Modulation of ankle muscle postural reflexes in stroke: influence of weight-bearing load

Delayed cortical engagement associated with balance dysfunction after stroke

Cortical resources are typically engaged for balance and mobility in older adults, but these resources are impaired post-stroke. Although slowed balance and mobility after stroke have been well-characterized, the effects of unilateral cortical lesions due to stroke on neuromechanical control of balance is poorly understood. Our central hypothesis is that stroke impairs the ability to rapidly and effectively engage the cerebral cortex during balance and mobility behaviors, resulting in asymmetrical contributions of each limb to balance control. Using electroencephalography (EEG), we assessed cortical N1 responses evoked over fronto-midline regions (Cz) during balance recovery in response to backward support-surface perturbations loading both legs, as well as posterior-lateral directions that preferentially load the paretic or nonparetic leg. Cortical N1 responses were smaller and delayed in the stroke group. While older adults exhibited weak or absent relationships between cortical responses and clinical function, stroke survivors exhibited strong associations between slower N1 latencies and slower walking, lower clinical mobility, and lower balance function. We further assessed kinetics of balance recovery during perturbations using center of pressure rate of rise. During backward support-surface perturbations that loaded the legs bilaterally, balance recovery kinetics were not different between stroke and control groups and were not associated with cortical response latency. However, lateralized perturbations revealed slower kinetic reactions during paretic loading compared to controls, and to non-paretic loading within stroke participants. Individuals post stroke had similar nonparetic-loaded kinetic reactions to controls implicating that they effectively compensate for impaired paretic leg kinetics when relying on the non-paretic leg. In contrast, paretic-loaded balance recovery revealed time-synchronized associations between slower cortical responses and slower kinetic reactions only in the stroke group, potentially reflecting the limits of cortical engagement for balance recovery revealed within the behavioral context of paretic motor capacity. Overall, our results implicate individuals after stroke may be uniquely limited in their balance ability by the slowed speed of their cortical engagement, particularly under challenging balance conditions that rely on the paretic leg. We expect this neuromechanical insight will enable progress toward an individualized framework for the assessment and treatment of balance impairments based on the interaction between neuropathology and behavioral context.

Recovery of Quiet Standing Balance and Lower Limb Motor Impairment Early Poststroke: How Are They Related?

BACKGROUND

Recovery of quiet standing balance early poststroke has been poorly investigated using repeated measurements.

OBJECTIVE

To investigate (1) the time course of steady-state balance in terms of postural stability and inter-limb symmetry, and (2) longitudinal associations with lower limb motor recovery in the first 3 months poststroke.

METHODS

Forty-eight hemiparetic subjects (age: 58.9 ± 16.1 years) were evaluated at weeks 3, 5, 8, and 12 poststroke. Motor impairments concerned the Fugl-Meyer assessment (FM-LE) and Motricity Index total score (MI-LE) or ankle item separately (MI-ankle). Postural stability during quiet 2-legged stance was calculated as the net center-of-pressure area (COPArea) and direction-dependent velocities (COPVel-ML and COPVel-AP). Dynamic control asymmetry (DCA) and weight-bearing asymmetry (WBA) estimated inter-limb symmetries in balance control and loading. Linear mixed models determined (1) time-dependent change and (2) the between- and within-subject associations between motor impairments and postural stability or inter-limb symmetry.

RESULTS

Time-dependent improvements were significant for FM-LE, MI-LE, MI-ankle, COPArea, COPVel-ML, and COPVel-AP, and tended to plateau by week 8. DCA and WBA did not exhibit significant change. Between-subject analyses yielded significant regression coefficients for FM-LE, MI-LE, and MI-ankle scores with COPArea, COPVel-ML, and COPVel-AP up until week 8, and with WBA until week 12. Within-subject regression coefficients of motor recovery with change in COPArea, COPVel-ML, COPVel-AP, DCA, or WBA were generally non-significant.

CONCLUSIONS

Postural stability improved significantly in the first 8 weeks poststroke, independent of lower limb motor recovery at the most affected side within subjects. Our findings suggest that subjects preferred to compensate with their less affected side, making metrics reflecting inter-limb asymmetries in balance invariant for change early poststroke.Clinical Trial Registration: Clinicaltrials.gov. unique identifier NCT03728036.

Impairments in the mechanical effectiveness of reactive balance control strategies during walking in people post-stroke

People post-stroke have an increased risk of falls compared to neurotypical individuals, partly resulting from an inability to generate appropriate reactions to restore balance. However, few studies investigated the effect of paretic deficits on the mechanics of reactive control strategies following forward losses of balance during walking. Here, we characterized the biomechanical consequences of reactive control strategies following perturbations induced by the treadmill belt accelerations. Thirty-eight post-stroke participants and thirteen age-matched and speed-matched neurotypical participants walked on a dual-belt treadmill while receiving perturbations that induced a forward loss of balance. We computed whole-body angular momentum and angular impulse using segment kinematics and reaction forces to quantify the effect of impulse generation by both the leading and trailing limbs in response to perturbations in the sagittal plane. We found that perturbations to the paretic limb led to larger increases in forward angular momentum during the perturbation step than perturbations to the non-paretic limb or to neurotypical individuals. To recover from the forward loss of balance, neurotypical individuals coordinated reaction forces generated by both legs to decrease the forward angular impulse relative to the pre-perturbation step. They first decreased the forward pitch angular impulse during the perturbation step. Then, during the first recovery step, they increased the backward angular impulse by the leading limb and decreased the forward angular impulse by the trailing limb. In contrast to neurotypical participants, people post-stroke did not reduce the forward angular impulse generated by the stance limb during the perturbed step. They also did not increase leading limb angular impulse or decrease the forward trailing limb angular impulse using their paretic limb during the first recovery step. Lastly, post-stroke individuals who scored poorer on clinical assessments of balance and had greater motor impairment made less use of the paretic limb to reduce forward momentum. Overall, these results suggest that paretic deficits limit the ability to recover from forward loss of balance. Future perturbation-based balance training targeting reactive stepping response in stroke populations may benefit from improving the ability to modulate paretic ground reaction forces to better control whole-body dynamics.

Impact of pathological conditions on postural reflex latency and adaptability following unpredictable perturbations: A systematic review and meta-analysis

BACKGROUND

Pathological conditions can impair responses to postural perturbations and increase risk of falls.

RESEARCH QUESTION

To what extent are postural reflexes impaired in people with pathological conditions and can exercise interventions shorten postural reflexes?

METHODS

MEDLINE, EMBASE, Scopus, SportDiscus and Web of Science were systematically searched for articles comparing muscle activation onset latency in people with pathological conditions to healthy controls following unpredictable perturbations including the effect of exercise interventions (registration: CRD42020170861).

RESULTS

Fifty-three articles were included for systematic review. Significant delays in muscle activity onset following perturbations were evident in people with multiple sclerosis (n = 7, mean difference [MD]: 22 ms, 95% confidence interval [CI]: 11, 33), stroke (n = 10, MD: 34 ms, 95% CI: 19, 49), diabetes (n = 2, MD: 19 ms, 95% CI: 10, 27), HIV (n = 3, MD: 9 ms, 95% CI: 4, 14), incomplete spinal cord injury (n = 2, MD: 57 ms, 95% CI: 33, 80) and back and knee pain (n = 7, MD: 12 ms, 95% CI: 6, 18), but not in people with Parkinson's disease (n = 10) or cerebellar dysfunction (n = 4). Following exercise interventions, the paretic limb of stroke survivors (n = 3) displayed significantly faster muscle activation onset latency compared to pre-exercise (MD: -13 ms, 95% CI: -24, -4), with no significant changes in Parkinson's disease (n = 3).

CONCLUSIONS

This systematic review demonstrated that postural reflexes are significantly delayed in people with multiple sclerosis (+22 ms), stroke (+34 ms), diabetes (+19 ms), HIV (+9 ms), incomplete spinal cord injury (+57 ms), back and knee pain (+12 ms); pathological conditions characterized by impaired sensation or neural function. In contrast, timing of postural reflexes was not impaired in people with Parkinson's disease and cerebellar dysfunction, confirming the limited involvement of supraspinal structures. The meta-analysis showed exercise interventions can significantly shorten postural reflex latencies in stroke survivors (-14 ms), but more research is needed to confirm this finding and in people with other pathological conditions.

