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Purpose: The Berg Balance Scale (BBS) is a performance-based measure of standing balance commonly used by clinicians working with individuals post-stroke. Performance on the BBS can be influenced by compensatory strategies, but measures derived from two force plates can isolate compensatory strategies and thus better indicate balance impairment. This study examined BBS scores that reflect "normal" and disordered balance with respect to dual force-plate measures of standing balance in individuals post-stroke. Methods: BBS and force-plate measures were extracted from 75 patient charts. Individuals were classified by BBS score with respect to (1) age-matched normative values and (2) values that suggested increased risk of falls. Multiple analysis of variance was used to examine the effect of group assignment on force-plate measures of standing balance. Results: Individuals with BBS scores within and below normative values did not differ in force-plate measures. Individuals with BBS scores below the falls risk cutoff loaded their affected leg less than individuals with BBS scores above the cutoff. There were no other differences in force-plate measures between these two groups. Conclusions: BBS scores indicating either normal or disordered balance function are not necessarily associated with normal or disordered quiet standing-balance control measured by two force plates. This finding suggests that the BBS may reflect a capacity for compensation rather than any underlying impairments.

To investigate the role of vision in stair locomotion, young adults descended a seven-step staircase during unrestricted walking (CONTROL), and while performing a concurrent visual reaction time (RT) task displayed on a monitor. The monitor was located at either 3.5 m (HIGH) or 0.5 m (LOW) above ground level at the end of the stairway, which either restricted (HIGH) or facilitated (LOW) the view of the stairs in the lower field of view as participants walked downstairs. Downward gaze shifts (recorded with an eye tracker) and gait speed were significantly reduced in HIGH and LOW compared with CONTROL. Gaze and locomotor behaviour were not different between HIGH and LOW. However, inter-individual variability increased in HIGH, in which participants combined different response characteristics including slower walking, handrail use, downward gaze, and/or increasing RTs. The fastest RTs occurred in the midsteps (non-transition steps). While gait and visual task performance were not statistically different prior to the top and bottom transition steps, gaze behaviour and RT were more variable prior to transition steps in HIGH. This study demonstrated that, in the presence of a visual task, people do not look down as often when walking downstairs and require minimum adjustments provided that the view of the stairs is available in the lower field of view. The middle of the stairs seems to require less from executive function, whereas visual attention appears a requirement to detect the last transition via gaze shifts or peripheral vision., Competing Interests: The authors declare that they have no conflict of interest.

This paper proposes a novel approach for gait analysis from wearable sensing, based on an adaptive periodic model of any gait signal. The proposed method learns a model of the gait cycle during online measurement, using a continuous representation that can adapt to inter and intra-personal variability by creating an individualized model. Once the algorithm has converged to the input signal, key gait events can be identified relative to the estimated gait phase; these events can then be used to calculate gait parameters. The approach is implemented and tested on a human motion dataset where heel impact and toe takeoff events are extracted with an average error of 0.04 cycles.

Objective: To identify patient characteristics that influence physiotherapist's decisions on enrollment and attendance in a structured aerobic exercise program early after stroke., Design: Retrospective chart review., Setting: Rehabilitation hospital., Participants: Consecutive sample of people (N=345) admitted to inpatient stroke rehabilitation over a 2-year period., Interventions: Not applicable., Main Outcome Measures: Patient demographic characteristics, preexisting medical conditions, and poststroke outcome variables (neurological deficit, physical impairment, balance control, and functional mobility and independence) were compared between individuals enrolled and not enrolled in a structured aerobic exercise program. The rate of attendance was calculated for the enrolled group., Results: One hundred twenty-nine patients (38%) were enrolled in the structured aerobic exercise program. Patients who were older (P=.0093) and had cardiac disease (P=.012), cardioembolic sources (P=.0094), and arthritis (P=.031) were less likely to be enrolled in the structured aerobic exercise program. Poststroke outcome variables were not associated with enrollment. Among those enrolled, the rate of attendance was positively correlated with the FIM cognitive rating (r=.27; P=.0031)., Conclusions: Enrollment in structured aerobic exercise programs during inpatient stroke rehabilitation can be limited by safety concerns related to patients' cardiovascular and musculoskeletal status. Barriers associated with the perception of cardiovascular risk factors should be confronted because they do not preclude participation in cardiac rehabilitation. In addition, poststroke deficits do not limit participation in adapted aerobic exercise early after stroke. It is likely that the characteristics of the structured aerobic exercise program were integral to accommodate the breadth of poststroke deficits encountered in this study. Future research investigating physiotherapist and practice environment factors that influence the decision to prescribe and implement aerobic exercise is warranted., (Copyright © 2015 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Objective: To develop a video-based system, mounted on a rollator to quantify the step width values of rollator users in the community., Subjects: A total of 5 able-bodied young adults, age range 24-28 years., Methods: A digital video camera system was mounted on the rollator frame to capture the position of the participant's feet during overground walking. A method of estimating step width from the video data was developed and evaluated against the output from a concurrently recording Vicon MX motion capture system., Results: Mean step widths of the rollator and motion capture systems were 14.40 cm (standard deviation (SD) 4.64) and 14.37 cm (SD 4.34), respectively, revealing a strong level of agreement; intra-class correlation coefficient 0.999 (95% confidence interval (95% CI) 0.987-1.000; and root-mean-square difference 0.70 cm., Conclusion: The video-based system mounted on a rollator to collect foot placement data enabled accurate measurement of step width during rollator use. The ability to record foot placement measurements outside the laboratory setting, characterizing foot placement patterns occurring in the community, will enable research into how these assistive devices influence mobility during everyday use.

