Particular adaptations to potentially slippery surfaces: The effects of friction on consecutive postural adjustments (CPA)

The role(s) of "Simultaneous Postural Adjustments" (SPA) during Single Step revealed with the Lissajous method

Dynamical phenomena in the postural chain occur before, during and after the voluntary movement. These phenomena correspond to anticipatory (APA), simultaneous (SPA), and consecutive (CPA) postural adjustments, respectively. APA and, more recently, CPA, have been extensively investigated in the literature. SPA have surprisingly received much less attention. The aim of the present study was to examine the role(s) of SPA associated with a single step task (SST). Ten healthy young adults performed series of SST on a force-plate. A 2-DOF mechanical model was used to separate the dynamics of the swing leg and the dynamics of the rest of the body, corresponding to the focal and the postural component of the SST, respectively. The postural component was plotted against the focal one during SPA (from heel-off to foot-contact), and this plot was modelled as a Lissajous ellipse. Result showed that this ellipse systematically ran through the same three quadrants of the diagram. For each of these quadrants, the role of the postural component in regards to the focal one was interpreted according to the relative orientation of the postural and focal dynamics. Results thus showed that SPA ensured the following successive roles: counter-perturbation of swing leg dynamics following heel-off, propulsion of swing leg, counter-perturbation of swing leg dynamics again, and then braking swing leg movement. These new findings contribute to a better knowledge of postural adjustments properties, and may provide new insights for understanding balance troubles with aging and in neurological patients (e.g. people with Parkinson's disease).

Standing balance on inclined surfaces with different friction

Working and walking environments often involve standing positions on different surfaces with inclination and different friction. In this study, standing balance of thirteen participants during sudden and irregular external perturbation to calf muscles was investigated. The aim of the study was to evaluate the combined effect of surface inclination and friction on standing balance. The main findings when eyes closed revealed that the standing utilised coefficient of friction (μ_SUCOF) increased when the surface was inclined for both high and low friction materials. The anterior-posterior torque increased more anteriorly when the surface was inclined toes down and when the surface friction was low. The results indicate that the anterior-posterior torque is a sensitive parameter when evaluating standing balance ability and slip risk. On inclined surface, particularly on the surface with lower friction, the potential slip and fall risk is higher due to the increase of standing utilised coefficient of friction and increased forward turning torque.

Role of point of application of perturbation in control of vertical posture

The role of point of application of perturbation in the anticipatory (APAs) and compensatory (CPAs) postural control was studied. Twelve healthy participants stood on a sliding board (that was either locked and as such motionless or unlocked and as such free to move in the anterior-posterior direction). The body perturbations were applied either to the shoulders (by a pendulum impact) or the feet (by the movement of the sliding board). Electromyographic activity (EMG) of the trunk and lower extremity muscles was recorded. Latencies, integrals of EMG and muscle co-contraction (C) and reciprocal (R) activation indices were calculated and analyzed within the intervals typical for the APAs and CPAs. Higher EMG integrals were seen in the APAs phase when perturbation was applied to the shoulders. Reciprocal activation of muscles was seen in the APAs phase in the shoulders perturbation condition, while co-contraction was seen in the feet perturbation condition. Co-contraction was observed within the CPA phase in both experimental conditions. Higher C values were found in the feet perturbation condition in the CPA phase. The results suggest that different motor control strategies are employed by the central nervous system when encounter perturbations of similar magnitude but applied to different parts of the body. The outcome highlights the importance of investigation of the role of the point of application of the perturbation.

Comparison of base of support size during gait initiation using force-plate and motion-capture system: A Bland and Altman analysis

This study aimed to estimate the error made by investigators when force-plate data are used to approximate base of support size during gait initiation. Step length and step width obtained with a method based on motion capture system (Kinematics method, considered the "gold standard") and with a method based on the centre of pressure traces obtained from a force-plate (Force-plate method) were purposely compared using descriptive statistics and the Bland and Altman (BA) method. Participants (N=19) performed series of gait initiation in Spontaneous and Maximal Velocity Conditions (SVC and MVC, respectively). BA analysis showed that 1) step length and width biases, corresponding to the difference between the two methods, were very small (<2.1%) in both velocity conditions and 2) the 95% limits of agreement of the BA plots ranged between 10% and 15% in absolute value. Repeated measures ANOVA showed that step length was significantly larger in MVC than in SVC, with no velocity X method interaction. There was no significant effect of the method on both step parameters. The present results suggest that the Force-plate method is sufficiently accurate to compare step parameters across conditions. However, researchers should be aware that non-negligible errors might occur when considering individual data.