The effects of physical and temporal certainty on human locomotion with discrete underfoot perturbations.

Foot placement can be selected to anticipate upcoming perturbations, but it is unclear how this anticipatory strategy is influenced by available response time or precise knowledge of the perturbation's characteristics. This study investigates anticipatory and reactive locomotor strategies for repeated underfoot perturbations with varying levels of temporal certainty, physical certainty, and available response time. Thirteen healthy adults walked with random underfoot perturbations from a mechanized shoe. Temporal certainty was challenged by presenting the perturbations with or without warning. Available response time was challenged by adjusting the timing of the warning before the perturbation. Physical certainty was challenged by making perturbation direction (inversion or eversion) unpredictable for certain conditions. Linear-mixed effects models assessed the effect of each condition on the percentage change of margin of stability and step width. For perturbations with one stride or less of response time, we observed few changes to step width or margin of stability. As response time increased to two strides, participants adopted wider steps in anticipation of the perturbation (P=0.001). Physical certainty had little effect on gait for the step of the perturbation, but participants recovered normal gait sooner when the physical nature of the perturbation was predictable (P<0.001). Despite having information about the timing and direction of upcoming perturbations, individuals do not develop perturbation-specific feedforward strategies. Instead, they use feedback control to recover normal gait after a perturbation. However, physical certainty appears to make the feedback controller more efficient and allows individuals to recover normal gait sooner.

Reactive Balance Responses After Mild Traumatic Brain Injury: A Scoping Review.

OBJECTIVE: Balance testing after concussion or mild traumatic brain injury (mTBI) can be useful in determining acute and chronic neuromuscular deficits that are unapparent from symptom scores or cognitive testing alone. Current assessments of balance do not comprehensively evaluate all 3 classes of balance: maintaining a posture; voluntary movement; and reactive postural response. Despite the utility of reactive postural responses in predicting fall risk in other balance-impaired populations, the effect of mTBI on reactive postural responses remains unclear. This review sought to (1) examine the extent and range of available research on reactive postural responses in people post-mTBI and (2) determine whether reactive postural responses (balance recovery) are affected by mTBI. DESIGN: Scoping review. METHODS: Studies were identified using MEDLINE, EMBASE, CINAHL, Cochrane Library, Dissertations and Theses Global, PsycINFO, SportDiscus, and Web of Science. Inclusion criteria were injury classified as mTBI with no confounding central or peripheral nervous system dysfunction beyond those stemming from the mTBI, quantitative measure of reactive postural response, and a discrete, externally driven perturbation was used to test reactive postural response. RESULTS: A total of 4747 publications were identified, and a total of 3 studies (5 publications) were included in the review. CONCLUSION: The limited number of studies available on this topic highlights the lack of investigation on reactive postural responses after mTBI. This review provides a new direction for balance assessments after mTBI and recommends incorporating all 3 classes of postural control in future research.

Interadministrator Reliability of a Modified Instrumented Push and Release Test of Reactive Balance.

CONTEXT: Traditional assessments of reactive balance require sophisticated instrumentation to ensure objective, highly repeatable paradigms. This instrumentation is clinically impractical. The Push and Release test (P&R) is a well-validated clinical test that examines reactive balance, and the application of wearable inertial measurement units (IMU) enables sensitive and objective assessment of this clinically feasible test. The P&R relies on administrator experience and may be susceptible to interadministration reliability concerns. The purpose of this study was to evaluate the interadministrator reliability of objective outcomes from an instrumented, modified version of the P&R test. DESIGN: Crossover interadministrator design. METHODS: Twenty healthy adults (20-35 y) completed the P&R in 4 directions with 2 different administrators. Measures quantified using IMUs included step latency, step length, and time to stability. Lean angle (LA) at release was used as a measure of administration consistency. The intraclass correlation coefficient (ICC) estimate was used to assess interadministrator reliability in each direction. To determine consistency of LA within and across administrators, we calculated the SDs for each rater by direction and the interadministrator reliability of LA using ICC. RESULTS: Across individual directions, the ICC for agreement between raters ranged from .16 to .39 for step latency, from .52 to .62 for time to stability, and from .48 to .84 for step length. Summary metrics across all 4 directions produced higher ICC values. There was poor to moderate consistency in administration based on LA, but LA did not significantly affect any of the outcomes. CONCLUSION: The modified P&R yields moderate interadministrator reliability and high validity. Summary metrics over all 4 directions (the maximum step latency, the median time to stability, and the median step length) are likely more reliable than direction-specific scores. Variations in body size should also be considered when comparing populations.