Research examining age-related changes in dynamic stability during stepping has recognised the importance of the restabilisation phase, subsequent to foot-contact. While regulation of the net ground reaction force (GRFnet) line of action is believed to influence dynamic stability during steady-state locomotion, such control during restabilisation remains unknown. This work explored the origins of age-related decline in mediolateral dynamic stability by examining the line of action of GRFnet relative to the centre of mass (COM) during restabilisation following voluntary stepping. Healthy younger and older adults (n=20 per group) performed three single-step tasks (varying speed and step placement), altering the challenge to stability control. Age-related differences in magnitude and intertrial variability of the angle of divergence of GRFnet line of action relative to the COM were quantified, along with the peak mediolateral and vertical GRFnet components. The angle of divergence was further examined at discrete points during restabilisation, to uncover events of potential importance to stability control. Older adults exhibited a reduced angle of divergence throughout restabilisation. Temporal and spatial constraints on stepping increased the magnitude and intertrial variability of the angle of divergence, although not differentially among the older adults. Analysis of the time-varying angle of divergence revealed age-related reductions in magnitude, with increases in timing and intertrial timing variability during the later phase of restabilisation. This work further supports the idea that age-related challenges in lateral stability control emerge during restabilisation. Age-related alterations during the later phase of restabilisation may signify challenges with reactive control., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Objective: To determine the responsiveness to change of spatial-temporal gait parameters among stroke survivors for 3 different variability measures: SD, coefficient of variation (CV), and median absolute deviation (MAD)., Design: Retrospective chart review., Setting: Clinical laboratory in a Canadian hospital., Participants: Stroke survivors (N=74) receiving inpatient rehabilitation., Interventions: Not applicable., Main Outcome Measures: Spatial-temporal gait variability was calculated for step length, step width, stance time, swing time, and double support time. Responsiveness to change was determined by comparing (1) trials without versus trials with a concurrent cognitive task and (2) admission to discharge from rehabilitation., Results: Variability estimators (SD, CV, and MAD) increased with the addition of a cognitive task and decreased from admission to discharge of rehabilitation. However, these changes were not statistically significant when change in gait velocity was included as a covariate. The effect size values were similar for all variability estimators with a trend toward a greater SD response to temporal parameters. The CV displayed a larger response to change for step length than did the SD and MAD. Although gait variability decreased between admission and discharge, the effect size was larger for the condition without the cognitive task than for the condition with the cognitive task., Conclusions: Our results show that gait variability estimators demonstrate a similar responsiveness to a concurrent cognitive task and improved walking ability with recovery from stroke. Future work may focus on evaluating the clinical utility of these measures in relation to informing therapy and response to gait-specific training protocols., (Copyright © 2014 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Objective: To determine whether attending an aerobic fitness program during inpatient stroke rehabilitation is associated with increased participation in physical activity after discharge., Design: This was a prospective cohort study. Patients who received inpatient stroke rehabilitation and were discharged into the community (n = 61; mean age, 65 years) were recruited. Thirty-five participants attended a standardized aerobic fitness program during inpatient rehabilitation, whereas 26 did not. The Physical Activity Scale for Individuals with Physical Disabilities (PASIPD) and adherence to the American College of Sports Medicine (ACSM) guidelines were assessed up to 6 months after discharge., Results: Participants in the fitness group had PASIPD scores and adherence to ACSM guidelines similar to those of participants in the nonfitness group up to 6 months after discharge. There was no significant correlation between volume of exercise performed during the inpatient program and amount of physical activity after discharge., Conclusion: Participation in an inpatient fitness program did not increase participation in physical activity after discharge in individuals with stroke. A new model of care that encourages patients to pursue physical activity after discharge and reduces the potential barriers to participation should be developed.

Dual-task gait allows assessment of impaired executive function and mobility control in older individuals, which are risk factors of falls. This study investigated gait changes in older individuals due to the addition of a cognitive load, using wearable pressure-sensing insole and tri-axial accelerometer measures. These wearable sensors can be applied at the point-of-care. Eleven elderly (65 years or older) individuals walked 7.62 m with and without a verbal fluency cognitive load task while wearing FScan 3000E pressure-sensing insoles in both shoes and a Gulf Coast X16-1C tri-axial accelerometer at the pelvis. Plantar-pressure derived parameters included center of force (CoF) path and temporal measures. Acceleration derived measures were descriptive statistics, Fast Fourier Transform quartile, ratio of even-to-odd harmonics, and maximum Lyapunov exponent. Stride time, stance time, and swing time all significantly increased during dual-task compared to single-task walking. Minimum, mean, and median CoF stance velocity; cadence; and vertical, anterior-posterior, and medial-lateral harmonic ratio all significantly decreased during dual-task walking. Wearable plantar pressure-sensing insole and lower back accelerometer derived-measures can identify gait differences between single-task and dual-task walking in older individuals and could be used in point-of-care environments to assess for deficits in executive function and mobility impairments.

