Why did he say "Horeini Hashem Derech Chukecha"?
Malbim: Even though I run in Derech ha'Mitzvos (32), and I am expert in it, I do not know Derech ha'Chukim. Just like Chukim themselves are hidden - "Lamedeni Chukecha" (12), also their way is not known. Also refer to 119:33:2:5.
What is the meaning of "v'Etzrenah Ekev"?
Rashi: I will guard it in all its circuits and minor paths.
Radak citing Ibn Ezra: I will guard it because it has Ekev, i.e. reward 1 , like "b'Shamram Ekev Rav" (19:12).
Radak citing his father: Ekev is reward; the reward that You will inform me the ways of Your Chukim is that I will guard it.
Radak: Ekev is the end; the same applies to "l'Olam Ekev" (112) and "v'Hu Yagud Akev" (Bereishis 49:19). Because the Ekev (heel) is at the end of the body, the end of a matter is called Ekev. Also reward is called Ekev - "v'Hayah Ekev Tishme'un", for it is the end of deed. I will guard until the end of my days, i.e. as long as I live.
Malbim: I cannot step with my heel on a path that is hidden from me. Therefore he requested "inform me the way of Your Chukim", so I can guard to place my heel on this path.
Radak: This is wrong. Chazal said, do not be like servants who serve in order to receive reward! (Avos 1:3)!
