to altered reflex regulation across the joint by examining reflexes in the muscle shortened by a joint perturbation. Reflexes were recorded from electromyographic activity in elbow flexors and extensors while positional perturbations to the elbow joint were applied. Perturbations were delivered during isolated activation of the flexor or extensor muscles as well as during flexor and extensor co-contraction. Across the group, the shortening reflex in the elbow extensor switched from suppression during isolated extensor muscle activation to facilitation during co-contraction. The shortening reflex in the elbow flexor remained suppressive during co-contraction but was significantly smaller compared to the response obtained during isolated elbow flexor activation. This response in the shortened muscle was graded by the level of activation in the lengthened muscle. The lengthening reflex did not change during co-contraction. These results support the idea that reflexes are regulated across multiple muscles around a joint. We speculate that the facilitatory response in the shortened muscle arises through a fast-conducting oligosynaptic pathway involving Ib interneurons.