Evoked autonomic nervous system (ANS) activity may be an important modulator of rapid reactions, generated in the face of urgency and may serve to augment the parallel somatosensory processing to adjust speed of processing. The primary objective of the current study was to temporally pair auditory stimuli with whole body perturbations to determine if conditioning could 'prime' the central nervous system (CNS) to respond faster and with greater ANS reactivity to the auditory stimulus alone. Healthy young participants (n = 19) were seated in a custom chair, which tilted backwards upon the release of an electromagnet and were instructed to reach to grasp a handle located in front of their arm as fast as possible following an auditory cue. Three conditions were completed in the following order: (1) baseline-auditory cue alone (5 trials); (2) paired-auditory cue, followed by a chair tilt 110 ms later (20 trials); and (3) post-pairing-auditory cue alone (5 trials). Participants were not informed of the switch from paired to auditory-only stimuli in the first trial of the post-pairing task condition. Reaction time was measured using electromyography, and autonomic nervous system activity was monitored via the electrodermal response (EDR). The first trial post-pairing had significantly faster reaction time (Δ = 21 ms) and significantly greater EDR amplitude compared to the last trial prior to pairing (baseline). The amplitude of contraction and overall time to handle contact were not significantly different between the first trial post-pairing and the last trial prior to pairing. This study demonstrates that the CNS can be 'primed' to generate rapid reactions and an elevated autonomic response in the absence of whole body instability. This indicates that afferent volume generated following whole body instability is not the only determinant of rapid reactions and emphasizes the importance of physiologic measures of autonomic activity with respect to stimulus-evoked reaction time.

Onset and execution of compensatory reaches are faster than the most rapid voluntary reaches. With onset latencies near 100 ms, it is proposed that initial control of compensatory reaches cannot rely on visual information obtained after perturbation onset; rather, they rely on a visuospatial map acquired prior to instability. In natural conditions, it is not practical to direct gaze toward every potential support surface in preparation for a perturbation, suggesting that peripheral vision may be uniquely important. This study aimed to determine whether visuospatial mapping achieved using only peripheral visual information could be used to control reach-to-grasp reactions. Participants sat in an unstable chair. Whole body perturbations were used to evoke rapid reach-to-grasp reactions. A handle was positioned at midline or to the right of the participant. Gaze was directed toward the center or right to view the handle in peripheral or central visual fields. Electromyographic and kinematic data were recorded. Peripheral information acquired prior to perturbation was sufficient for successful execution of reach-to-grasp without delay. Differences in reach kinematics, however, did exist between vision conditions (e.g., maximum lateral wrist displacement and magnitude of hand overshoot relative to the handle were greater for peripheral vs. central vision). Handle location led to target-specific differences in initial muscle recruitment revealing information acquired prior to perturbation were used to guide initial limb trajectory. Results reveal the capacity to rely on a visuospatial map constructed from peripheral visual information for compensatory reaching but also highlight limitations leading to more conservative reach trajectories.

Objective: Community exercise programs are effective in improving aerobic capacity after stroke, but individual variability in training responses has not previously been studied. This study examined explanatory factors for post-program responses in aerobic capacity (VO2peak) with respect to training parameters and participant characteristics., Design: Secondary analysis of data from a prospective cohort study., Subjects: Thirty-two participants, mean age 63 years (standard deviation (SD) 13), 29 months (SD 26) post-stroke., Method: Following a 6-month community exercise program, multivariate regression analysis was performed to assess for correlates with change in VO2peak with respect to training parameters (exercise duration and intensity) and participant characteristics (lower limb and balance impairment, aerobic and walking capacity)., Results: The mean improvement in VO2peak was 11% (SD 19), but individually, changes ranged from 32% decline to 56% improvement. Regression analysis revealed that only 2 variables, baseline aerobic capacity and balance impairment, accounted for the training-related change in VO2peak (R2 = 0.20, p = 0.045)., Conclusions: There is considerable inter-individual variability in stroke exercise program responses in aerobic capacity. Lower baseline aerobic capacity and better balance were associated with greater training-related improvements in VO2peak. Identifying individuals who are more likely to respond to exercise programs may inform future work in establishing effective interventions suited for different subsets of the stroke population.

Objective: To investigate the determinants of limb preference for initiating compensatory stepping poststroke., Design: Retrospective chart review., Setting: Inpatient rehabilitation., Participants: Convenience sample of individuals admitted to inpatient rehabilitation with poststroke hemiparesis., Interventions: Not applicable., Main Outcome Measures: Compensatory stepping responses were evoked using a lean-and-release postural perturbation. The limb used to initiate compensatory stepping was determined. The relationships between stepping with the paretic limb and premorbid limb dominance, weight bearing on the paretic limb in quiet standing, ability to bear weight on the paretic limb, preperturbation weight bearing on the paretic limb, and lower-limb motor recovery scores were determined., Results: The majority (59.1%) of responses were steps initiated with the nonparetic limb. Increased lower-limb motor recovery scores and preperturbation weight bearing on the nonparetic limb were significantly related to increased frequency of stepping with the paretic limb. When the preferred limb was physically blocked, an inappropriate response was initiated in 21% of trials (ie, nonstep responses or an attempt to step with the blocked limb)., Conclusions: This study reveals the challenges that individuals with poststroke hemiparesis face when executing compensatory stepping responses to prevent a fall after a postural perturbation. The inability or challenges to executing a compensatory step with the paretic limb may increase the risk for falls poststroke., (Copyright © 2012 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Availability of fingertip touch onto a stable surface reduces body sway for subjects standing with eyes closed. This is largely associated with sensory feedback from the fingertip when mechanical load is limited. Here, it is possible that the central nervous system facilitates cortical sensory processing to augment feedback to control upright stance. To test this, we compared cortical sensory excitability between tasks with and without light finger touch while standing. Subjects stood in tandem on a force plate with eyes closed while lightly touching a stable surface with the index finger. This was, in two different studies, compared to: (1) no haptic contact or (2) light touch on an object not referenced to balance. Throughout testing, the median nerve was stimulated and electroencephalography was used to measure somatosensory evoked potentials (SEPs). As expected, availability of stable light touch reduced medial-lateral COP sway. Peak amplitudes for SEP components revealed reduced P100 (48%), but increased P50 (31%), N140 (80%), and P200 (20%) during stable touch versus no touch. The modulation of P50 and N140 was no longer present when comparing stable to control (touch), which suggested that attending to touch on either surface, regardless of stability reference, accounted for these changes. Conversely, P200 was increased (19%) when touching the stable surface. Our data show SEP modulation during a standing balance task related to hand contact. Facilitation of P200 in particular may indicate task-specific regulation of the cortical representation of fingertip afferent input when it is relevant to providing stable cues for static balance control.

