Impact of Support Surface Characteristics on Postural Sway During Standing

Abstract

Background and Aim: Various types of support surfaces are used to evaluate balance control [1] . Foam pads are effective for challenging somatosensory input during standing and different foam pad properties may influence balance performances [2] . However, foam pads are less suitable for dynamic tasks like gait. In such cases, insoles could serve as an alternative. Until now, most research focused on the balance-enhancing effects of insoles, rather than on their capacity to challenge the somatosensory system [3] . Therefore, this study aimed to measure the effects of foam pads and insoles with varying densities on postural sway in bipedal and unipedal stances.

Methods: Five different support surfaces were evaluated: a) a blue foam pad (48x40x6 cm/Airex®, Switzerland), b) a gray foam pad (40x33x5 cm/Jianzhongxidianzishangwu Co., Ltd., China), c) an insole made from the blue foam (2cm), d) an insole made from the gray foam (2cm), and e) a firm surface. The characteristics of the support surfaces were evaluated using the Indentation Force Deflection (IFD) Test and density calculations. The densities of blue and gray foam were 66.0 kg/m³ and 41.2 kg/m³, respectively. IFD testing showed Young’s modulus was 76.78 N/m² for blue foam and 244.45 N/m² for gray foam, indicating the blue foam was softer. Then, healthy young adults (n= 20, 26.7± 3.3 years, 70% female) without any comorbidities were recruited and were asked to stand on the support surfaces in a random order. Center of pressure (CoP) data were collected using K-force plates® (Kinvent®, Montpellier, France) during 30-second trials of bipedal and unipedal stances (dominant leg). The CoP area was calculated using the 95% confidence interval ellipse area.

Results: During bipedal stance, significant differences in CoP area were found between the different support surfaces (p<0.001). More specifically the CoP area was significantly larger while standing on blue foam (Z=2.817, p=0.005), gray foam (Z=2.199, p=0.028), and gray insole (Z=2.455, p=0.014) compared to on firm surface. Furthermore, there was no significant difference between the blue foam and gray insole (Z=-1.067, p=0.286). However, the CoP area significantly differed in blue insoles in comparison to blue foam pads (Z=-3.157, p=0.002) (Figure 1). No significant difference was found in the unipedal stance trials (p=0.116), and posthoc analysis did not reveal any differences between the unipedal conditions (p>0.05 for all). Conclusions: While no significant differences were observed in the unipedal stance, comparisons of different support surfaces during the bipedal stance showed that both foam pads and gray insoles produced a significant perturbation effect on postural sway. These findings suggest that blue foam pads, commonly used to evaluate balance control during standing, could potentially be replaced by gray insoles to assess balance control during more dynamic tasks such as gait, given their similar effect in unipedal stance as well.