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Background The characteristics of experimentally induced slips (low-friction surfaces and non-motorized platforms) in laboratory settings are influenced by participant gait velocity, contact surface area, and level of friction between the foot and surface. However, motorized platforms that could account for these factors during slip-like paradigms have not been extensively used. Research question How does slip-like perturbations evoked via a motorized platform change gait characteristics and postural stability during overground walking? Methods Ten healthy young adults performed 4 overground, self-paced walking trials, with the 4th trial including an unexpected forward support surface translation at heel-strike during steady state walking. Kinematic and kinetic data were collected, with step characteristics (time, distance, velocity) and postural stability calculated to compare between normal gait and slip-like trials. Slip foot characteristics were also determined. Results Peak slipping foot velocity variability was considerably smaller compared to previously reported low-friction and non-motorized perturbations. The centre of mass was shifted more posteriorly (thus in a less stable location) by the end of the platform acceleration phase compared to the same time point post-heel strike during normal gait trials. Participants successfully responded to every slip-like perturbation by significantly increasing step time, decreasing step distance, and decreasing step velocity. Significance Our results demonstrate the repeatability and consistency of a motorized support surface paradigm to induce slip-like perturbations. Furthermore, stability and step characteristic results confirm posterior shifts in stability and appropriate stepping responses, mimicking how participants would react if responding to a real world slip.

Maintaining the centre of mass (CoM) of the body within the base of support is a critical component of upright balance; the ability to accurately quantify balance recovery mechanisms is critical for many research teams.The purpose of this study was to investigate how exclusion of specific body segments in an anthropometric CoM model influenced a dynamic measure of postural stability, the margin of stability (MoS), following a support-surface perturbation.Healthy young adults (n = 10) were instrumented with kinematic markers and a safety harness. Sixteen support-surface translations, scaled to ensure responses did not involve a change in base of support, were then issued (backwards, forwards, left, or right). Whole-body CoM was estimated using four variations of a 13-segment anthropometric model: i) the full-model (WFM), and three simplified models, ii) excluding upper limbs (NAr); iii) excluding upper and lower limbs (HTP); iv) pelvis CoM (CoMp). The CoM calculated for each variant was then used to estimate extrapolated CoM (xCoM) position and the resulting MoS within the plane of postural disturbance.Comparisons of simplified models to the full model revealed significant differences (p 0.05) in MoS for all models in each perturbation condition; however, the largest differences were following sagittal plane based perturbations. Poor estimates of WFM MoS were most evident for HTP and CoMp models; these were associated with the greatest values of RMS/maximum error, poorest correlations, etc. The simplified models provided low-error approximates for frontal plane perturbations.Findings suggest that simplified calculations of CoM can be used by researchers without reducing MoS measurement accuracy; however, the degree of simplification should be context-dependent. For example, CoMp models may be appropriate for questions pertaining to frontal plane MoS; sagittal plane MoS necessitates inclusion of lower limb and HTP segments to prevent underestimation of postural stability.

Investigating an ecologically relevant upper limb task, such as manually transporting an object with a concurrent lateral change in support (sidestepping alongside a kitchen counter), may provide greater insight into potential deficits in postural stability, variability and motor coordination in older adults. Nine healthy young and eleven older, community dwelling adults executed an upper limb object transport task requiring a lateral change in support in two directions at two self-selected speeds, self-paced and fast-paced. Dynamic postural stability and movement variability was quantified via whole-body center of mass motion. The onset of lead lower limb movement in relation to object movement onset was quantified as a measure of motor coordination. Older adults demonstrated similar levels of stability and variability as their younger counterparts, but at slower peak movement velocity and increased task duration. Furthermore, older adults demonstrated asymmetrical motor coordination between left and right task directions, while younger adults remained consistent regardless of task direction. Thus, older adults significantly modulated movement speed and motor coordination to maintain similar levels of stability and variability compared to their younger counterparts.