The Effects of Auditory Feedback Gait Training Using Smart Insole on Stroke Patients

This study aimed to assess the effect of the auditory feedback gait training (AFGT) using smart insole on the gait variables, dynamic balance, and activities of daily living (ADL) of stroke patients. In this case, 45 chronic stroke patients who were diagnosed with a stroke before 6 months and could walk more than 10 m were included in this study. Participants were randomly allocated to the smart insole training group (n = 23), in which the AFGT system was used, or to the general gait training group (GGTG) (n = 22). Both groups completed conventional rehabilitation, including conventional physiotherapy and gait training, lasting 60 min per session, five times per week for 4 weeks. Instead of gait training, the smart insole training group received smart insole training twice per week for 4 weeks. Participants were assessed using the GAITRite for gait variables and Timed Up and Go test (TUG), Berg Balance Scale (BBS) for dynamic balance, and Modified Barthel Index (MBI) for ADL. The spatiotemporal gait parameters, symmetry of gait, TUG, BBS, and MBI in the smart insole training group were significantly improved compared to those in the GGTG (p < 0.05). The AFGT system approach is a helpful method for improving gait variables, dynamic balance, and ADL in chronic stroke patients.

Kinect-based rapid movement training to improve balance recovery for stroke fall prevention: a randomized controlled trial

Background

Falls are more prevalent in stroke survivors than age-matched healthy older adults because of their functional impairment. Rapid balance recovery reaction with adequate range-of-motion and fast response and movement time are crucial to minimize fall risk and prevent serious injurious falls when postural disturbances occur. A Kinect-based Rapid Movement Training (RMT) program was developed to provide real-time feedback to promote faster and larger arm reaching and leg stepping distances toward targets in 22 different directions.

Objective

To evaluate the effectiveness of the interactive RMT and Conventional Balance Training (CBT) on chronic stroke survivors’ overall balance and balance recovery reaction.

Methods

In this assessor-blinded randomized controlled trial, chronic stroke survivors were randomized to receive twenty training sessions (60-min each) of either RMT or CBT. Pre- and post-training assessments included clinical tests, as well as kinematic measurements and electromyography during simulated forward fall through a “lean-and-release” perturbation system.

Results

Thirty participants were recruited (RMT = 16, CBT = 14). RMT led to significant improvement in balance control (Berg Balance Scale: pre = 49.13, post = 52.75; P = .001), gait control (Timed-Up-and-Go Test: pre = 14.66 s, post = 12.62 s; P = .011), and motor functions (Fugl-Meyer Assessment of Motor Recovery: pre = 60.63, post = 65.19; P = .015), which matched the effectiveness of CBT. Both groups preferred to use their non-paretic leg to take the initial step to restore stability, and their stepping leg’s rectus femoris reacted significantly faster post-training (P = .036).

Conclusion

The RMT was as effective as conventional balance training to provide beneficial effects on chronic stroke survivors’ overall balance, motor function and improving balance recovery with faster muscle response.

Trial registration: The study was registered at Clinicaltrials.gov (https://clinicaltrials.gov/ct2/show/NCT03183635, NCT03183635) on 12 June 2017.

Application of neuromuscular electrical stimulation on the support limb during reactive balance control in persons with stroke: a pilot study

The aim of the present study was to investigate the effect of the application of neuromuscular electrical stimulation to the quadriceps muscle of the paretic limb during externally induced stance perturbations on reactive balance control and on fall outcomes in people with chronic stroke. Ten participants experienced 12 stance treadmill perturbation trails, 6 forward balance perturbation trials and 6 backward balance perturbation trials. For each perturbation condition, three perturbation trials were delivered synchronized with neuromuscular electrical stimulation applied to the quadriceps of the paretic limb and three perturbation trials were delivered without stimulation. Behavioral outcome measures, such as incidence of laboratory falls and number of compensatory steps, kinematic outcome measures, such as margin of stability and minimum hip high values after the perturbation, step initiation time, step execution time and step length of the stepping leg were analyzed. The application of neuromuscular electrical stimulation on the paretic quadriceps between the range of 50 and 500 ms after stance forward and backward perturbations reduced the laboratory falls incidence (p < 0.05), improved stability values (p < 0.05) and reduced the hip height descent (p < 0.05) compared to the experimental condition in which participants were exposed to stance perturbations without neuromuscular electrical stimulation. Additionally, step initiation time of the recovery step was lower in neuromuscular electrical stimulation condition during the forward balance perturbation protocol. Our results showed that the application of neuromuscular electrical stimulation on the knee extensor muscles of the paretic limb reduces the incidence of laboratory falls, enhances reactive stability control and reduces vertical limb collapse after stance forward and backward perturbations in people with chronic stroke.

Characterization of Motor-Evoked Responses Obtained with Transcutaneous Electrical Spinal Stimulation from the Lower-Limb Muscles after Stroke

An increasing number of studies suggests that a novel neuromodulation technique targeting the spinal circuitry enhances gait rehabilitation, but research on its application to stroke survivors is limited. Therefore, we investigated the characteristics of spinal motor-evoked responses (sMERs) from lower-limb muscles obtained by transcutaneous spinal cord stimulation (tSCS) after stroke compared to age-matched and younger controls without stroke. Thirty participants (ten stroke survivors, ten age-matched controls, and ten younger controls) completed the study. By using tSCS applied between the L1 and L2 vertebral levels, we compared sMER characteristics (resting motor threshold (RMT), slope of the recruitment curve, and latency) of the tibialis anterior (TA) and medial gastrocnemius (MG) muscles among groups. A single pulse of stimulation was delivered in 5 mA increments, increasing from 5 mA to 250 mA or until the subjects reached their maximum tolerance. The stroke group had an increased RMT (27–51%) compared to both age-matched (TA: p = 0.032; MG: p = 0.005) and younger controls (TA: p < 0.001; MG: p < 0.001). For the TA muscle, the paretic side demonstrated a 13% increased latency compared to the non-paretic side in the stroke group (p = 0.010). Age-matched controls also exhibited an increased RMT compared to younger controls (TA: p = 0.002; MG: p = 0.007), suggesting that altered sMER characteristics present in stroke survivors may result from both stroke and normal aging. This observation may provide implications for altered spinal motor output after stroke and demonstrates the feasibility of using sMER characteristics as an assessment after stroke.

Posterior fall-recovery training applied to individuals with chronic stroke: A single-group intervention study

BACKGROUND

To assess the effects of the initial stepping limb on posterior fall recovery in individuals with chronic stroke, as well as to determine the benefits of fall-recovery training on these outcomes.

METHODS

This was a single-group intervention study of 13 individuals with chronic stroke. Participants performed up to six training sessions, each including progressively challenging, treadmill-induced perturbations from a standing position. Progressions focused on initial steps with the paretic or non-paretic limb. The highest perturbation level achieved, the proportion of successful recoveries, step and trunk kinematics, as well as stance-limb muscle activation about the ankle were compared between the initial stepping limbs in the first session. Limb-specific outcomes were also compared between the first and last training sessions.

FINDINGS

In the first session, initial steps with the non-paretic limb were associated with a higher proportion of success and larger perturbations than steps with the paretic limb (p = 0.02, Cohen's d = 0.8). Paretic-limb steps were wider relative to the center of mass (CoM; p = 0.01, d = 1.3), likely due to an initial standing position with the CoM closer to the non-paretic limb (p = 0.01, d = 1.4). In the last training session, participants recovered from a higher proportion of perturbations and advanced to larger perturbations (p < 0.05, d > 0.6). There were no notable changes in kinematic or electromyography variables with training (p > 0.07, d < 0.5).

INTERPRETATION

The skill of posterior stepping in response to a perturbation can be improved with practice in those with chronic stroke, we were not able to identify consistent underlying kinematic mechanisms behind this adaptation.

