What is "Umlal"?
Rashi: It is destroyed and weak, like "ha'Yehudim ha'Amemalim" (Nechemyah 1 3:34). Malbim - therefore, David requests Chen (a free gift).
Radak: It is To'ar (a kind of person) even though there is a Patach (under the Lamed), and similarly "Lev Husal Hitahu" (Yeshayah 44:20). It refers to being detached and cut off - "v'Rabas Banim Umlalah" (Shmuel I, 2:5), "ha'Yehudim ha'Amemalim" (Nechemyah 3:34), "Umlalah Nefesh ha'Shiv'ah" (Yirmeyah 15:9) and similar verses.
Rashi calls this Ezra. Ezra and Nechemyah are really one Sefer. They were separated into two (as were the Seforim Shmuel, Melachim and Divrei ha'Yamim) via Nochrim, and it became accepted also within Yisrael (Ohr Yisrael, Monsey 5758:12, p.144-152).
Why did he say "Nivhalu Atazamai"?
Radak #1: The bones are the foundation of the body.
Radak #2: This refers to the pain of the limbs.
Malbim: The illness reached the bones.