Stair walking is a challenging locomotor task, and visual information about the steps is considered critical to safely walk up and down. Despite the importance of such visual inputs, there remains relatively little information on where gaze is directed during stair walking. The present study investigated the role of vision during stair walking with a specific focus on gaze behaviour relative to (1) detection of transition steps between ground level and stairs, (2) detection of handrails, and (3) the first attempt to climb an unfamiliar set of stairs. Healthy young adults (n = 11) walked up and down a set of stairs with 7 steps (transitions were defined as the two top and bottom steps). Gaze behaviour was recorded using an eye tracker. Although participants spent most part of the time looking at the steps, gaze fixations on stair features covered less than 20% of the stair walking time. There was no difference in the overall number of fixations and fixation time directed towards transitions compared to the middle steps of the stairs. However, as participants approached and walked on the stairs, gaze was within 4 steps ahead of their location. The handrail was rarely the target of gaze fixation. It is noteworthy that these observations were similar even in the very first attempt to walk on the stairs. These results revealed the specific role of gaze behaviour in guiding immediate action and that stair transitions did not demand increased gaze behaviour in comparison with middle steps. These findings may also indicate that individuals may rely on a spatial representation built from previous experience and/or visual information other than gaze fixations (e.g. dynamic gaze sampling, peripheral visual field) to extract information from the surrounding environment.

Changes in gait parameters induced by the concomitant performance of one of two cognitive tasks activating working memory and spatial attention, was examined in healthy young adults (YA) and older adults (OA). There was a main effect of task condition on gait-speed (p = .02), stride-length (p < .001) and double-support time (p = .04) independent of the group. There were no significant differences between working memory and spatial attention associated gait changes. Working-memory and spatial-attention dual-tasking led to a decrease in gait-speed (p = .09 and 0.01) and stride-length (p = .04 and 0.01) and increase in double-support time (p = .01 and 0.03) in YA and decrease in stride-length (p = .04 and 0.01) alone in OA. Cognitive task associated changes in gait may be a function of limited attentional resources irrespective of the type of cognitive task.

Background: Although the consequences of spinal cord injury (SCI) within the spinal cord and peripheral nervous system have been studied extensively, the influence of SCI on supraspinal structures during recovery remains largely unexplored., Objective: To assess temporal changes in cortical sensorimotor representations beginning in the subacute phase following SCI and determine if an association exists between the plastic changes within cortical sensorimotor areas and recovery of movement postinjury., Methods: Functional magnetic resonance imaging (fMRI) was used to study 6 SCI patients for 1 year, beginning shortly postinjury, and 10 healthy control individuals. During fMRI, individuals performed a simple self-paced wrist extension motor task. Recovery of movement was assessed using the American Spinal Injury Association (ASIA) Standard Neurological Classification of SCI., Results: In the subacute period post-SCI, during impaired movement, little task-related activation within the primary motor cortex (M1) was present, whereas activation in associated cortical sensorimotor areas was more extensive than in controls. During motor recovery, a progressive enlargement in the volume of movement-related M1 activation and decreased activation in associated cortical sensorimotor areas was seen. When movement was performed with little to no impairment, the overall pattern of cortical activation was similar to that observed in control individuals., Conclusions: This study provides the first report of the temporal progression of cortical sensorimotor representational plasticity during recovery following traumatic SCI in humans and suggests an association between movement-related fMRI activation and motor recovery postinjury. These findings have implications on current and future rehabilitative interventions for patients with SCI.