Compensatory control between the legs in automatic postural responses to stance perturbations under single-leg fatigue

In response to sudden perturbations of stance stability, muscles of both legs are activated for balance recovery. In conditions that one of the legs has a reduced capacity to respond, the opposite leg is predicted to compensate by responding more powerfully to restore stable upright stance. In this investigation, we aimed to evaluate between-leg compensatory control in automatic postural responses to sudden perturbations in a situation in which plantar flexor muscles of a single leg were fatigued. Young participants were evaluated in response to a series of perturbations inducing forward body sway, with a focus on activation of plantar flexor muscles: lateral and medial gastrocnemii and soleus. Muscular responses were analyzed through activation magnitude and latency of muscular activation onset. For evaluation of balance and postural stability, we also analyzed the center of pressure and upper trunk displacement and weight-bearing asymmetry between the legs. Responses were assessed in three conditions: pre-fatigue, under single-leg fatigue, and following the recovery of muscular function. Results showed (a) compensation of the non-fatigued leg through the increased magnitude of muscular activation in the first perturbation under fatigue; (b) adaptation in the non-fatigued leg over repetitive perturbations, with a progressive decrement of muscular activation over trials; and (c) maintenance of increased muscular activation of the non-fatigued leg following fatigue dissipation. These findings suggest that the central nervous system is able to modulate the descending motor drive individually for each leg's muscles apparently based on their potential contribution for the achievement of the behavioral aim of recovering stable body balance following stance perturbations.

A Randomized Controlled Study Assessing the Effects of a Shoe Lift Under the Nonparetic Leg on Balance Performance in Individuals With Chronic Stroke

BACKGROUND AND PURPOSE

Improvement of balance and postural stability is an important goal in stroke rehabilitation. The purpose of this study was to investigate the effects of a shoe lift under the nonparetic leg on balance function and balance confidence in persons with chronic stroke.

METHODS

Thirty-six individuals with chronic stroke (21 males and 15 females), who were able to walk independently and showed stance asymmetry, were randomized to a shoe insert and a control group. The interventions included a 6-week balance training program, in conjunction with a shoe lift under the nonaffected leg (shoe insert group, n = 18), or balance training alone (control group, n = 18). The outcome measures were weight-bearing asymmetry (WBA), root mean square (RMS) of anterior-posterior (AP) and medial-lateral (ML) center-of-pressure (COP) velocity asymmetry, Berg Balance Scale (BBS), and Activities-specific Balance Confidence (ABC) Scale. These were measured in both groups at baseline, after the intervention, and at a 3-month follow-up. A repeated-measure multivariate analysis of variance was conducted to evaluate the impact of 2 different interventions on balance measures, across the 3 periods.

RESULTS AND DISCUSSION

No significant between-group differences were found for demographics and stroke-related characteristics of participants (P > .05). The outcome measures between the 2 groups were not significantly different at baseline (P > .05). There were between-group differences for WBA and the RMS of AP COP velocity asymmetry after the intervention and at the 3-month follow-up (P < .05). No significant difference in the RMS of ML COP velocity asymmetry, BBS, and ABC was identified between the 2 groups after the intervention and at the 3-month follow-up (P > .05).

CONCLUSION

The results indicated that the use of a shoe lift under the nonaffected leg in the context of a balance training program could result in a greater improvement in static standing balance as compared with balance training alone in an individual with chronic stroke.

TRIAL REGISTRATION

The study was retrospectively registered in the Iranian Registry of Clinical Trials (IRCT20190603043808N1).

Perturbation-Induced Stepping Post-stroke: A Pilot Study Demonstrating Altered Strategies of Both Legs

Introduction: Asymmetrical sensorimotor function after stroke creates unique challenges for bipedal tasks such as walking or perturbation-induced reactive stepping. Preference for initiating steps with the less-involved (preferred) leg after a perturbation has been reported with limited information on the stepping response of the more-involved (non-preferred) leg. Understanding the capacity of both legs to respond to a perturbation would enhance the design of future treatment approaches. This pilot study investigated the difference in perturbation-induced stepping between legs in stroke participant and non-impaired controls. We hypothesized that stepping performance will be different between groups as well as between legs for post-stroke participants.

Methods: Thirty-six participants (20 persons post-stroke, 16 age matched controls) were given an anterior perturbation from three stance positions: symmetrical (SS), preferred asymmetrical (PAS−70% body weight on the preferred leg), and non-preferred asymmetrical (N-PAS−70% body weight on the non-preferred leg). Kinematic and kinetic data were collected to measure anticipatory postural adjustment (APA), characteristics of the first step (onset, length, height, duration), number of steps, and velocity of the body at heel strike. Group differences were tested using the Mann-Whitney U-test and differences between legs tested using the Wilcoxon signed-rank test with an alpha level of 0.05.

Results: Stepping with the more-involved leg increased from 11.5% of trials in SS and N-PAS up to 46% in PAS stance position for participants post-stroke. Post-stroke participants had an earlier APA and always took more steps than controls to regain balance. However, differences between post-stroke and control participants were mainly found when stance position was modified. Compare to controls, steps with the preferred leg (N-PAS) were earlier and shorter (in time and length), whereas steps with the non-preferred leg (PAS) were also shorter but took longer. For post-stroke participants, step duration was longer and utilized more steps when stepping with the more-involved leg compared to the less-involved leg.

Conclusions: Stepping with the more-involved leg can be facilitated by unweighting the leg. The differences between groups, and legs in post-stroke participants illustrate the simultaneous bipedal role (support and stepping) both legs have in reactive stepping and should be considered for reactive balance training.

Insufficient Balance Recovery Following Unannounced External Perturbations in Persons With Stroke

Background. Persons with stroke (PwS) are at increased risk of falls, especially toward the paretic side, increasing the probability of a hip fracture. The ability to recover from unexpected loss of balance is a critical factor in fall prevention. Objectives. We aimed to compare reactive balance capacity and step kinematics between PwS and healthy controls. Methods. Thirty subacute PwS and 15 healthy controls were exposed to forward, backward, right, and left unannounced surface translations in 6 increasing intensities while standing. Single step threshold, multiple step threshold, and fall threshold (ie, perturbation intensity leading to a fall into harness system) were recorded as well as reactive step initiation time, step length, and step velocity. Results. Twenty-five PwS fell into harness system during the experiment while healthy controls did not fall. Fourteen out of 31 falls occurred in response to surface translations toward the nonparetic side, that is, falling toward the paretic side. Compared with healthy controls, PwS demonstrated significantly lower fall threshold and multiple step threshold in response to forward, backward, and lateral surface translations. Impairments were more pronounced in response to forward surface translation and toward the nonparetic side (ie, loss of balance toward the paretic side). A trend toward significant shorter step length in response to lateral surface translations was found in PwS compared with healthy controls. Conclusions. Findings highlight the importance of assessing reactive balance capacity in response to perturbations in different directions and intensities in addition to the routine assessment in PwS.

Cryotherapy reduces muscle hypertonia, but does not affect lower limb strength or gait kinematics post-stroke: a randomized controlled crossover study

BACKGROUND

Based on the premise that spasticity might affect gait post-stroke, cryotherapy is among the techniques used to temporarily reduce spasticity in neurological patients. This effective technique would enhance muscle performance, and ultimately, functional training, such as walking. However, understanding whether a decrease in spasticity level, if any, would lead to improving muscle performance and gait parameters is not based on evidence and needs to be clarified.

OBJECTIVES

to investigate the immediate effects of cryotherapy, applied to spastic plantarflexor muscles of subjects post-stroke, on tonus level, torque generation capacity of plantarflexors and dorsiflexors, and angular/spatiotemporal gait parameters.

METHODS

Sixteen chronic hemiparetic subjects participated in this randomized controlled crossover study. Cryotherapy (ice pack) or Control (room temperature sand pack) were applied to the calf muscles of the paretic limb. The measurements taken (before and immediately after intervention) were: 1) Tonus according to the Modified Ashworth Scale; 2) Torque assessments were performed using an isokinetic dynamometer; and 3) Spatiotemporal and angular kinematics of the hip, knee, and ankle (flexion/extension), obtained using a tridimensional movement analysis system (Qualisys).

