# Effects of Wearing a Swimming Cap and Goggles on Phoria and Fusional Vergence
## 4 Discussion
### Relationship between Swimming Equipment and Visual Changes  
When we evaluated the relationships between changes in phoria and fusional vergence when wearing a cap and goggles, a correlation of change in phoria and PFV was found at distance but not at near, and the correlation of change in NFV was found at near but not at distance. Therefore, in terms of the correlation of cap and goggles with phoria, it is likely that wearing this swimming equipment affects PFV at distance and NFV at near, regardless of phoria at near. For fusion of both the eyes when wearing a cap and goggles, interaction between phoria and PFV at distance seems to be related to voluntary convergence (moving the eyes inwards to see closer objects clearly), and the interaction between phoria and NFV at near seems to be related to voluntary divergence (moving the eyes outward to see objects that are further away) [47].  
To evaluate the relative contribution of the cap and the goggles when wearing both a cap and goggles, the explanatory power was defined by the coefficient of determination and evaluated by linear regression analysis. The explanatory power for wearing both a cap and goggles was greater than for wearing goggles or for wearing a cap, except for the NFV recovery point at distance and PFV recovery point at near. These results suggest that the effects of the periocular pressure exerted by the goggles were greater than that exerted by pulling of the eyelids by the cap. As previously discussed, it appears that the pressure exerted in the orbital tissue by the rubber seal on the goggle frames [38] is greater than that exerted by pulling of the eyelids by the cap [34].  
The effect of the load on phoria was low or insignificant but did have an effect when wearing goggles. In particular, the effect of wearing goggles alone was greater than that when wearing both a cap and goggles at near. This finding can be explained by the fact that the pulling effect of the cap and the pressure effect of the goggles are slightly offset when wearing both a cap and goggles, whereas wearing only goggles prevents the common natural phenomenon of narrowing of the palpebal fissures on downgaze at near fixation [48,49].  
In this study, changes in phoria and fusional vergence when wearing swimming equipment were affected by the wearing position, the pressure exerted, and the fixation distance, and the influence of goggles was considered to be substantial. The discomfort experienced by our study participants is consistent with that in another study, in which subjects reported subjective symptoms of poor visual acuity, asthenopia, and gait disturbance when wearing swimming goggles [50]. It is noteworthy that our present findings are based on data obtained over a short period of time. Actual swimming sessions last much longer, and the visual discomfort caused by swimming caps and googgles is likely to be worse than that detected in this study.  
This study has some limitations. We did not evaluate changes in the effect of full correction rather than the spherical equivalent power on phoria and fusional vergence while swimming. Furthermore, we did not investigate whether these changes returned to baseline levels after removal of the swimming equipment. However, these limitations are unlikely to have influenced our results because the rotation of the astigmatism axis with full correction and the dynamic movements involved in swimming are likely to have acted as factors of change. In addition, the vergence demands on the wearer's fusion system may create more discomfort to the swimmers than simply the wearing of the swimming equipment.