Objectives: To establish the feasibility and reliability of graded maximal exercise testing, suitability of standard indexes of maximum oxygen consumption (Vo(2)max), and evidence of trial-to-trial practice effects in subacute stroke., Design: Descriptive, cross-sectional study., Setting: Rehabilitation hospital., Participants: Consecutive sample of 35 participants (mean age, 65.7+/-2.6 y; mean days poststroke, 17.6+/-2.2 d)., Interventions: Not applicable., Main Outcome Measures: Graded maximal exercise test using semirecumbent cycle ergometry. A subset (n=20) performed repeated tests to determine test-retest reliability and presence of practice effects., Results: Thirty-one (89%) participants completed the exercise test without symptomatic responses (mean peak level of oxygen consumed [Vo(2)peak], 10.7 mL.kg(-1).min(-1); peak work rate, 39 W). Twelve (34%), 15 (44%), and 3 (9%) participants achieved oxygen consumption per unit time (Vo(2)) plateau, respiratory exchange ratio, and heart rate criteria, respectively. Although test-retest correlations were high (intraclass correlation coefficient range, .67-.87), higher Vo(2)peak (1.0 mL.kg(-1).min(-1), P=.04) and work rate (7.3 W, P=.01) were observed with repeated testing, with no associated increase in reaching standard criteria for Vo(2)max., Conclusions: This work has important implications for developing guidelines for measuring aerobic capacity early after stroke. Although maximal exercise testing using semirecumbent cycle ergometry is feasible, standard Vo(2)max criteria are not consistently appropriate. At least 1 practice trial is recommended before the actual evaluation is performed.

This study investigated the effects of postural set on the cortical response evoked by an external perturbation to human upright stance. Postural set was manipulated by providing either predictable or unpredictable whole body perturbations which required balance corrections to maintain upright stability. Unpredictable perturbations evoked a large negative potential (e.g., CZ: -19.9+/-5.1 microV) that was similar in timing (e.g., CZ: 98.9+/-5.5 ms) and shape to that reported in previous studies. This large negative potential was not discernable for perturbations with predictable onset timing and direction in spite of the presence of significant compensatory balance reactions. Importantly, when a surprise perturbation was presented following a series of predictable perturbations, the large negative potential occurred on this trial even though subjects expected a predictable stimulus onset. This suggests that the large negative potential was dependent on a dissociation between expected and actual stimuli rather than on a tonic central state defined by task conditions. These results suggest that cortical events may be linked to error detection that is independent of sensory or motor events associated with evoked balance reactions.

Background and Purpose: Hemiparesis is the commonest disabling deficit caused by stroke. In animals, dextroamphetamine (AMPH) paired with training enhances motor recovery, but its clinical efficacy is uncertain., Methods: In a randomized, double-blind, placebo-controlled trial, 71 stroke patients were stratified by hemiparesis severity and randomly assigned to 10 sessions of physiotherapy coupled with either 10 mg AMPH or placebo. Study treatments were administered by 1 physiotherapist, beginning 5 to 10 days after stroke and continuing twice per week for 5 weeks. Outcomes were assessed by 1 physiotherapist at baseline, after each treatment session, at 6 weeks, and at 3 months. The primary outcome was motor recovery (impairment level) on the Fugl-Meyer (FM) scale. Secondary outcomes assessed mobility, ambulation, arm/hand function, and independence in activities of daily living., Results: Baseline hemiparesis was severe overall (mean FM score 27.7+/-20.0). Motor scores improved during treatment in both groups (mean change, baseline to 3 months 29.5+/-16.6). Repeated-measures ANOVA revealed no significant differences in recovery between the treatment groups for the entire cohort (n=67) or for subgroups with a severe hemiparesis (n=43), moderate hemiparesis (n=24), or cortically based stroke (n=26). In the moderate subgroup, there was a significant drug x time interaction for upper extremity motor recovery (F=5.14; P<0.001), although there was a significant baseline imbalance in motor scores in this subgroup., Conclusions: In stroke patients with a severe motor deficit, 10 mg AMPH coupled with physiotherapy twice per week for 5 weeks in the early poststroke period provided no additional benefit in motor or functional recovery compared with physiotherapy alone. Patients with moderate severity hemiparesis deserve further investigation. Increased intensity and longer duration drug/therapy dosing regimens should be explored, targeting the upper and lower limbs separately.

In many situations successful execution of a balance-recovery reaction requires visual information about the environment. In particular, reactions that involve rapid limb movements, such as stepping, must be controlled to avoid obstacles and accommodate other constraints on limb trajectory. However, it is unknown whether the central nervous system can acquire the necessary visuospatial information prior to perturbation onset or must, instead, redirect gaze at the floor during the execution of the stepping reaction. To study this we examined gaze behaviour, during rapid forward-directed stepping reactions triggered by unpredictable platform perturbation, in 12 healthy young adults. We also monitored switching of attention, as inferred from onset of significant error in performing a concurrent visuomotor tracking task. Obstacles and/or step targets were used as constraints, to increase demands for accurate foot movement. Downward gaze shifts towards the floor almost never occurred during stepping reactions when foot motion was unconstrained but did occur more frequently as the demands for accurate foot movement increased. Nonetheless, even in the most challenging condition (target plus obstacle), downward redirection of gaze occurred in less than 40% of the trials, and subjects were commonly well able to clear the obstacle and land the foot on the target without redirecting their gaze towards the floor. An apparent switching of attention, subsequent to perturbation onset, occurred frequently (>80% of trials) in all task conditions, independent of the gaze shifts. The findings indicate that visual fixation of the foot or floor was not essential for accurate control of the foot movement, nor was the apparent switching of attention that followed perturbation onset linked, in any consistent way, to overt changes in visual fixation. Spatial features of the support surface were apparently "remembered" prior to perturbation onset, thereby allowing both vision and attention to be directed to other demands during the execution of the balance reaction.

