What is "Min ha'Metzar"? `
Radak #1: If the Mizmor discusses David, he thanks Hashem for taking him from straits 1 to expanse. When he was fleeing, he was constricted; he hid in caves and fortresses.
Radak #2: If the Mizmor discusses Yisrael (in the days of Mashi'ach), the Metzar is Galus.
Malbim: [Chizkiyah] was ill, and about to die. Hashem answered him amidst expanse - the danger already passed, and this was a sign for him that He will save him also from Melech Ashur - "umi'Kaf Melech Ashur Atzilecha" (Melachim II, 20:6).
Radak: Metzar is a noun. The root is Tzar, and sometimes the Reish is doubled. It is like Metzarim, u'Metzarei She'ol (116:3), "Ki Tzorerim Hem Lachem" (Bamidbar 25:18).