RESULTS

Cryotherapy decreased plantarflexor tonus but did not change muscle torque generation capacity and did not affect spatiotemporal or angular parameters during gait compared to control application. These findings contribute to the evidence-based approach to clinical rehabilitation post-stroke.

CONCLUSIONS

The findings of this study suggest that cryotherapy applied to the calf muscles of subjects with chronic hemiparesis reduces muscle hypertonia but does not improve dorsiflexors and plantarflexors performance and gait parameters.

Does severity of motor impairment affect reactive adaptation and fall-risk in chronic stroke survivors?

Background

A single-session of slip-perturbation training has shown to induce long-term fall risk reduction in older adults. Considering the spectrum of motor impairments and deficits in reactive balance after a cortical stroke, we aimed to determine if chronic stroke survivors could acquire and retain reactive adaptations to large slip-like perturbations and if these adaptations were dependent on severity of motor impairment.

Methods

Twenty-six chronic stroke participants were categorized into high and low-functioning groups based on their Chedoke-McMaster-Assessment scores. All participants received a pre-training, slip-like stance perturbation at level-III (highest intensity/acceleration) followed by 11 perturbations at a lower intensity (level-II). If in early phase, participants experienced > 3/5 falls, they were trained at a still lower intensity (level-I). Post-training, immediate scaling and short-term retention at 3 weeks post-training was examined. Perturbation outcome and post-slip center-of-mass (COM) stability was analyzed.

Results

On the pre-training trial, 60% of high and 100% of low-functioning participants fell. High-functioning group tolerated and adapted at training-intensity level-II but low-functioning group were trained at level-I (all had > 3 falls on level-II). At respective training intensities, both groups significantly lowered fall incidence from 1st through 11th trials, with improved post-slip stability and anterior shift in COM position, resulting from increased compensatory step length. Both groups demonstrated immediate scaling and short-term retention of the acquired stability control.

Conclusion

Chronic stroke survivors are able to acquire and retain adaptive reactive balance skills to reduce fall risk. Although similar adaptation was demonstrated by both groups, the low-functioning group might require greater dosage with gradual increment in training intensity.

Right in Comparison to Left Cerebral Hemisphere Damage by Stroke Induces Poorer Muscular Responses to Stance Perturbation Regardless of Visual Information

OBJECTIVE

Fast and scaled muscular activation is required to recover body balance following an external perturbation. An issue open to investigation is the extent to which the cerebral hemisphere lesioned by stroke leads to asymmetric deficits in postural reactive responses. In this experiment, we aimed to compare muscular responses to unanticipated stance perturbations between individuals who suffered unilateral stroke either to the right or to the left cerebral hemisphere.

METHODS

Stance perturbations were produced by releasing a load attached to the participant's trunk, inducing fast forward body oscillation. Electromyography was recorded from the gastrocnemius medialis and biceps femoris muscles. Muscular activation from age-matched healthy individuals was taken as reference.

RESULTS

Analysis indicated that damage to the right hemisphere induced delayed activation onset, and lower rate and magnitude of activation of the proximal and distal muscles of the paretic leg. Those deficits were associated with stronger activation of the nonparetic leg. Comparisons between left hemisphere damage and controls showed deficits limited to activation of the biceps femoris of the paretic leg. Manipulation of visual information led to no significant effects on muscular responses.

CONCLUSIONS

These results suggest that right cerebral hemisphere damage by stroke leads to more severe deficits in the generation of reactive muscular responses to stance perturbation than damage to the left cerebral hemisphere regardless of visual information.

Reactive balance to unanticipated trip-like perturbations: a treadmill-based study examining effect of aging and stroke on fall risk

The purpose of this study was to examine the mechanism of fall risk in community-dwelling ambulatory hemiplegic stroke survivors when exposed to a sudden, trip-like support surface perturbation in standing. Participants (n = 14 / group) included stroke survivors, Age-similar older controls (AC), and Young controls (YC) experienced trip-like perturbation on a motorized treadmill. The primary outcomes were COM state control (measured as COM position (X_COM/BOS) and velocity (V_COM/BOS) relative to the base of support (BOS)) and the vertical limb support recorded as the extent of hip descent. All participants demonstrated forward loss of balance (FLOB) followed by an equal first compensatory step length. At step touchdown, stroke survivors demonstrated lower COM state stability and increased trunk flexion than the YC group. Stroke survivors also demonstrated greater hip descent than YC and AC groups, as they first stepped with their non-paretic limb. For the second compensatory step, the stroke survivors stepped with their paretic limb. However, unlike the AC group, they were unable to control V_COM/BOS despite a longer compensatory step. In conclusion, poor control of COM state, impaired trunk control and inability of the paretic limb to provide vertical limb support may explain the higher fall-risk in stroke survivors.

Direction-Specific Instability Poststroke Is Associated With Deficient Motor Modules for Balance Control

Defective muscle coordination for balance recovery may contribute to stroke survivors' propensity for falling. Thus, we investigated deficits in muscle coordination for postural control and their association to body sway following balance perturbations in people with stroke. Specifically, we compared the automatic postural responses of 8 leg and trunk muscles recorded bilaterally in unimpaired individuals and those with mild to moderate impairments after unilateral supratentorial lesions (>6 months). These responses were elicited by unexpected floor translations in 12 directions. We extracted motor modules (ie, muscle synergies) for each leg using nonnegative matrix factorization. We also determined the magnitude of perturbation-induced body sway using a single-link inverted pendulum model. Whereas the number of motor modules for balance was not affected by stroke, those formed by muscles with long latency responses were replaced by atypically structured paretic motor modules (atypical muscle groupings), which hints at direct cerebral involvement in long-latency feedback responses. Other paretic motor modules had intact structure but were poorly recruited, which is indicative of indirect cerebral control of balance. Importantly, these paretic deficits were strongly associated with postural instability in the preferred activation direction of the impaired motor modules. Finally, these deficiencies were heterogeneously distributed across stroke survivors with lesions in distinct locations, suggesting that different cerebral substrates may contribute to balance control. In conclusion, muscle coordination deficits in the paretic limb of stroke survivors result in direction-specific postural instability, which highlights the importance of targeted interventions to address patient-specific balance impairments.

A startling acoustic stimulus facilitates voluntary lower extremity movements and automatic postural responses in people with chronic stroke

A startling acoustic stimulus (SAS) involuntary releases prepared movements at accelerated latencies, known as the StartReact effect. Previous work has demonstrated intact StartReact in paretic upper extremity movements in people after stroke, suggesting preserved motor preparation. The question remains whether motor preparation of lower extremity movements is also unaffected after stroke. Here, we investigated StartReact effects on ballistic lower extremity movements and on automatic postural responses (APRs) following perturbations to standing balance. These APRs are particularly interesting as they are critical to prevent a fall following balance perturbations, but show substantial delays and poor muscle coordination after stroke. Twelve chronic stroke patients and 12 healthy controls performed voluntary ankle dorsiflexion movements in response to a visual stimulus, and responded to backward balance perturbations evoking APRs. Twenty-five percent of all trials contained a SAS (120 dB) simultaneously with the visual stimulus or balance perturbation. As expected, in the absence of a SAS muscle and movement onset latencies at the paretic side were delayed compared to the non-paretic leg and to controls. The SAS accelerated ankle dorsiflexion onsets in both the legs of the stroke subjects and in controls. Following perturbations, the SAS accelerated bilateral APR onsets not only in controls, but for the first time, we also demonstrated this effect in people after stroke. Moreover, APR inter- and intra-limb muscle coordination was rather weak in our stroke subjects, but substantially improved when the SAS was applied. These findings show preserved movement preparation, suggesting that there is residual (subcortical) capacity for motor recovery.

Bilateral early activity in the hip flexors associated with falls in stroke survivors: Preliminary evidence from laboratory-induced falls

OBJECTIVE

Falls are the most common and expensive medical complication following stroke. Hypermetric reflexes have been suggested to impact post-stroke balance but no study has evaluated reflex amplitudes under real conditions of falls in this population. Our objective was to quantify the early reflexive responses during falls induced in the laboratory.