Previous studies have suggested that early cortical potentials (e.g. N1) that are evoked by perturbations to upright stance are associated with sensory processing of the initial perturbation and that later potentials may represent cognitive processing of this perturbation. However, it has also been suggested that later cortical potentials could reflect sensory and motor processing of later phases of the postural reaction. The current study set out to provide additional insight into the association between perturbation-evoked cortical potentials and postural reactions evoked by whole-body perturbations. By altering the deceleration onset of the perturbation, which altered the timing of later postural responses, we determined whether changes in later postural responses were associated with changes in later potentials. Based on previous work, we hypothesized that later potentials would not be associated with changes in later postural responses. During stance, seven healthy young adults were instructed to maintain their balance following two types of perturbations: (1) acceleration phase immediately followed by a deceleration phase (TASK 1), and (2) acceleration phase followed by a delayed deceleration phase (TASK 2). In spite of profound task differences in later postural responses, results revealed no significant differences in later potentials. This work provides additional support for the idea that latter elements of perturbation-evoked cortical responses are likely independent of evoked motor reactions required to maintain stability.

Background and Purpose: Patients with hemiparetic stroke have impaired balance control. Some patients ("pushers") are resistant to accepting weight on and actively "push" away from the nonparetic side. This research identified pushers from stroke patients with moderate to severe hemiparesis and examined longitudinal changes in symptoms, level of impairment, and functional independence., Methods: Prospective sample of hemiparetic stroke patients (n=65) located in Toronto, Canada. Detailed clinical assessments were performed within 10 days postonset, at 6 weeks, and at 3 months., Results: At 1 week after stroke, 63% of patients demonstrated features of pushing. In 62% of pushers, symptoms resolved by 6 weeks, whereas in 21%, pushing symptoms persisted at 3 months. Motor recovery and functional abilities at 3 months were significantly lower among the pushers compared with the nonpushers. Pushers also had a significantly longer hospital length of stay (89 days versus 57 days). It is noteworthy that motor and functional recovery improved significantly over the 3-month study period for both pushers and nonpushers., Conclusions: Identification of stroke patients with pushing symptoms has prognostic implications for recovery. In light of this potential recovery, rehabilitation specialists need to refine treatment approaches for the pushers to further improve functional outcome.

The ability to reach and "grasp" (grip or touch) structures for support in reaction to instability is an important element of the postural repertoire. It is unclear, however, how the central nervous system (CNS) resolves the potential conflict between holding an object and the need to release the held object and grasp alternative support, particularly if the held object is perceived to be relevant to the task of stabilizing the body, e.g. an assistive device. This study examined whether compensatory grasping is inhibited when holding an object, and whether the influence differs when holding an assistive device (cane) versus a task-irrelevant object (top handle portion of a cane). We also investigated the influence of preloading the assistive device, to determine whether conflicting demands for arm-muscle activation (requiring disengagement of ongoing agonist or antagonist activity) would influence the inhibition of compensatory grasping. Unpredictable forward and backward platform translations were used to evoke the balancing reactions in 16 healthy young adults. A handrail was mounted to the right and foot motion was constrained by barriers, with the intent that successful balance recovery would (in large-perturbation trials) require subjects to release the held object and contact the rail with the right hand. Results showed that grasping reactions were commonly used to recover equilibrium when the hand was free (rail contact in 71% of large-perturbation trials). However, holding either the cane or canetop had a potent modulating effect: although early biceps activation was almost never inhibited completely (significant activity within 200 ms in 98% of trials), the average activation amplitude was attenuated by 30-64% and the average frequency of handrail contact was reduced by a factor of two or more. This reduced use of the rail occurred even though the consequence often involved falling against a safety harness or barriers. Handrail contact occurred least frequently when holding the cane during forward loss of balance: subjects persisted in pushing on the cane (failing to use the rail) in 93% of trials, even when the perturbations were too large to allow this strategy to be successful. Prior contraction (preloading the cane) did not influence any of these findings. Complex strategies (e.g. partial release of object) were often adopted to allow balance to be recovered without dropping the held object. Remarkably, it appears that the CNS may give priority to the ongoing task of holding an object, even when it has no stabilizing value (cane during backward falls) or any intrinsic value whatsoever (canetop).

Background: Although previous studies suggest that postural control requires attention and other cognitive resources, the central mechanisms responsible for this relationship remain unclear. To address this issue, we examined the effects of altered attention on cortical activity and postural responses following mechanical perturbations to upright stance. We hypothesized that cortical activity would be attenuated but not delayed when mechanical perturbations were applied during a concurrent performance of a cognitive task (i.e. when attention was directed away from the perturbation). We also hypothesized that these cortical changes would be accompanied by alterations in the postural response, as evidenced by increases in the magnitude of anteroposterior (AP) centre of pressure (COP) peak displacements and tibialis anterior (TA) muscle activity. Healthy young adults (n = 7) were instructed to continuously track (cognitive task) or not track (control task) a randomly moving visual target using a hand-held joystick. During each of these conditions, unpredictable translations of a moving floor evoked cortical and postural responses. Scalp-recorded cortical activity, COP, and TA electromyographic (EMG) measures were collected., Results: Results revealed a significant decrease in the magnitude of early cortical activity (the N1 response, the first negative peak after perturbation onset) during the tracking task compared to the control condition. More pronounced AP COP peak displacements and EMG magnitudes were also observed for the tracking task and were possibly related to changes in the N1 response., Conclusion: Based on previous notions that the N1 response represents sensory processing of the balance disturbance, we suggest that the attenuation of the N1 response is an important central mechanism that may provide insight into the relationship between attention and postural control.

