# Impact of Differing Instability Devices on Postural
## 4 Discussion  
This study aimed to compare the sway parameters between instability devices using block and foam conditions. Our results indicated no significant differences in the sway parameters between the foam and block conditions. However, there was a significant difference in the sway parameters between the control and both of the instability devices. This supports previous investigations where no differences were seen between foam pad conditions on measures of postural stability [7,9]. While the instability device challenges one's ability to maintain postural control, it also raises the CoM above that of the control condition. When one increases the height of the CoM, postural control will decrease. This could partially explain the differences between the experimental and control conditions within the current investigation. Previous research has explored the effects of altering the location of the CoM on postural sway. Using two instability devices, [PERSON] and [PERSON] [20] observed the postural sway differences in construction workers. There was a significant increase in postural sway as the heights of the instability devices increased. [PERSON] et al. [21] used the angle of hanging mass and the projected sway to observe the relationship between the sway and the height of the CoM. It was found that the height of the CoM was directly correlated to the projected sway. They found that as the height increased from 50 cm to 100 cm, the mean of the CoM displacement continually increased. A potential explanation for the lack of differences between the foam and block conditions may be that the height differences between the devices were not large enough (block 8.89 cm vs. pad 6.51 cm). The results of the current investigation showed no significant differences between the two devices; the height difference was not sufficient to induce an increase in postural sway for the block condition. Furthermore, the block was constructed with a top layer made of wood. Thus, differences in the direct feedback to the somatosensory system may have occurred, which offset any differences in the height of the two devices [9,10]. As for other differences between the two experimental devices, the block was narrower than the foam pad. However, no differences between the block and foam conditions regarding the mediolateral displacement or peak sway velocity were present. The finding of no differences with the narrower block in the ML direction is supported by [PERSON] et al. [6], where no differences were seen between the half-foam roller and the foam pad. This could potentially be due to the block, while narrower, still being greater than the width of the foot itself for both the current investigation and many of the instability devices used in previous investigations. Additionally, it is of interest when examining the foam pad in the amount of surface area that is in contact with the foot. Previous investigations have shown that softer foam conditions produce lower postural sway velocities than firmer and more dense foams [8,11]. Within the present study, the construction of the block could again potentially explain why no differences were seen. The wooden upper layer of the block again could provide a differing feedback response from the bottom of the foot compared to the foam pad, thus creating a similar response through differing mechanisms. While the density of the two experimental conditions in the present study was not explored, further investigation is warranted as to whether a firmer foam pad would elicit greater sway than a block. Our data show no significant differences in the sway parameters between the foam and block conditions. [PERSON] et al. [9] found similar results when observing the differences between two foams commonly used by both practitioners and researchers. They concluded that there were no significant differences between the instability devices, which could be used interchangeably in a clinical setting. [PERSON] et al. [7] also found similar results, as both foam pad conditions produced similar sway parameters.