METHODS

Sixteen stroke survivors were exposed to posteriorly directed treadmill perturbations that required a forward step to maintain a balance. Perturbations differed in terms of treadmill translation displacement, velocity, and acceleration. EMG amplitudes were compared between Fall/Recovery trials, as well as Fallers/Non-Fallers at two different time windows: 50-75 and 75-100 ms.

RESULTS

Sixteen of 86 trials resulted in falls by nine subjects (Fallers). While no differences were found between 50 and 75 ms, EMG amplitude in the paretic rectus femoris muscle was larger between 75 and 100 ms during Fall trials. Further, a bilateral increase in RF activity was seen in Fallers but not Non-Fallers. Interestingly, the bilateral increase was related to perturbation intensity (larger EMG activity with larger perturbations) in Fallers, but again not in Non-Fallers.

CONCLUSIONS

Heightened early recovery hip flexor activity between 75 and 100 ms is associated with falls and Fallers post-stroke.

SIGNIFICANCE

Though requiring replication and expanded subject pools, these preliminary results reflect a possible clinically meaningful relationship between heightened reflexive responses and fall risk. Future work should evaluate the underlying mechanisms driving these heightened reflexes (e.g. stretch, startle) such that future rehabilitation techniques can address this abnormal response.

Stroke

Stroke, or cerebrovascular accident, involves injury to the central nervous system as a result of a vascular cause, and is a leading cause of disability worldwide. People with stroke often experience sensory, cognitive, and motor sequelae that can lead to difficulty walking, controlling balance in standing and voluntary tasks, and reacting to prevent a fall following an unexpected postural perturbation. This chapter discusses the interrelationships between stroke-related impairments, problems with control of balance and gait, fall risk, fear of falling, and participation in daily physical activity. Rehabilitation can improve balance and walking function, and consequently independence and quality of life, for those with stroke. This chapter also describes effective interventions for improving balance and walking function poststroke, and identifies some areas for further research in poststroke rehabilitation.

Lateral Perturbation-Induced Stepping: Strategies and Predictors in Persons Poststroke

BACKGROUND AND PURPOSE

Falls commonly occur as weight is transferred laterally, and impaired reactive stepping responses are associated with falls after stroke. The purpose of this study was to examine differences in and the determinants of mediolateral (M-L) protective stepping strategies when pulled off balance toward the paretic and nonparetic sides.

METHODS

Eighteen individuals more than 6 months poststroke were pulled in the M-L direction by a lateral waist-pull perturbation system. Step type (crossover, medial, and lateral) and count were recorded, along with first-step initiation time, length, and clearance. Sensorimotor variables including hip adductor/abductor and ankle plantar flexor/dorsiflexor peak isokinetic torques, paretic foot plantar cutaneous sensation, and motor recovery were used to predict step type by discriminant function analyses (DFAs).

RESULTS

Regardless of pull direction, nearly 70% of trials required 2 or more recovery steps, with more frequent nonparetic leg first steps, 63.5%. The step type was significantly different for pull direction (P = 0.005), with a greater percentage of lateral steps when pulled toward the nonparetic side (45.1%) compared with the paretic side (17.5%). The M-L step length of the lateral step was increased (P < 0.001), with a reduced step clearance (P = 0.05), when pulled toward the paretic side compared with a pull toward the nonparetic side. DFAs revealed that nonparetic and paretic-side pulls could respectively classify step type 64% and 60% of the time, with foot cutaneous sensation discriminating for pull direction.

DISCUSSION AND CONCLUSIONS

Balance recovery initiated with the nonparetic leg occurred more frequently in response to M-L perturbations, and paretic foot cutaneous sensation was an important predictor of the stepping response regardless of the pull direction.Video Abstract available for more insights from the authors (see Video, Supplementary Digital Content 1, http://links.lww.com/JNPT/A190).

The effect of immediate decreasing of weight bearing asymmetry on quiet standing postural control in individuals with chronic stroke

The main patterns characterizing standing posture of hemiparetic patients include: weight-bearing asymmetry (WBA), larger postural sway, asymmetrical contribution of lower limbs to balance control, and increased visual dependency to balance control. The aim of this study was to evaluate the effect of decreasing WBA with the use of a shoe lift, on quiet standing postural control in patients with chronic stroke. Twenty-seven patients participated in this study. Patients completed two tests: 1) quiet standing; and 2) quiet standing while a lift was placed under the non-paretic limb. The following tests were completed on force plates for evaluation: asymmetry of the balance measures (weight bearing, root mean square (RMS) of anterior-posterior (AP) and medial-lateral (ML) center of pressure (COP) velocity), RMS of total AP and ML COP velocity, and AP and ML Romberg quotients. Paired t-tests were used to analyze the data. The mean value of WBA index decreased significantly after using a lift (p < 0.05). However, the changes of the mean value of other postural control parameters were not significant (p > 0.05). The results indicate that there may not be an association between decreased WBA and improved postural control during quiet standing in patients with stroke.

The Next Step in Understanding Impaired Reactive Balance Control in People With Stroke: The Role of Defective Early Automatic Postural Responses

Background and objective. Postural muscle responses are often impaired after stroke. We aimed to identify the contribution of deficits in very early postural responses to poorer reactive balance capacity, with a particular focus on reactive stepping as a key strategy for avoiding falls. Methods. A total of 34 chronic stroke survivors and 17 controls were subjected to translational balance perturbations in 4 directions. We identified the highest perturbation intensity that could be recovered without stepping (single stepping threshold [SST]) and with maximally 1 step (multiple stepping threshold [MST]). We determined onset latencies and response amplitudes of 7 leg muscles bilaterally and identified associations with balance capacity. Results. People with stroke had a lower MST than controls in all directions. Side steps resulted in a higher lateral MST than crossover steps but were less common toward the paretic side. Postural responses were delayed and smaller in amplitude on the paretic side only. We observed the strongest associations between gluteus medius (GLUT) onset and amplitude and MST toward the paretic side (R² = 0.33). Electromyographic variables were rather weakly associated with forward and backward MSTs (R² = 0.10-0.22) and with SSTs (R² = 0.08-0.15). Conclusions. Delayed and reduced paretic postural responses are associated with impaired reactive stepping after stroke. Particularly, fast and vigorous activity of the GLUT is imperative for overcoming large sideways perturbations, presumably because it facilitates the effective use of side steps. Because people with stroke often fall toward the paretic side, this finding indicates an important target for training.

Reactive Stepping After Stroke: Determinants of Time to Foot Off in the Paretic and Nonparetic Limb

BACKGROUND AND PURPOSE

Impaired features of reactive stepping, specifically delays in the early time to foot off (TFO) phase, are associated with increased fall rates after stroke. This study aimed to determine differences in, and determinants of, paretic and nonparetic limb TFO, and to determine whether both paretic and nonparetic TFO were associated with perturbation-evoked falls.

METHODS

Retrospective chart review of 105 individuals with stroke was performed within an inpatient rehabilitation setting; each had received a standardized assessment of reactive balance control (in response to a perturbation) at time of discharge.

RESULTS

There were no significant differences in paretic (351 ms) and nonparetic (365 ms) TFO. The capacity to maximally load the nonparetic limb, the amplitude of the perturbation, and the capacity to load the paretic limb were all negatively associated with paretic step TFO, explaining 23.8% of the variance. The amplitude of the perturbation and the preperturbation load under the nonparetic stepping limb were, respectively, negatively and positively associated with nonparetic step TFO, explaining 22.7% of the variance. The likelihood of a perturbation-evoked fall was associated with mean nonparetic limb TFO but not paretic limb TFO.

DISCUSSION AND CONCLUSIONS

Unique stroke-related impairments of dynamic balance control and limb-load asymmetry may differentially influence paretic and nonparetic reactive step TFO, in response to a loss of balance. The amplitude of the perturbation influences reactive step TFO in both limbs. The results of the current study have implications for the future development of standardized clinical assessment methodologies and training strategies to evaluate and remediate reactive stepping and reduce fall risk.Video Abstract available for more insights from the authors (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A133).

