# Impact of Differing Instability Devices on Postural
## 4 Discussion
The findings of this investigation support previous recommendations indicating that foam pads can be used interchangeably to induce increased postural instability, as no statistically significant differences were found between the two devices. In contrast, the current results differ from those of [PERSON] et al. [5], who found statistically significant differences between instability devices. They evaluated the differences between a BOSU ball, an Airex foam pad, a half-foam roller, and a Dyna Disc. Notably, differences were seen between the BOSUball and all other conditions. The BOSU ball elevated the CoM of the participants to a much greater height than the other three devices used. This could explain why [PERSON] et al. [5] found a significantly greater center of pressure area and sway velocity for the BOSU ball conditions. Based on these findings, [PERSON] et al. [4] recommended that clinicians and practitioners implementing balance training protocols be aware of patients' position in the rehabilitation process when selecting unstable devices. For example, since the BOSU ball produces a higher sway velocity, a patient should not train on a BOSU ball at the beginning of a rehabilitation program. However, when looking at the two foam conditions in [PERSON] et al. [5], the results support the findings of the current investigation. No differences were seen between the foam pad and half-foam roller conditions. This points to the need for further investigation into the materials used in the construction of the instability device and when a given device should be implemented in a training protocol. While no significant differences were found between the foam pad and block, the variability was greater in the block for both the path length and average sway velocity. As the average sway velocity was calculated as the path length over time, any increase in path length variability would also be present in the average sway velocity. [PERSON] et al. [9] showed that when using a similar foam pad to the one used in this investigation, the path length displayed fair to excellent reliability (ICC 0.41-0.81). As a rather large range of reliability has previously been reported, it is an important consideration in the interpretation of the results. Within the current study, the use of the randomized crossover study design limited the order effect that could have occurred. Future investigations should examine order effects when using instability devices as this could create potentially greater between-subject variability. It Is of interest to note that no statistically significant differences were found between the conditions for the peak AP displacement, peak AP sway velocity, and peak ML sway velocity, although moderate effect sizes were present. Across all three of these variables, the instability devices demonstrated greater values. This is similar to the variables that did reach statistical significance, where the control condition had lower postural sway. While the lack of differences in AP displacement could intuitively be explained by the unilateral stance, both instability conditions had greater displacement. This could potentially be explained by changes in the ML direction not being strictly in the frontal plane but across both sagittal and frontal planes. This also would explain the differences seen in the 95% sway area. Interestingly, previous investigations that have used unilateral balance training interventions found changes in the AP direction post training [17]. This would support the findings of the current investigation that, during a unilateral stance, there are still changes in the AP direction, although it has a greater base of support than the ML direction. Previous investigations have shown differences in patient perception and clinically relevant outcomes after short-term training programs using various instability devices [13; 17; 18; 22]. However, there is a gap in the evidence when examining training outcomes across different devices.