During self-motion, the spatial and temporal properties of the optic flow input directly influence the body sway. Men and women have anatomical and biomechanical differences that influence the postural control during visual stimulation. Given that recent findings suggest a peculiar role of each leg in the postural control of the two genders, we investigated whether the body sway during optic flow perturbances is lateralized and whether anteroposterior and mediolateral components of specific center of pressure (COP) parameters of the right and left legs differ, reexamining a previous experiment (Raffi et al. (2014)) performed with two, side-by-side, force plates. Experiments were performed on 24 right-handed and right-footed young subjects. We analyzed five measures related to the COP of each foot and global data: anteroposterior and mediolateral range of oscillation, anteroposterior and mediolateral COP velocity, and sway area. Results showed that men consistently had larger COP parameters than women. The values of the COP parameters were correlated between the two feet only in the mediolateral axis of women. These findings suggest that optic flow stimulation causes asymmetry in postural balance and different lateralization of postural controls in men and women.
The human upright stance is characterized by continuous movements of the body similar to an inverted pendulum [
Until now, several studies have focused on the maintenance of balance control, looking at the variation of the center of pressure (COP) trajectory. The COP analysis with bilateral force plate can be useful for assessing postural behavior related to each foot in healthy individuals [
In a previous paper, we showed that foveal, peripheral, and full field optic flow stimulations evoke different muscular activations in the right and left leg and different directions of oscillation in men and women [
For this study, we reexamined the data of the experiments performed on 24 right-handed and right-footed subjects [
The experimental paradigm and visual stimuli are identical to those described in a previous publication [
Optic flow stimuli consisted of white dots (1.3 cd/m2) of 0.4° size, which moved at a speed of 5°/s. The stimuli were expanding and contracting flows originating from a central FOE. The fixation point consisted of a white dot of 0.6° always positioned in the middle of the screen. The focus of expansion was always in the center of the screen. Expansion and contraction optic flows were presented as full field (Exp and Contr, resp., Figures
Optic flow stimuli. Arrows represent the velocity vectors of moving dots. (a) Full field expansion. (b) Full field contraction. (c) Foveal expansion. (d) Foveal contraction. For the foveal stimuli, the stimulated area had a radius of 7°. (e) Peripheral expansion. (f) Peripheral contraction. For the peripheral stimuli, the blank area in the center had a radius of 20°. (g) Random motion stimulus. (h) Baseline (fixation in the dark).
The stabilometric data were acquired using two Kistler force platforms (number 9286BA). Subjects were instructed to place a foot on each platform before the beginning of each trial. The platforms were marked to normalize posture and to control the subject’s distance from the screen. Subjects had to look at a white fixation point (0.6°), which was also the FOE of the optic flow stimuli, always positioned in the center of the screen and adjusted to the height of each subject. The stimulus was present during the entire trial duration and trial onset was determined by the stimulus onset.
We acquired 5 trials for each stimulus condition and 4 trials at baseline (i.e., fixation in the dark without visual stimulation, Figure
In this study, we computed five measures referring to the COP of each foot and COPglobal:
We first computed the percentage of loading in the right and left foot using Smart-Analyzer software (BTS Bioengineering Inc.) and MATLAB (The MathWorks Inc.). The values of the percentage of loading were then analyzed with a multivariate ANOVA (within-subject factor: stimuli; between-subject factors: side and gender).
Then, we analyzed the COP parameters APO, MLO, VelAP, VelML, and Area using Sway and Smart-Analyzer software (BTS Bioengineering Inc.) and MATLAB (The MathWorks Inc.). The analysis was performed separately for measurements of each limb and global. To analyze the influence of optic flow stimuli on postural control, we performed a repeated-measure ANOVA in which optic flow stimuli and side (right, left, and global) were the within-subject factors, while gender was the between-subjects factor.
After having assessed the effects of stimuli, side, and gender, we then analyzed in depth the relationship between the left and right feet in response to visual stimuli using a bivariate Pearson linear correlation analysis.
Lastly, we looked at the degree of variation of the right and left foot in the five COP parameters using the coefficient of variation (CV) computed as the ratio of the standard deviation to the mean. The CV was computed for each trial of each stimulus in each subject. Then, values for all subjects in each condition and group were averaged.