Physiological arousal accompanying postural responses to external perturbations after stroke

OBJECTIVE

The purpose of this study was to examine simultaneously the level of physiological arousal and the postural response to external perturbations in people post-stroke compared to age-matched controls to build a more comprehensive understanding of the effect of stroke on postural control and balance self-efficacy.

METHODS

Participants stood with each foot on separate force platforms. Ten applications of loads of 2% body weight at the hips perturbed the participant anteriorly under two conditions: investigator-triggered or self-triggered (total 20). Electrodermal activity (EDA; measurement of physiological arousal), electromyography (EMG) of the ankle plantarflexor muscles and anterior-posterior center of pressure measurements were taken pre-perturbation (anticipatory) and post-perturbation (response) and compared between the initial (first two) and final (last two) perturbations.

RESULTS

Participants post-stroke demonstrated significantly higher levels of anticipatory EDA and anticipatory paretic plantarflexor EMG during both self- and investigator-triggered conditions compared to controls. Anticipatory EDA levels were higher in the final perturbations in participants post-stroke in both conditions, but not in controls. Habituation of the EDA responses post-perturbation was exhibited in the self-triggered perturbations in controls, but not in participants post-stroke.

CONCLUSIONS

Physiological arousal and postural control strategies of controls revealed habituation in response to self-triggered perturbations, whereas this was not seen in participants post-stroke.

SIGNIFICANCE

Understanding the physiological arousal response to challenges to standing balance post-stroke furthers our understanding of postural control mechanisms post-stroke.

The effect of weight-bearing asymmetry on dynamic postural stability in people with chronic stroke

After stroke, weight-bearing asymmetry (WBA) towards the non-paretic side is associated with postural instability. It remains unknown whether WBA is a cause or consequence of postural instability, as both phenomena depend on stroke severity. We investigated the effect of WBA on the ability to recover from balance perturbations in people with stroke. Fourteen people in the chronic phase of stroke underwent multidirectional translational perturbations at three levels of initial WBA (0, 10 and 20% of body weight unloading of the paretic leg). We iteratively determined the highest perturbation intensity that could be sustained with a feet-in-place response (i.e. stepping threshold) for each WBA condition and in four perturbation directions (forward, backward, towards paretic and towards non-paretic side). For perturbations above the stepping threshold we determined the choice of stepping leg. WBA increased the stepping threshold for perturbations towards the paretic side, whereas it decreased the stepping threshold for perturbations towards the non-paretic side (p<0.05). No effects of WBA were found on forward or backward stepping thresholds. Yet, the frequency of stepping with the paretic leg in the anteroposterior directions increased with greater WBA. Similarly, greater initial WBA resulted in a larger number of side steps towards the paretic side. In conclusion, the results suggest that people with stroke can benefit from some paretic leg unloading when perturbed towards the paretic side. It remains to be investigated, however, to what extent these benefits outweigh the potentially detrimental effects of WBA that were observed when recovering from perturbations in the other directions.

Characteristics and adaptive strategies linked with falls in stroke survivors from analysis of laboratory-induced falls

Falls are the most common and expensive medical complication in stroke survivors. There is remarkably little information about what factors lead to a fall in stroke survivors. With few exceptions, the falls literature in stroke has focused on relating metrics of static balance and impairment to fall outcomes in the acute care setting or in community. While informative, these studies provide little information about what specific impairments in a stroke-survivor's response to dynamic balance challenges lead to a fall. We identified the key kinematic characteristics of stroke survivors' stepping responses following a balance disturbance that are associated with a fall following dynamic balance challenges. Stroke survivors were exposed to posteriorly-directed translations of a treadmill belt that elicited a stepping response. Kinematics were compared between successful and failed recovery attempts (i.e. a fall). We found that the ability to arrest and reverse trunk flexion and the ability to perform an appropriate initial compensatory step were the most critical response contributors to a successful recovery. We also identified 2 compensatory strategies utilized by stroke survivors to avoid a fall. Despite significant post-stroke functional impairments, the biomechanical causes of trip-related falls by stroke survivors appear to be similar to those of unimpaired older adults and lower extremity amputees. However, compensatory strategies (pivot, hopping) were observed.

Does aging with a cortical lesion increase fall-risk: Examining effect of age versus stroke on intensity modulation of reactive balance responses from slip-like perturbations

We examined whether aging with and without a cerebral lesion such as stroke affects modulation of reactive balance response for recovery from increasing intensity of sudden slip-like stance perturbations. Ten young adults, older age-match adults and older chronic stroke survivors were exposed to three different levels of slip-like perturbations, level 1 (7.75m/s(2)), Level II (12.00m/s(2)) and level III (16.75m/s(2)) in stance. The center of mass (COM) state stability was computed as the shortest distance of the instantaneous COM position and velocity relative to base of support (BOS) from a theoretical threshold for backward loss of balance (BLOB). The COM position (XCOM/BOS) and velocity (ẊCOM/BOS) relative to BOS at compensatory step touchdown, compensatory step length and trunk angle at touchdown were also recorded. At liftoff, stability reduced with increasing perturbation intensity across all groups (main effect of intensity p<0.05). At touchdown, while the young group showed a linear improvement in stability with increasing perturbation intensity, such a trend was absent in other groups (intensity×group interaction, p<0.05). Between-group differences in stability at touchdown were thus observed at levels II and III. Further, greater stability at touchdown positively correlated with anterior XCOM/BOS however not with ẊCOM/BOS. Young adults maintained anterior XCOM/BOS by increasing compensatory step length and preventing greater trunk extension at higher perturbation intensities. The age-match group attempted to increase step length from intensity I to II to maintain stability however could not further increase step length at intensity III, resulting in lower stability on this level compared with the young group. Stroke group on the other hand was unable to modulate compensatory step length or control trunk extension at higher perturbation intensities resulting in reduced stability on levels II and III compared with the other groups. The findings reflect impaired modulation of recovery response with increasing intensity of sudden perturbations among stroke survivors compared with their healthy counter parts. Thus, aging superimposed with a cortical lesion could further impair reactive balance control, potentially contributing toward a higher fall risk in older stroke survivors.

Reactive Balance in Individuals With Chronic Stroke: Biomechanical Factors Related to Perturbation-Induced Backward Falling

BACKGROUND

An effective compensatory stepping response is the first line of defense for preventing a fall during sudden large external perturbations. The biomechanical factors that contribute to heightened fall risk in survivors of stroke, however, are not clearly understood. It is known that impending sensorimotor and balance deficits poststroke predispose these individuals to a risk of fall during sudden external perturbations.

OBJECTIVE

The purpose of this study was to examine the mechanism of fall risk in survivors of chronic stroke when exposed to sudden, slip-like forward perturbations in stance.

DESIGN

This was a cross-sectional study.

METHODS

Fourteen individuals with stroke, 14 age-matched controls (AC group), and 14 young controls (YC group) were exposed to large-magnitude forward stance perturbations. Postural stability was computed as center of mass (COM) position (XCOM/BOS) and velocity (ẊCOM/BOS) relative to the base of support (BOS) at first step lift-off (LO) and touch-down (TD) and at second step TD. Limb support was quantified as vertical hip descent (Zhip) from baseline after perturbation onset.

RESULTS

All participants showed a backward balance loss, with 71% of the stroke group experiencing a fall compared with no falls in the control groups (AC and YC groups). At first step LO, no between-group differences in XCOM/BOS and ẊCOM/BOS were noted. At first step TD, however, the stroke group had a significantly posterior XCOM/BOS and backward ẊCOM/BOS compared with the control groups. At second step TD, individuals with stroke were still more unstable (more posterior XCOM/BOS and backward ẊCOM/BOS) compared with the AC group. Individuals with stroke also showed greater peak Zhip compared with the control groups. Furthermore, the stroke group took a larger number of steps with shorter step length and delayed step initiation compared with the control groups.

