Rashi: When one's feet are together, he can fall easily. (He widened my feet, so I will not fall.) And so it says "b'Lechtecha Lo Yetzar Tza'adecha" (Mishlei 4:12).

Malbim (Tehilim 18:37) #1: In the first method, he stays stationary. In place of this, in this method he chases the enemy. (Even in this method, Hashem widens his steps; he does not do so himself.)

Malbim (Tehilim 18:37) #2: It is as if You widen my steps under me, i.e. in my place. It is as if Hashem's wind takes him miraculously fast (Kefitzas ha'Derech). My feet did not slip, for I stand in my place.

Radak: This is like Tachtai (under me, and so it says in Tehilim 18:37). Normally, Nun and Yud do not come together (in a suffix).

Rashi (Tehilim 18:37): They did not slip.

Rashi: My ankles.

Radak: My legs. The Targum of "Asher Lo Kera'im" (Vayikra 11:21) is 'd'Lei Karsulin.'

Radak citing Targum Yonasan: Rechuvosai (my knees).