A "reach and transport object" task that represents common activities of daily living may provide improved insight into dynamic postural stability and movement variability deficits in older adults compared to previous lean to reach and functional reach tests. Healthy young and older, community dwelling adults performed three same elevation object transport tasks and two multiple elevation object transport tasks under two self-selected speeds, self-paced and fast-paced. Dynamic postural stability and movement variability was quantified by whole-body center of mass motion. Older adults demonstrated significant decrements in frontal plane stability during the multiple elevation tasks while exhibiting the same movement variability as their younger counterparts, regardless of task speed. Interestingly, older adults did not exhibit a tradeoff in maneuverability in favour of maintaining stability throughout the tasks, as has previously been reported. In conclusion, the multi-planar, ecologically relevant tasks employed in the current study were specific enough to elucidate decrements in dynamic stability, and thus may be useful for assessing fall risk in older adults with suspected postural instability.

Rapid motor learning may occur in situations where individuals perceive a threat if they do not perform a task well. This rapid motor learning may be facilitated by improved motor performance and, consequently, more errorless practice. As a first step towards understanding the role of perceived threat on rapid motor learning, the purpose of this study was to determine how performance of a motor task is affected in situations where perceived threat is high. We hypothesized that perceived threat in a virtual environment would result in improved performance of a walking task (i.e., walking on a narrow beam). Results demonstrated that increased perceived threat did not yield statistically significantly greater balance performance in the high-threat virtual environment (median percentage of successful steps: 78.8%, 48.3%, and 55.2% in the real low-threat, virtual low-threat, and virtual high-threat environments, respectively). These results may be partially attributed to habituation to threat over time and practice. If implemented carefully, virtual reality technology can be an effective tool for investigating walking balance in environments that are perceived as threatening.

Dysequilibrium of cervicogenic origin can result from pain and injury to cervical paraspinal tissues post-whiplash; however, the specific physiological mechanisms still remain unclear. Central sensitization is a neuradaptive process which has been clinically associated with conditions of chronic pain and hypersensitivity. Strong links have been demonstrated between pain hypersensitivity and postural deficits post-whiplash; however, the precise mechanisms are still poorly understood. The purpose of this study was to explore the mechanisms of cervicogenic disequilibrium by investigating the effect of experimentally induced central sensitization in the cervical spine on postural stability in young healthy adults. Sixteen healthy young adults (7 males (22.6±1.13 years) and 9 females (22±2.69 years)) performed 30-s full-tandem stance trials on an AMTI force plate under normal and centrally sensitized conditions. The primary outcome variables included the standard deviation of the center of pressure (COP) position in medio-lateral (M-L) and antero-posterior (A-P) directions; sway range of the COP in M-L and A-P directions and the mean power frequency (MPF) of the COP and horizontal ground shear forces. Variability and sway range of the COP decreased with experimental induction of central sensitization, accompanied by an increase in MPF of COP displacement in both M-L and A-P directions, suggesting an increase in postural stiffening post-sensitization versus non-sensitized controls. Future studies need to further explore this relationship in clinical (whiplash, chronic pain) populations.

This study aimed to determine the validity of the centre of mass position (COM) position and extrapolated COM (XCOM), relative to the base of support, for predicting medio-lateral stability during a walking task where the base of support width is limited. Nine young healthy participants walked on a narrow beam. Three-dimensional motion capture was used to calculate the COM and XCOM relative to the base of support. Steps were classified as having either the COM or XCOM inside or outside the base of support, and were classified as successful (stable - foot placed on the beam) or failed (unstable - foot stepped off the beam). If the COM or XCOM are valid measures of stability, they should be within the base of support for successful steps and outside the base of support for failed steps. Classifying the COM and XCOM inside or outside the base of support correctly predicted successful or failed steps in 69% and 58% of cases, respectively. When the COM or XCOM were outside the base of support, walking faster seemed to help participants to maintain stability. The further the COM or XCOM were outside the base of support during a successful step, the more likely participants were to fail on a subsequent step. The results of this study suggest that both COM and XCOM are valid measures of stability during a beam walking task, but that classifying COM and XCOM as inside or outside the base of support may be over-simplistic.