LIMITATIONS

Although the study highlights the reactive balance deficits increasing fall risk in survivors of stroke compared with healthy adults, the study was restricted to individuals with chronic stroke only. It is likely that comparing compensatory stepping responses across different stages of recovery would enable clinicians to identify reactive balance deficits related to a specific stage of recovery.

CONCLUSIONS

These findings suggest the inability of the survivors of stroke to regain postural stability with one or more compensatory steps, unlike their healthy counterparts. Such a response may expose them to a greater fall risk resulting from inefficient compensatory stepping and reduced vertical limb support. Therapeutic interventions for fall prevention, therefore, should focus on improving both reactive stepping and limb support.

The effect of weight-bearing asymmetry on dynamic postural stability in healthy young individuals

BACKGROUND

In people with lateralized disorders, such as stroke, Weight-Bearing Asymmetry (WBA) is common. It is associated with postural instability, however, WBA is one of several abnormalities that may affect postural stability in these disorders. Therefore, we investigated the isolated effects of WBA on dynamic postural stability in healthy individuals.

METHODS

Ten young participants were subjected to multidirectional stance perturbations by support surface translations at three levels of WBA (0, 10 and 20% of body weight unloading of one leg). The stepping threshold was determined iteratively for each condition and in four perturbation directions (forward, backward, leftward and rightward). The stepping threshold was defined as the highest perturbation intensity recovered from with a feet-in-place response. The Margin of Stability (MOS) at the stepping threshold was defined as the smallest distance between the vertical projection of the Extrapolated Center of Mass (XCOM) and the edge of the base of support.

RESULTS

WBA decreased the stepping threshold (stability decreased) for perturbations towards the loaded side (translations towards the unloaded side), whereas it increased stepping thresholds for perturbations towards the unloaded side. No significant effects of WBA were found on the MOS. WBA increased the frequency of stepping with the unloaded leg upon forward and backward perturbations.

CONCLUSION

WBA increased dynamic stability towards the unloaded leg following external balance perturbations and resulted in a greater probability of stepping with this leg. Future studies are needed to evaluate the functional significance of these WBA-related effects on postural stability in people with lateralized disorders.

Behavior of medial gastrocnemius motor units during postural reactions to external perturbations after stroke

OBJECTIVE

This study investigated the behavior of medial gastrocnemius (GM) motor units (MU) during external perturbations in standing in people with chronic stroke.

METHODS

GM MUs were recorded in standing while anteriorly-directed perturbations were introduced by applying loads of 1% body mass (BM) at the pelvis every 25-40s until 5% BM was maintained. Joint kinematics, surface electromyography (EMG), and force platform measurements were assessed.

RESULTS

Although external loads caused a forward progression of the anterior-posterior centre of pressure (APCOP), people with stroke decreased APCOP velocity and centre of mass (COM) velocity immediately following the highest perturbations, thereby limiting movement velocity in response to perturbations. MU firing rate did not increase with loading but the GM EMG magnitude increased, reflecting MU recruitment. MU inter spike interval (ISI) during the dynamic response was negatively correlated with COM velocity and hip angular velocity.

CONCLUSIONS

The GM utilized primarily MU recruitment to maintain standing during external perturbations. The lack of MU firing rate modulation occurred with a change in postural central set. However, the relationship of MU firing rate with kinematic variables suggests underlying long-loop responses may be somewhat intact after stroke.

SIGNIFICANCE

People with stroke demonstrate alterations in postural control strategies which may explain MU behavior with external perturbations.

The intra-rater reliability and agreement of compensatory stepping thresholds of healthy subjects

The purpose of this study was to evaluate the test-retest, intra-rater reliability and agreement of compensatory stepping thresholds. A protocol was developed to establish anteroposterior single-stepping thresholds, anteroposterior multiple-stepping thresholds, and lateral single-stepping thresholds. Healthy, young subjects stood on a microprocessor-controlled treadmill, and responded to three series of progressively challenging surface translations. Subjects were instructed to "try not to step" when establishing single-stepping thresholds or "try to take only one step" when establishing multiple-stepping thresholds. Stepping thresholds were defined as the minimum disturbance magnitude that consistently elicited a single or second compensatory step. Thresholds were expressed as the ankle torque necessary to maintain upright posture. Thresholds studied included anterior single-stepping thresholds (τ = 273.0 ± 82.3 N m), posterior single-stepping, thresholds (τ = 235.5 ± 98.0 N m), anterior multiple-stepping thresholds (τ = 977.0 ± 416.3 N m), posterior multiple-stepping thresholds (τ = 701.9 ± 237.5 N m), stability-side lateral single-stepping thresholds (τ = 225.7 ± 77.7 Nm), and mobility-side lateral single-stepping thresholds (τ = 236.8 ± 85.4 N m). Based on intraclass correlation coefficients (ICC) and Bland-Altman plots, all thresholds demonstrated excellent reliability (ICC(2,1) = 0.87-0.97) and agreement. These results suggest that compensatory stepping thresholds have sufficient repeatability to be used in clinical and research-related assessments of fall-risk. Additional study is needed to determine the intra- and inter-rater reliabilities and validity of thresholds specific to the patient populations of interest.

The Relationship Between Spatiotemporal Gait Asymmetry and Balance in Individuals With Chronic Stroke

Falls are common after stroke and often attributed to poor balance. Falls often occur during walking, suggesting that walking patterns may induce a loss of balance. Gait after stroke is frequently spatiotemporally asymmetric, which may decrease balance. The purpose of this study is to determine the relationship between spatiotemporal gait asymmetry and balance control. Thirty-nine individuals with chronic stroke walked at comfortable and fast speeds to calculate asymmetry ratios for step length, stance time, and swing time. Balance measures included the Berg Balance Scale, step width during gait, and the weight distribution between legs during standing. Correlational analyses determined the relationships between balance and gait asymmetry. At comfortable and fast gait speeds, step width was correlated with stance time and swing time asymmetries (r= 0.39−0.54). Berg scores were correlated with step length and swing time asymmetries (r= –0.36 to –0.63). During fast walking, the weight distribution between limbs was correlated with stance time asymmetry (r= –0.41). Spatiotemporal gait asymmetry was more closely related to balance measures involving dynamic tasks than static tasks, suggesting that gait asymmetry may be related to the high number of falls poststroke. Further study to determine if rehabilitation that improves gait asymmetry has a similar influence on balance is warranted.

Is weight-bearing asymmetry associated with postural instability after stroke? A systematic review

Introduction. Improvement of postural stability is an important goal during poststroke rehabilitation. Since weight-bearing asymmetry (WBA) towards the nonparetic leg is common, training of weight-bearing symmetry has been a major focus in post-stroke balance rehabilitation. It is assumed that restoration of a more symmetrical weight distribution is associated with improved postural stability. Objective. To determine to what extent WBA is associated with postural instability in people after stroke. Methods. Electronic databases were searched (Cochrane, MEDLINE, EMBASE, and CINAHL) until March 2012. Main Eligibility Criteria. (1) Participants were people after stroke. (2) The association between WBA and postural stability was reported. Quality of reporting was assessed with the STROBE checklist and a related tool for reporting of confounding. Results. Nine observational studies met all criteria. Greater spontaneous WBA was associated with higher center of pressure (COP) velocity and with poorer synchronization of COP trajectories between the legs (two and one studies, resp.). Evidence for associations between WBA and performance on clinical balance tests or falls was weak. Conclusion. Greater WBA after stroke was associated with increased postural sway, but the current literature does not provide evidence for a causal relationship. Further studies should investigate whether reducing WBA would improve postural stability.

Control of fast squatting movements after stroke

OBJECTIVE

Little is known about how residual motor impairments after stroke affect the motor control of fast movements, particularly those that combine postural control and limb movement. The purpose of this study was to examine the influence of stroke on the motor control of fast squatting movements.

METHODS

Seventeen individuals with hemiparesis and seventeen age- and sex-matched controls performed fast squatting movements. Force platform data, knee acceleration, and electromyographic activity from rectus femoris, biceps femoris, tibialis anterior, soleus, were collected.

