One way in which the narrator is confined is through being forbidden to do anything except "rest." The narrator is open about this in the exposition of the short story:
My brother is also a physician, and also of high standing, and he says the same thing [as the husband].
So I take phosphates or phospites -- whichever it is, and tonics, and journeys, and air, and exercise, and am absolutely forbidden to "work" until I am well again.
The confinement of the narrator to not "work" and live in isolation from the social order is an essential component of her confinement. the only thing she is really able to do is to describe the setting around her. The only thing she can do is "rest," a condition in which the narrator is forbidden to write or articulate her thoughts in any way. The rationale behind this was that the narrator is suffering from a case of "nerves" and needs to "rest" in order to get better. A diagnosis that indicates a rather limited view of women, a misdiagnosed nervous disorder is the rationale behind her confinement.
As the narrative progresses, so do the confinements imposed upon the narrator. As the narrator wishes to go to other parts of the house, the husband denies this: "Then he took me in his arms and called me a blessed little goose, and said he would go down to the cellar, if I wished, and have it whitewashed into the bargain." The belief here is that the more limited environment she experiences, the better she will be. This confinement is what drives her to intensely focus on the wallpaper for it is the only thing she can do. The increased emphasis on her limitations and rest is the irrational belief, the illogical rationale, that the diagnosis is valid and is working. The erroneous rationales amplify her confinement and accelerate the narrator's digression.
The bed being nailed down increases the confinement the narrator experiences. The confinement increases with her not being able to move or walk around, being told to remain. The rationales behind her confinement arise from invalid diagnosis, mistaken social concepts, and undervaluation of women.