To quantify the asymmetry, we first computed the limb loading. Mean values of the percentage of loading are shown in Figure
Average values of left and right percentage of loading in the right and left foot of men and women. Data are shown for all stimuli and baseline. Each data point shows mean ± standard error (SE). ContrF: foveal contraction, Contr: full field contraction, ContrP: peripheral contraction, ExpF: foveal expansion, Exp: full field expansion, and ExpP: peripheral expansion.
All COP parameters showed significant main effects of stimuli, side, and gender as summarized in Table
Full statistical information for the repeated-measure ANOVA in which optic flow stimuli and side (right, left, and global) were the within-subject factors, while gender was the between-subjects factor.
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Significant values are in bold and marked with an asterisk.
The results of the between-subjects analysis (ANOVA, see Methods) showed that, among the COP parameters, MLO showed more differences between men and women. The gender effect was examined in each stimulus of the right and left leg allowing the analysis in 16 conditions. A significant effect was found in almost all stimuli (14/16). The two nonsignificant effects were found in expansion
Average values of COP parameters in the left and right limb and global data. Values are shown for men and women during optic flow stimuli and baseline. (a) Anteroposterior range of oscillation (APO). (b) Mediolateral range of oscillation (MLO). (c) Anteroposterior velocity (VelAP). (d) Mediolateral velocity (VelML). (e) Sway area (Area). Each data point shows mean ± standard error (SE). Conventions are as in Figure
A bivariate Pearson correlation was used to test whether the relationship between the right and left foot in each COP parameter was linear. The analysis was performed separately for men and women on left versus right foot for all stimuli and baseline values of each COP parameter. In women (Figure
Correlation coefficients for the correlation analysis between the right and left foot. (a) Women. (b) Men. Asterisks indicate significant values (bivariate Pearson correlation,
To examine the variability of postural adjustments during optic flow stimulation, we computed the CV for the five COP parameters in the right and left foot. MLO consistently showed greater variability than APO. Baseline stimuli always had the highest CV, indicating that the absence of visual stimulation caused a greater instability. In women, different variability was observed in the left and right foot: MLOleft always showed higher CV than MLOright, while, in almost all stimuli, APOright showed higher CV than APOleft (Figure
Coefficients of variations of COP parameters across the right and left feet in men and women. (a) Women anteroposterior range of oscillation (APO) and mediolateral range of oscillation (MLO). (b) Male APO and MLO. (c) Women anteroposterior velocity (VelAP) and mediolateral velocity (VelML). (d) Men VelAP and VelML. (e) Female sway Area. (f) Male sway Area.
As these observations on the CV were largely descriptive, the CV values were further analyzed to quantify the variability related to gender and foot. A one-way ANOVA, with side as between-subject factor and stimuli as within-subject factor, was performed separately for men and women. Significant differences between the left and right feet were found only in women in VelML for all visual stimuli (foveal contraction:
The optic flow is a key input for maintaining postural stability during self-motion [
An important issue in studying postural asymmetry is limb loading. Some evidence seems to support the idea that healthy subjects unequally distribute their weight across the two feet in conditions of eyes open and closed [
Footedness entails postural asymmetry [
The present results suggest that optic flow stimuli produced different COP oscillations, velocities, and area dimensions. These results point out important characteristics of the feet asymmetry; the fact that the two feet exhibit different values in distinct parameters may indicate that each foot has its own role in balance control. As suggested by Anker and coworkers [
Gender differences in brain asymmetry are well documented and may explain the different postural strategies exhibited by men and women. The brain of adult women is, from the functional point of view, less asymmetrical than that of men [
This study provides new evidence on the postural strategy used by men and women in the control of stance under visual optic flow stimulation. The feet asymmetry observed during optic flow stimulation causes specific inter-leg coordination dynamics necessary to maintain the control of posture. This might suggest that the postural control system uses various mechanisms within each leg to produce the most appropriate postural response to interact with the extrapersonal environment.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Authors are grateful to Dr. Andrea Giovanardi for technical assistance in the experiments. This work was supported by University of Bologna and Italian Ministry for University and Scientific Research (MIUR).