Rashi: In Arame'ic this is like Sadna b'Sadnei Yasiv (Pesachim 28a; the carpenter who made the ring in which a prisoner's foot is put, his foot will be put in it).

Ibn Ezra: This is [plaster,] like "v'Sadta Osam ba'Sid", so people will see all my ways.

Malbim: In addition to locking my foot so I cannot move, You guard all paths on which I go. Even though I cannot go with my power, for my foot is locked, i.e. my actions are forced, You punish me as if I had choice.

Ibn Ezra: My thighs.

Rashi: You fix [my steps and actions].

Ibn Ezra: You fix a border.

Malbim: [If You would] investigate [the source of my foot's motion].