Rashi: It is a bed, like "v'Hinei Arso Eres Barzel" (Devarim 3:11). Hashem sustains him when he is sick (and bedridden).

Rashi citing Shocher Tov: It is the seventh ¹ day, when a Choleh is very ill.

Radak: It is when he lies bedridden due to harsh Madvim (illnesses).

Malbim: It is one who is afflicted due to evils caused by his body's mix; Hashem sustains him when no one else can.

Rashi: Even when his illness is severe, that all his rest was upset [Hashem sustains him].

Radak #1: Hashem gives to him strength to turn from side to side.

Radak #2: You, Hashem, who switched his bed in his illness, You sustain him. When he is healthy, his bed is rest; You switched it to affliction. It says "Kol Mishkavo", for he lies day and night.

Radak #3: This is a Tefilah - may You reverse his bed to health and serenity!

Malbim #1: Sometimes one must switch a Choleh's bed when he is wet in his illness. Even if he has no one to do so for him, You do so. I.e. You supervise that [the moisture] will not harm him.

Malbim #2: You reverse his bed via his illness. The illness itself does so; You supervise over him to cure him.