Aging-related temporal constraints to stability and instability in postural control

Measuring the Reliability of Postural Sway Measurements for a Static Standing Task: The Effect of Age

Background: A force plate is used to determine the ability to balance ability. However, only some medical centers or laboratories are equipped with force plates because they are costly so a low-cost force plate is required for home care or health care institutes. Few studies compare the reliability of postural sway measurements in terms of age. This study proposes a low-cost force plate to select reliable parameters to evaluate postural sway.

Objectives: To determine the intra-rater reliability of a novel force plate and the effect of age difference on the intra-rater test-retest reliability for the center of pressure (COP).

Methods: Forty participants were enrolled for this study: 20 youths and 20 older adults. Participants stood on a custom-made and low-cost force plate with eyes opened and eyes closed to measure COP-related parameters. The within-day test-retest reliability was measured at two sessions on the same day and the between-days reliability was measured on two different days. The COP-related parameters include the average velocity of COP, the average velocity in the antero-posterior and medio-lateral directions, the mean distance of COP and the mean distance in the antero-posterior and medio-lateral directions. An intra-class correlation coefficient test with one-way random model was performed to determine the reliability of different variables within-days and between-days. The results were presented in single measurement of intraclass correlation coefficient (ICC), the standard error of measurements, and the minimal detectable changes of each COP-related parameters.

Results: The novel low-cost force plate demonstrates excellent reliability in terms of the COP velocity related parameters for within- and between-day measurements. The ICC of COP distance related parameters were good to excellent reliability for between-day measurements (range: 0.43–0.84). Older adults demonstrated excellent reliability in terms of the mean distance for antero-posterior and the results were better than those for younger participants for the eyes-opened and eyes-closed conditions. The reliability in terms of the mean distance for medio-lateral was poor to good for older adults (range: 0.38–0.55), and excellent for younger participants.

Conclusion: The novel and low-cost force plate reliably measured balance and age affects the reliability of different COP variables, so the results of this study were pertinent to the selection of COP measures.

Effects of two passive back-support exoskeletons on postural balance during quiet stance and functional limits of stability

While occupational back-support exoskeletons (BSEs) are considered as potential workplace interventions, BSE use may compromise postural control. Thus, we investigated the effects of passive BSEs on postural balance during quiet upright stance and functional limits of stability. Twenty healthy adults completed trials of quiet upright stance with differing levels of difficulty (bipedal and unipedal stance; each with eyes open and closed), and executed maximal voluntary leans. Trials were done while wearing two different BSEs (SuitX™, Laevo™) and in a control (no-BSE) condition. BSE use significantly increased center-of-pressure (COP) median frequency and mean velocity during bipedal stance. In unipedal stance, using the Laevo™ was associated with a significant improvement in postural balance, especially among males, as indicated by smaller COP displacement and sway area, and a longer time to contact the stability boundary. BSE use may affect postural balance, through translation of the human + BSE center-of-mass, restricted motion, and added supportive torques. Furthermore, larger effects of BSEs on postural balance were evident among males. Future work should further investigate the gender-specificity of BSE effects on postural balance and consider the effects of BSEs on dynamic stability.

Recurrence dynamics reveals differential control strategies to maintain balance on sloped surfaces

BACKGROUND

Studies on postural control have primarily focused on the maintenance of balance in quiet upright standing on flat horizontal support surfaces that can reveal only a subset of the potential postural stability/instability configurations in everyday contexts.

OBJECTIVES

Here we investigated the nature of dynamical properties of postural coordination in an upright standing task as a function of the systematic scaling of seven support surface angles, +20°, +10° dorsiflexion (+), 0 °Flat, -10°, -20°, -30°, -35° plantarflexion (-), mounted on a force plate.

METHODS

The center of pressure (CoP) and virtual time-to-contact (VTC) were analyzed to examine the spatial and spatio-temporal aspects of postural coordination dynamics, respectively. Recurrence quantification analysis (RQA) was used to characterize the dynamic postural control strategies as a function of slope surface angle.

RESULTS

The recurrence findings showed that on a flat surface the postural CoP dynamic are recurrent with a largely deterministic process and higher Shannon entropy compared to elevated slope angles in dorsiflexion and plantarflexion. There were asymmetrical patterns between similar slope angles for dorsiflexion and plantarflexion postures. The recurrence measures revealed that VTC operates on a higher embedding dimension than that of CoP.

SIGNIFICANCE

VTC showed an enhanced sensitivity to detection of postural instability in relation to the stability boundary that was magnified on the flat surface but progressively reduced over larger surface angles for both the dorsiflexion and plantarflexion postures.

Postural Stability Margins as a Function of Support Surface Slopes

This investigation examined the effects of slope of the surface of support (35°, 30°, 20°, 10° Facing(Toe) Down, 0° Flat and 10°, 20°, 25° Facing (Toe) Up) and postural orientation on the margins of postural stability in quiet standing of young adults. The findings showed that the center of pressure—CoP (displacement, area and length) had least motion at the baseline (0° Flat) platform condition that progressively increased as a function of platform angle in both facing up and down directions. The virtual time to collision (VTC) dynamics revealed that the spatio-temporal margins to the functional stability boundary were progressively smaller and the VTC time series also more regular (SampEn–Sample Entropy) as slope angle increased. Surface slope induces a restricted stability region with lower dimension VTC dynamics that is more constrained when postural orientation is facing down the slope. These findings provide further evidence that VTC acts as a control variable in standing posture that is influenced by the emergent dynamics of the individual-environment-task interaction.

Spinal cord injury and time to instability in seated posture

OBJECTIVE

To investigate seated postural control in persons with spinal cord injury (SCI) compared with age-matched controls.

DESIGN

Cohort.