Consequences of changing planned foot placement on balance control and forward progress.

While walking humans generally plan foot placement two steps in advance. However, it is often necessary to rapidly alter foot placement position just before stepping due to the appearance of a new obstacle. While humans are quite capable of rapidly altering foot placement position, such changes can have major effects on centre of mass dynamics. We investigated how rapid changes to planned foot placement impact centre of mass dynamics, and how such changes influence the control of balance and forward progress, during both straight- and turning-gait. Thirteen young adults walked along a virtually projected walkway with precision footholds oriented either in a straight line or with a single 60°, 90° or 120° turn. On a subset of trials, participants were required to rapidly avoid stepping on select footholds. We found that if the centre of mass was disrupted such that it interfered with task success (i.e. staying upright and continuing along the planned path), walkers were more likely to sacrifice forward progress than the upright stability. Further, walkers appear to control centre of mass dynamics differently following inhibited steps during step turns than during spin turns, which may reflect a larger threat to task success when spin turns are interrupted.

The Effect of Intranasal Plus Transcranial Photobiomodulation on Neuromuscular Control in Individuals with Repetitive Head Acceleration Events.

Objective: This proof-of-concept study was to investigate the relationship between photobiomodulation (PBM) and neuromuscular control. Background: The effects of concussion and repetitive head acceleration events (RHAEs) are associated with decreased motor control and balance. Simultaneous intranasal and transcranial PBM (itPBM) is emerging as a possible treatment for cognitive and psychological sequelae of brain injury with evidence of remote effects on other body systems. Methods: In total, 43 (39 male) participants, age 18-69 years (mean, 49.5; SD, 14.45), with a self-reported history of concussive and/or RHAE and complaints of their related effects (e.g., mood dysregulation, impaired cognition, and poor sleep quality), completed baseline and posttreatment motor assessments including clinical reaction time, grip strength, grooved pegboard, and the Mini Balance Evaluation Systems Test (MiniBEST). In the 8-week interim, participants self-administered itPBM treatments by wearing a headset comprising four near-infrared light-emitting diodes (LED) and a near-infrared LED nasal clip. Results: Posttreatment group averages in reaction time, MiniBEST reactive control subscores, and bilateral grip strength significantly improved with effect sizes of g = 0.75, g = 0.63, g = 0.22 (dominant hand), and g = 0.34 (nondominant hand), respectively. Conclusion: This study provides a framework for more robust studies and suggests that itPBM may serve as a noninvasive solution for improved neuromuscular health.

The choice of reference frame alters interpretations of turning gait and stability.

Humans regularly follow curvilinear trajectories during everyday ambulation. However, globally-defined and locally-defined reference frames fall out of alignment during turning gait, which complicates spatiotemporal and biomechanical analyses. Thus, the choice of the locally-defined reference frame is an important methodological consideration. This study investigated how different definitions of reference frame change the results and interpretations of common gait measures during turning. Nine healthy adults completed two walking trials around a circular track. Kinematic data were collected via motion capture and used to calculate step length, step width, anteroposterior margin of stability, and mediolateral margin of stability using three different locally-defined reference frames: walkway-fixed, body-fixed, and trajectory-fixed. Linear-mixed effects models compared the effect of reference frame on each gait measure, and the effect of reference frame on conclusions about a known effect of turning gait - asymmetrical stepping patterns. All four gait measures differed significantly across the three reference frames. A significant interaction of reference frame and step type (i.e. inside vs outside step) on step length (p < 0.001), anteroposterior margin of stability (p < 0.001), and mediolateral margin of stability (p < 0.001) indicated conclusions about asymmetry differed based on the choice of reference frame. The choice of reference frame will change the calculated gait measures and may alter the conclusions of studies investigating turning gait. Care should be taken when comparing studies that used different reference frames, as results cannot be easily harmonized. Future studies of turning gait need to justify and detail their choice of reference frame.