In order to recover balance by grasping an object for support, the CNS must rapidly move the hand toward a specific target (handhold) in the environment. The early latency (80-140 ms) of these grasping reactions would seem to preclude a role for online visual feedback in the control of the initial limb movement; however, some studies have shown that vision can influence initiation of lower-limb postural reactions at similar latency. This study explored the role of vision in controlling the initial trajectory of grasping reactions triggered by sudden unpredictable medio-lateral platform translation. Healthy young adults were instructed to recover balance by grasping a marked section of a handrail, located to their right. To reinforce a dependence on arm reactions, movement of the feet was prevented by barriers. Liquid-crystal goggles were used to occlude vision during response initiation (200 ms interval starting at perturbation onset, PO). Results showed that the initial grasping trajectory (first 100 ms) and associated muscle activation were heavily modulated to take into account the direction and speed of the perturbation-induced body motion in relation to the handrail. This modulation was unaffected by occlusion of vision at PO, indicating that information about the rail location obtained prior to PO was incorporated into the control. These findings are consistent with the view that the CNS tunes the initial arm trajectory by combining an egocentric spatial map, formed prior to PO, with online feedback about the body motion from non-visual inputs (somatosensory and/or vestibular). This prevents potential delays associated with visual processing and ensures very rapid onset of arm movement that is directed appropriately even though the position of the body is perturbed unpredictably with respect to the target.

Using functional magnetic resonance imaging (fMRI), responses in human somatosensory cortex were quantified in response to changes in the amplitude of peripheral stimulation during (1) passive vibration and (2) an attention-demanding tactile tracking task whereby changes in vibration amplitude were used to guide motor behaviour. Functional MRI was conducted using a scanner operating at 1.5 T, and vibration was delivered to the volar surface of the right index finger with a custom-built magnetomechanical device. Results showed that primary somatosensory cortex (SI) reliably reflects changes in vibration amplitude applied to the finger during passive vibration and also in the presence of a task that modulates the activity in SI. Secondary somatosensory cortex did not reveal any clear relationship with vibration amplitude but was more often activated during the attention demanding tracking task compared with passive vibration. The present study supports an increasing stimulus-response relationship between vibrotactile stimuli and activity in SI that persists during attentive, active states.

Stepping reactions are often triggered rapidly in response to loss of balance. It has been unclear whether spatial step parameters are defined at time of step-initiation or whether they can be modulated online, during step execution, in response to sensory feedback about the evolving state of instability. This study explored the capacity to actively alter step direction subsequent to step initiation in six healthy young-adult subjects. To elicit forward-step reactions, subjects were released suddenly from a tethered forward lean. A second perturbation (medio-lateral support-surface translation) was applied at lags of 0-200 ms. Active reaction to the second perturbation was determined primarily through analysis of swing-leg hip-abductor activation. In addition, to gauge the biomechanical consequence of the changes in muscle activation, we compared the measured medio-lateral swing-foot displacement to that predicted by a simple passive mechanical model. Perturbations at 0-100 ms lag evoked active medio-lateral swing-foot deviation, allowing balance to be recovered with a single step. However, when the second perturbation occurred near foot-off (200-ms lag), there was no evidence of active alteration of step direction and subjects typically required additional steps to recover balance. The results suggest that step direction can be reparameterized during early stages of stepping reactions, but that step direction was not actively modulated in response to perturbation arising near start of swing phase.

How the brain functions during behavioural tasks conducted in virtual reality (VR) remains largely unresolved. This issue is extremely important both in terms of establishing the benefits of VR through basic science, as well as for future optimization of tasks conducted in VR environments. Here, the authors describe their current work to develop a testing platform for conducting VR experiments that can be probed by functional magnetic resonance imaging (fMRI) to measure brain activity. Examples involving human spatial navigation and data glove operation illustrate the technical feasibility of the approach and introduce thought-provoking observations of brain activation patterns. Future research directions for combined use of VR and fMRI are also discussed.

Background and Purpose: In the undamaged brain, sensory input to the cortex is intricately controlled via sensory gating mechanisms. Given the role of corticothalamic pathways in this control, it was hypothesized that in patients recovering from thalamic stroke there would be evidence of disrupted sensory gating and that efficient control of cortical sensory inputs would emerge during recovery., Methods: Four patients were tested serially after stroke from 1 to 24 weeks after injury. Perceptual thresholds, somatosensory evoked potential amplitudes, and functional MRI activations under specific somatosensory stimulation conditions were measured., Results: All patients demonstrated comparable results, revealing disrupted threshold detection to vibrotactile stimuli in the presence of a concurrent competing, contralateral input. In contrast, threshold detection was comparable between the affected and unaffected sides when there were no competing stimuli. This compromised capacity to inhibit competing sensory inputs was paralleled by a reduction in the measured activation of cortical representation in the stroke-affected hemisphere (functional MRI and somatosensory evoked potential) during bilateral stimulation. After recovery, perceptual detection improvements during bilateral stimulation were paralleled by enhancements of primary somatosensory cortical activation in the stroke-affected hemisphere., Conclusions: These results provide insight into potential mechanisms that contribute to sensory gating and suggest that the ability to control sensory input through effective gating mechanisms, in addition to primary somatosensory representation, may be important for poststroke sensory recovery.