SETTING

University research laboratory.

PARTICIPANTS

Adults (N=36; mean age ± SD, 22.5 ± 3.2y): 7 persons with high SCI (HI group; injury level greater than T10), 11 persons with low SCI (LI group; injury level between T10 and L4), and 18 persons with non-SCI.

INTERVENTION

Not applicable.

MAIN OUTCOMES MEASURES

Participants sat on a force platform on a custom-built wooden box with their arms by their side. Postural control was quantified in several ways. Participants completed a functional reach test. The amount of postural sway was quantified by characterizing the center of pressure (COP) trajectory by determining median velocity and root mean square of the signal. In addition, the virtual time to contact to the functional boundary was quantified. Last, the instability index was determined as the ratio of the COP area to the functional boundary.

RESULTS

There were no group differences in COP-based metrics (P>.05). There was no difference between SCI groups in functional reach (P>.05). The HI group had a smaller virtual time to contact (VTC) than the control group (.50 ± .20s vs .98 ± .24s, P<.05). Both SCI groups had a greater instability index than the control group, with the HI group having the largest amount of instability (P<.05).

CONCLUSIONS

The observations suggest that VTC analysis is appropriate to investigate seated postural control. It is proposed that including VTC of seated postural control as an outcome measure will provide novel information concerning the effectiveness of various rehabilitation approaches and/or technologies aimed at improving seated postural control in persons with SCI.

Reliability of center of pressure measures of postural stability in healthy older adults: effects of postural task difficulty and cognitive load

Postural instability is a major risk factor of falling in the elderly. It is well documented that postural control may decline while performing a concurrent cognitive task and this effect increases with age. Despite the extensive use of dual tasking in balance assessment protocols, a lack of sufficient reliability information is evident. This study determines the reliability of the postural stability measures in older adults, assessed under single and dual-task conditions and different levels of postural difficulty. Sixteen older adults completed quiet stance postural measurements at three levels of difficulty (rigid surface-eyes open, rigid surface-eyes closed, and foam surface-eyes closed), with or without performing a concurrent backward counting task, in two sessions 1 week apart. Force plate data was used to calculate center of pressure (COP) parameters including mean velocity, phase plane portrait, area (95% confidence ellipse), standard deviation (SD) of amplitude, and SD of velocity. Intraclass correlation coefficient (ICC), standard error of measurement (SEM), coefficient of variation (CV), and minimal metrically detectable change (MMDC) were calculated for each COP measure in all test conditions. Mean velocity, total phase plane, phase plane in ML direction, and SD of velocity in ML direction were the most reliable COP measures across all test conditions. ICC values were consistently higher in ML direction compared with AP direction. In general, velocity-related COP measures in ML direction showed to be highly reliable. Further research may explore the predictive and evaluative value of these COP parameters.

Scaling of plantarflexor muscle activity and postural time-to-contact in response to upper-body perturbations in young and older adults

In this study, we describe and compare the compensatory responses of healthy young and older adults to sequentially increasing upper-body perturbations. The scaling of plantarflexor muscular activity and minimum time-to-contact (TtC(MIN)) was examined, and we determined whether TtC(MIN) predictions of instability (stepping transitions) for the older subjects were similar to those we previously reported for younger subjects (Hasson et al. in J Biomech 41:2121-2129, 2008). We found that the older subjects stepped at a lower perturbation level than the younger subjects; however, this response was appropriate based on their greater center of mass (CoM) accelerations, which may have been caused by differences in pre-perturbation states between the age groups. Although the CoM acceleration increased linearly with perturbation magnitude, the amount of gastrocnemius and soleus muscular activity increased nonlinearly in both age groups. There were no differences in the maximum plantarflexor torque responses, suggesting that the maximum torque capabilities of the older subjects were not limiting factors. As previously demonstrated in the younger subjects, the older subjects showed a quadratic decrease in TtC(MIN) with increasing perturbation magnitude. The vertices of the quadratics gave accurate predictions of stepping transitions in both age groups, even though the older subjects stepped at lower perturbation magnitudes. By probing the postural system's behavior through sequentially increasing upper-body perturbations, we observed a complementary nonlinear scaling of muscle activity and TtC(MIN), which suggests that subjects could use TtC or a correlate as an informational variable to help determine whether a step is necessary.

Predicting dynamic postural instability using center of mass time-to-contact information

Our purpose was to determine whether spatiotemporal measures of center of mass motion relative to the base of support boundary could predict stepping strategies after upper-body postural perturbations in humans. We expected that inclusion of center of mass acceleration in such time-to-contact (TtC) calculations would give better predictions and more advanced warning of perturbation severity. TtC measures were compared with traditional postural variables, which do not consider support boundaries, and with an inverted pendulum model of dynamic stability developed by Hof et al. [2005. The condition for dynamic stability. Journal of Biomechanics 38, 1-8]. A pendulum was used to deliver sequentially increasing perturbations to 10 young adults, who were strapped to a wooden backboard that constrained motion to sagittal-plane rotation about the ankle joint. Subjects were instructed to resist the perturbations, stepping only if necessary to prevent a fall. Peak center of mass and center of pressure velocity and acceleration demonstrated linear increases with postural challenge. In contrast, boundary-relevant minimum TtC values decreased nonlinearly with postural challenge, enabling prediction of stepping responses using quadratic equations. When TtC calculations incorporated center of mass acceleration, the quadratic fits were better and gave more accurate predictions of the TtC values that would trigger stepping responses. In addition, TtC minima occurred earlier with acceleration inclusion, giving more advanced warning of perturbation severity. Our results were in agreement with TtC predictions based on Hof's model, and suggest that TtC may function as a control parameter, influencing the postural control system's decision to transition from a stationary base of support to a stepping strategy.