Anticipatory and reactive responses to underfoot perturbations during gait in healthy adults and individuals with a recent mild traumatic brain injury.

BACKGROUND: Following mild traumatic brain injury, individuals often exhibit quantifiable gait deficits over flat surfaces, but little is known about how they control gait over complex surfaces. Such complex surfaces require precise neuromotor control to anticipate and react to small disturbances in walking surfaces, and mild traumatic brain injury-related balance deficits may adversely affect these gait adjustments. METHODS: This study investigates anticipatory and reactive gait adjustments for expected and unexpected underfoot perturbations in healthy adults (n = 5) and individuals with mild traumatic brain injury (n = 5). Participants completed walking trials with random unexpected or expected underfoot perturbations from a mechanized shoe and inertial measurement units collected kinematic data from the feet and sternum. Linear mixed-effects models assessed the effects of segment, group, and their interaction on standardized difference of accelerations between perturbation and non-perturbation trials. FINDINGS: Both groups demonstrated similar gait strategies when perturbations were unexpected. During late swing phase before expected perturbations, persons with mild traumatic brain injury exhibited greater lateral acceleration of their perturbed foot and less lateral movement of their trunk compared with unperturbed gait. Control participants exhibited less lateral foot acceleration and no difference in mediolateral trunk acceleration compared with unperturbed gait during the same period. A significant group*segment interaction (p < 0.001) during this part of the gait cycle suggests the groups adopted different anticipatory strategies for the perturbation. INTERPRETATION: Individuals with mild traumatic brain injury may be adopting cautious strategies for expected perturbations due to persistent neuromechanical deficits stemming from their injury.

Analysis of Free-Living Mobility in People with Mild Traumatic Brain Injury and Healthy Controls: Quality over Quantity.

Balance and mobility issues are common non-resolving symptoms following mild traumatic brain injury (mTBI). Current approaches for evaluating balance and mobility following an mTBI can be subjective and suboptimal as they may not be sensitive to subtle deficits, particularly in those with chronic mTBI. Wearable inertial measurement units (IMU) allow objective quantification of continuous mobility outcomes in natural free-living environments. This study aimed to explore free-living mobility (physical activity and turning) of healthy and chronic mild traumatic brain injury (mTBI) participants using a single IMU. Free-living mobility was examined in 23 healthy control (48.56 ± 23.07 years) and 29 symptomatic mTBI (40.2 ± 12.1 years) participants (average 419 days post-injury, persistent balance complaints) over 1 week, using a single IMU placed at the waist. Free-living mobility was characterized in terms of macro (physical activity volume, pattern and variability) and micro-level (discrete measures of turning) features. Macro-level outcomes showed those with chronic mTBI had similar quantities of mobility compared with controls. Micro-level outcomes within walking bouts showed that chronic mTBI participants had impaired quality of mobility. Specifically, people with chronic mTBI made larger turns, had longer turning durations, slower average and peak velocities (all p < 0.001), and greater turn variability compared with controls. Results highlighted that the quality rather than quantity of mobility differentiated chronic mTBI from controls. Our findings support the use of free-living IMU continuous monitoring to enhance understanding of specific chronic mTBI-related mobility deficits. Future work is required to develop an optimal battery of free-living measures across the mTBI spectrum to aid application within clinical practice.