Rashi: I admit to the truth, that I sinned. Tuchos are the kidneys, which are smooth (Metzudas Tziyon - Tuchos (smeared) and covered with Chelev). Radak - we find "Mi Shas ba'Tuchos Chachmah" (Iyov 38:36). The heart understands, and the kidneys counsel.

Rashi citing Menachem: Tuchos is like "ki'Metachavei Keshes" (Bereishis 21:16). Just like a bow has length (extends), also Da'as.

Radak: Even though I sinned, the Yetzer (lust) overpowered me. My heart and kidneys did not think to rebel against You and Your Mitzvos. I knew that I do evil in that act to transgress Your Mitzvos. I regret; this shows that I did not rebel!

Malbim: Hashem planted powers in man via which he can be saved from Avon and Chet. Via the power to Tevunah, he reaches Emes in investigation - against Avon (crooked intellect). Via Chachmah, he can overcome lust. You wanted that we reach the Emes via investigation.

Rashi: In the heart, which is sealed, You gave to me Chachmah to confess.

Radak: Inform me of the true path, and make me understand covered matters.

Malbim: Emes via investigation is plastered over with very thick mud. One cannot attain it by himself, only if Hashem informs him.