MSB efect

The MSB efect on postural stability was not afected by the WoP. The linear regression characteristics with and without MSB were similar (Figure 16), as shown by the intersec-tions in the confdence intervals of the regression coefcients and constants (Table 7).

Posterior LoS were signifcantly (p = 0.042, η² = 0.035) improved by 0.1 cm (0.5 % foot length) using the MSB (Figure 17B), while anterior LoS were not afected. Sway ampli-tude A-P signifcantly (p = 0.036, η² = 0.038) increased by 0.1 cm (Figure 19C) using the MSB while sway area, path length, sway amplitude M-L and sway velocity were not afected. In the rest position the MSB led to a signifcant (p = 0.003, η² = 0.075) posterior displacement of the CoP (Figure 18). On average the shift was 1.1 ± 0.4 % foot length ( ˆ= 2.6 ± 0.9 mm). No interaction between group and MSB condition was observed for any of the parameters (p > 0.05).

Figure 16: Anterior LoS (A), posterior LoS (B) and CoP locations in the rest position (C) in ninety pregnant women with and without MSB at diferent weeks of pregnancy. A CoP of 0 % foot length is located at the toes, 100 % foot length equates to the calcaneus.

Figure 17: Anterior (A) and posterior (B) LoS with and without MSB at diferent stages of pregnancy (T1, T2 and T3) and in non-pregnant controls (^∗signifcantly diferent from the controls; ^#signifcant diference between the MSB conditions).

Figure 18: CoP locations in the rest position with and without MSB at diferent stages of pregnancy (T1, T2 and T3) and in non-pregnant controls (^#signifcant diference between MSB conditions). A CoP of 0 % foot length is located at the toes, 100 % foot length equates to the calcaneus.

Table 7: Lower and upper limit of the 95 % confdence intervals of the regression while slightly increasing postural sway in the A-P direction, indicating impaired balance.

Due to the conficting results we cannot clearly reject or accept our hypothesis that the

Figure 19: Sway area (A), path length (B) and sway amplitudes in anterior-posterior (C) and medio-lateral direction (D) with and without MSB at diferent stages of pregnancy (T1, T2 and T3) and in non-pregnant controls (^#signifcant diference between the MSB conditions).

Our results, regarding a positive efect of an MSB on postural stability, are in line with one recent study conducted by (Cakmak et al., 2014) assessing postural stability in pregnant women using a Biodex Balance System. The authors of that study found a noticeable improvement in the stability scores with 16 % in A-P direction and 33 % in M-L direction using the MSB. The diference to our comparatively small improvement of 2 % in LoS performance might at least partially be explained by the diferent experimental design.

Signifcant balance improvements may also result from sequence respectively learning ef-fects related to the repeated execution of the balance test (Robbins et al., 2017; van Dieen et al., 2015). In order to avoid sequence efects interfering with postural stability measures in the current study, trials with and without using the MSB were performed in random order. However, it is questionable if an improvement in LoS performance of 2 % is physiologically relevant and if the MSB is an appropriate and meaningful strategy to increase balance in pregnant women. In comparison, an activity program with seniors

held weekly for eight weeks, with each session lasting about 1 h, led to an improvement in the LoS of 9 % (Brouwer et al., 2003). Another exercise program with seniors, which focused on perturbations and was conducted twice weekly for 1.5 h over 14 weeks, even led to an improvement in LoS performance of 24 % (Hamed et al., 2018). While being far more time-consuming, in terms of balance improvement these exercise interventions appear to be far more efective compared to the MSB.

Apart from postural stability, we have shown that the MSB also afects body posture during quiescent standing in pregnant and non-pregnant women. The MSB led to a small shift of the CoP in the posterior direction, which is with 2.6 ± 0.9 mm half as large as the pregnancy-related shift of 6.6 ± 0.9 mm between pregnancy (T2 & T3) and non-pregnant controls. This MSB related shift in the CoP location may result from the reduced pelvic mobility (Damen et al., 2002; Mens et al., 2006). Furthermore, the MSB has been sugge-sted to infuence proprioception and to increase mental focus on the pelvis (Flack et al., 2015). Individuals may therefore become more aware of their body posture leading to a changed stance. This may also be associated to its efect in reducing pelvic girdle and back pain (Carr, 2003; Flack et al., 2015). The MSB related posterior shift of the CoP may furthermore explain the improved posterior LoS performance in our study, moving the CoM closer to the heels.

In comparison to non-pregnant women, postural stability during pregnancy has repeated-ly been described to be reduced especialrepeated-ly in the LP (Butler et al., 2006; Inanir et al., 2014; Jang et al., 2008). This has led to the assumption that weight gain as well as chan-ges in body shape and composition during pregnancy may be relevant factors infuencing balance (Inanir et al., 2014; Jang et al., 2008; Nagai et al., 2009). In the current study the LoS performance was found to be reduced already in the EP, before BM signifcantly increased. Thus, other physiological (McCrory et al., 2010b) or psychological factors such as an increased anxiety (Nagai et al., 2009) seem likely to have a more pronounced efect on postural stability than BM. Furthermore, postural stability performance during preg-nancy might be task specifc and depend on the degree of physical challenge. This may explain why most studies (Nagai et al., 2009; Oliveira et al., 2009; Opala-Berdzik et al., 2015) did not detect any diferences when conducting a moderately challenging balance

test such as motionless standing with open eyes. Interestingly, when maintaining balance was exacerbated by instructing the subjects to close their eyes, balance performance was signifcantly reduced in pregnant compared to non-pregnant women.

7.6 Conclusion

While the fexible and elastic MSB was found to afect postural stability in pregnant and non-pregnant women, it remains debatable if it is a meaningful tool to increase balan-ce sinbalan-ce the detected changes were marginal. Pregnant women demonstrated a slightly reduced postural stability in exacerbated circumstances compared to non-pregnant wo-men. The mechanism behind this not progressing impairment in balance, which already occurred early in pregnancy before BM signifcantly increased, needs to be elucidated. Re-garding the magnitude of the balance impairment, further studies of postural stability in pregnant women need to assess which degree of balance impairment necessitates postural stability enhancing interventions.