RESULTS

Subjects after stroke performed the squats asymmetrically, with reduced velocity and acceleration compared to controls. Subjects with low motor recovery depended on the non-paretic leg to compensate for poor paretic muscle activation whereas subjects with high motor recovery activated muscles in the paretic leg in an adaptive manner, making the movement more symmetrical. Difficulty with postural control was evident by reduced coupling of the timing of the knee movement with the center of pressure excursion.

CONCLUSIONS

Slow performance of squatting movements was accompanied by altered muscle activation, coupled with impaired postural control.

SIGNIFICANCE

Fast squatting movements in standing require appropriate muscle activation and postural control, the latter of which can be measured easily with force platform and accelerometer data.

Quadrupedal coordination of bipedal gait: implications for movement disorders

During recent years, evidence has come up that bipedal locomotion is based on a quadrupedal limb coordination. A task-dependent neuronal coupling of upper and lower limbs allows one to involve the arms during gait but to uncouple this connection during voluntarily guided arm/hand movements. Hence, despite the evolution of a strong cortico-spinal control of hand/arm movements in humans, a quadrupedal limb coordination persists during locomotion. This has consequences for the limb coordination in movement disorders such as in Parkinson's disease (PD) and after stroke. In patients suffering PD, the quadrupedal coordination of gait is basically preserved. The activation of upper limb muscles during locomotion is strong, similar as in age-matched healthy subjects although arm swing is reduced. This suggests a contribution of biomechanical constraints to immobility. In post-stroke subjects a close interactions between unaffected and affected sides with an impaired processing of afferent input takes place. An afferent volley applied to a leg nerve of the unaffected leg leads to a normal reflex activation of proximal arm muscles of both sides. In contrast, when the nerve of the affected leg was stimulated, neither on the affected nor in the unaffected arm muscles EMG responses appear. Muscle activation on the affected arm becomes normalized by influences of the unaffected side during locomotion. These observations have consequences for the rehabilitation of patients suffering movement disorders.

Locomotion in stroke subjects: interactions between unaffected and affected sides

The aim of this study was to evaluate the sensorimotor interactions between unaffected and affected sides of post-stroke subjects during locomotion. In healthy subjects, stimulation of the tibial nerve during the mid-stance phase is followed by electromyography responses not only in the ipsilateral tibialis anterior, but also in the proximal arm muscles of both sides, with larger amplitudes prior to swing over an obstacle compared with normal swing. In post-stroke subjects, the electromyography responses were stronger on both sides when the tibial nerve of the unaffected leg was stimulated compared with stimulation of the affected leg. This difference was more pronounced when stimuli were applied prior to swing over an obstacle than prior to normal swing. This indicates an impaired processing of afferent input from the affected leg resulting in attenuated and little task-modulated reflex responses in the arm muscles on both sides. In contrast, an afferent volley from the unaffected leg resulted in larger electromyography responses, even in the muscles of the affected arm. Arm muscle activations were stronger during swing over an obstacle than during normal swing, with no difference in electromyography amplitudes between the unaffected and affected sides. It is concluded that the deficits of the affected arm are compensated for by influences from the unaffected side. These observations indicate strong mutual influences between unaffected and affected sides during locomotion of post-stroke subjects, which might be used to optimize rehabilitation approaches.

Changes in preferred postural patterns following stroke during intentional ankle/hip coordination

We compared the spatio-temporal postural organization between stroke patients and healthy controls in a bipedal standing task where participants had to intentionally produce two specific ankle/hip coordination patterns: in-phase and anti-phase. The pattern to reproduce was visually represented by a ankle-hip Lissajous figure, and a real-time biofeedback displayed the current coordination sur-imposed to the expected coordination. Contrary to the healthy participants who were successful at reproducing the two patterns, stroke patients were unable to produce the in-phase pattern. In addition, when the anti-phase pattern was required, a reduction of stability was observed for the stroke group. The impairment of postural capacities following stroke was thus accompanied by a disappearance of one of the two preferred patterns found in healthy participants, a result that have consequences for understanding the etiology of postural pattern formation and the elaboration of rehabilitation programs.

On the relative contribution of the paretic leg to the control of posture after stroke

BACKGROUND

Reduced postural steadiness and asymmetry of weight bearing are characteristic for posture after stroke.

OBJECTIVE

To examine the relative contribution of each leg to postural control in a cohort of 33 stroke patients at 5 stages during 3 months of inpatient rehabilitation, while taking clinical scores of sensory and motor impairments of the paretic leg into account.

METHODS

Participants were instructed to stand as symmetrically as possible under both sensory and cognitive manipulations, while a dual-plate force platform was used to assess the contribution of each leg to postural control, quantified by the amplitude, velocity, and regularity of recorded center-of-pressure trajectories. A greater contribution of the nonparetic leg was expected, particularly in patients with ankle clonus, disturbed sensibility, and lack of selective muscle control on the paretic side.

RESULTS

With follow-up assessments, weight-bearing asymmetry and postural steadiness improved. Patients strongly relied on visual information. When attention was distracted by having the patients perform an arithmetic task, weight-bearing asymmetry increased, suggesting that symmetric weight bearing was attention demanding. Patients with severe motor impairments of the paretic leg showed greater static (weight-bearing) and dynamic (lateralized control) asymmetries than patients with limited motor impairments, whereas postural steadiness did not differ between these subgroups. Disturbed sensation did not affect weight-bearing asymmetry, postural steadiness, or lateralized control.

CONCLUSION

Patients with severe motor impairments of the paretic leg employ an effective compensatory strategy consisting of asymmetric weight bearing and lateralized control.

Muscle reflexes and synergies triggered by an unexpected support surface height during walking

An important phase in the step cycle is foot contact. When the moment of foot contact differs from the one expected, a fast response is needed. Such a mismatch can be caused by hitting a support surface earlier or later than expected. To study this, experiments were performed with healthy young adults who walked on a platform that was unexpectedly at a lowered (5 cm) or at a level height. Glasses blocked the lower visual field. In the unexpectedly lowered trials, the absence of expected heel contact triggered responses in the ipsilateral anti-gravity muscles [ipsilateral medial gastrocnemius (MGi), ipsilateral rectus femoris (RFi)] and contralateral flexor muscles [contralateral tibialis anterior (TAc), contralaterial biceps femoris (BFc)] with latencies of 47-69 ms. After the delayed heel contact, enhanced activity was found in the MGi, RFi, and TAc muscles. This specific muscle synergy was presumably activated to arrest the forward propulsion of the body. In contrast, when the surface was unexpectedly at level height, the subjects expected to step down, and the leg briefly yielded. A muscle synergy was activated at 46-81 ms that flexed the ipsilateral knee (TAi, BFi, RFi) and extended the contralateral one (MGc, BFc) to unload the perturbed leg and delay the contralateral swing phase. Both conditions triggered a fast functionally relevant muscle synergy because of a mismatch between the expected and actual sensory feedback at the moment of foot contact. The results are consistent with an internal model that compares the expected with the actual sensory feedback. The short latency of the response suggests a subcortical, possibly cerebellar pathway.

Altered timing of postural reflexes contributes to falling in persons with chronic stroke

The purpose of this study was to determine differences in the timing of postural reflexes and changes in kinematics between those who fell (fallers) in response to standing platform translations and those who did not (non-fallers). Forty-four persons with stroke were exposed to unexpected forward and backward platform translations while standing. Surface electromyography from bilateral tibialis anterior, gastrocnemius, rectus femoris, and biceps femoris were recorded along with kinematic data. Those that fell in response to the translations were compared to those who did not fall in terms of (1) postural reflex onset latency, (2) the time interval between the activation of distal and proximal muscles (i.e. intralimb coupling), and (3) changes in joint angles and trunk motion. Approximately 85% of falls occurred in response to the forward translations. Postural reflex onset latencies were delayed and intralimb coupling durations were longer in the faller versus non-faller group. At the time that the platform completed the translating motion (300 ms), the faller group demonstrated higher trunk velocity, greater change in paretic ankle angle, and the trunk was further behind the ankle compared to the non-faller group. This study suggests that following platform translations, delays in the timing of postural reflexes and disturbed intralimb coupling result in changes in kinematics, which contribute to falls in persons with stroke.