Recent work has revealed the specific time course of attention shifts associated with balance control in a seated model using a dual-task paradigm. This work highlighted an initial "automatic" and later "attention-demanding" phase of the evoked balance reaction. The objective of the present study was to determine if comparable influences would be observed for performance of a visuomotor tracking task when responding to perturbations of upright stability. Small-amplitude floor translations were applied in the forward or backward direction to evoke stabilizing postural reactions. Balance reactions were evoked with and without the concurrent performance of a visuomotor tracking task using the right hand. Results showed significant disruptions (pauses) in tracking that invariably occurred after onset of the earliest balance reaction measured in ankle muscles. On average, there was a delay of 345 ms between ankle-muscle activation (average onset 144 ms) and the pause in visuomotor tracking. The concurrent tracking led to modest change in later phases of the balance reaction, as measured by an increase in center-of-pressure excursions, but did not affect the earliest phase of the reaction. These results support the view that compensatory balance reactions, even those evoked by small perturbations, are characterized by an initial "automatic" phase and subsequent control that may be more dependent on cognitive resources.

This study used environmental restrictions on foot movement to challenge the capacity of the central nervous system (CNS) to counter the lateral instability that arises after foot-lift during rapid triggered stepping reactions evoked by unpredictable postural perturbation. The objective was to determine the extent to which lateral stability could be regulated via modulation of the mediolateral (m-l) anticipatory postural adjustment (APA) that precedes foot-lift. A high frontal obstacle was used to double the required swing duration, and thereby increase the potential for the center of mass (COM) to fall laterally toward the unsupported side, during forward-step reactions. The capacity to use lateral step placement to recover lateral stability was restricted by means of lateral barriers. Six healthy young adults were tested. In obstacle-only trials, the APA was insufficient to prevent increased lateral COM motion during the prolonged swing phase; hence, lateral step placement was necessitated. However, when lateral stepping was obstructed, the CNS was able to upregulate the APA amplitude so as to prevent this increase in lateral COM motion. The swing foot was placed medially, with no detriment to clearing the frontal obstacle or recovering equilibrium. There was no change in step timing or anteroposterior (a-p) COM motion. While previous studies have suggested that the a-p COM progression may determine the extent to which the m-l APA is expressed or truncated during triggered stepping reactions evoked by unpredictable perturbation, the present findings demonstrate that prior knowledge of environmental demands can lead to predictive efforts to modulate the APA during such reactions. An apparent preference to underscale anticipatory efforts when lateral step placement is permitted suggests that the CNS may be acting to avoid some potential risk or cost associated with the execution of a large APA.

This study examined whether the stabilizing features of rapid triggered compensatory stepping reactions can be modulated to accommodate an environmental constraint, in the form of an obstacle placed in front of the subject. The compensatory stepping reactions were evoked, in 11 healthy young adults, by unpredictable multidirectional platform translation; the forward-step reactions evoked by large backward translations were analyzed. Clearance of the obstacle required a doubling of the usual (no obstacle) swing duration and therein presented significant challenges to both anteroposterior (a-p) and lateral stability, yet the central nervous system (CNS) was able to decelerate and stabilize the body's center of mass (COM) without taking additional steps. Findings that the response was modulated successfully without prior practice or exposure to the perturbation suggest that the CNS automatically incorporates exteroceptive information into the control of compensatory stepping. Control of a-p stability appeared primarily to involve an increase in forward step distance. Surprisingly, the control of lateral stability involved both an anticipatory postural adjustment (APA) and lateral step placement. By including a much larger APA prior to foot-lift (137% increase in amplitude and 50% increase in duration versus no-obstacle trials), the COM was propelled to a more stable position prior to swing phase. This was achieved with minimal (32 ms) delay in foot-lift, due, in part, to more rapid initiation and execution of limb unloading. In contrast to previous findings that APAs are commonly absent or severely reduced during compensatory stepping, the present results demonstrate that large APAs can be incorporated into these reactions when demanded by task conditions. However, these large APAs were still insufficient to counter the increased tendency of the COM to fall during the prolonged swing phase. The fact that the APA was included at all may indicate a hybrid control, in which predictive control (via APA) is used to reduce the anticipated lateral instability, and reactive control of the final foot placement, governed by sensory discharge related to the actual COM motion, provides any additional stabilization required.

Electrophysiological studies have shown that task-relevant somatosensory information leads to selective facilitation within the primary somatosensory cortex (SI). The purpose of the present study was (1) to further explore the relationship between the relevancy of stimuli and activation within the contralateral and ipsilateral SI and (2) to provide further insight into the specific sensory gating network responsible for modulating neural activity within SI. Functional MRI of 12 normal subjects was performed with vibrotactile stimuli presented to the pad of the index finger. In experiment 1, the stimulus was presented to either the left or the right hand. Subjects were required to detect transient changes in stimulus frequency. In experiment 2, stimuli were presented to either the right hand alone or both hands simultaneously. Stimuli were applied either (A) passively or (B) when subjects were asked to detect frequency changes that occurred to the right hand only. In experiment 1, task-relevant somatosensory stimulation led not only to enhanced contralateral SI activity, but also to a suppression of activity in the ipsilateral SI. In experiment 2, SI activation was enhanced when stimuli were task-relevant, compared to that observed with passive input. When stimuli were presented simultaneously to both hands, only those that were task-relevant increased SI activation. This was associated with recruitment of a network of cortical regions, including the right prefrontal cortex (Brodmann area 9). We conclude that SI modulation is dependent on task relevancy and that this modulation may be regulated, at least in part, by the prefrontal cortex.