I am gone like the shadow when it declines: I am shaken off like the locust.
Read Chapter 109
Augustine of Hippo
23. "My knees are weak through fasting" (ver. 23). We read, that our Lord Christ underwent a fast of forty days: but had fasting so great power over Him, that His knees were weakened? Or is this more suitably understood of His members, that is, of His saints? "And my flesh is changed because of the oil;" because of spiritual grace, Whence Christ was so called from the Greek word, chrisma, which signifies unction. But the flesh was changed through the oil, not for the worse, but for the better, that is, rising from the dishonour of death to the glory of immortality. ...His flesh was not yet changed. But whether the Holy Spirit be represented by water through the notion of ablution or irrigation, or by oil through that of exultation and the inflaming of charity; It doth not differ from Itself, because Its types are different. For there is a great difference between the lion and the lamb, and yet Christ is represented by both. ...
For oil, propter oleum. The meaning is, my flesh is changed, being perfectly emaciated, and dried up, as having lost all its oil, or fatness. (Challoner)
Or, "on account of the privation of perfumes "(Berthier) which the Jews looked upon as necessary. David and Jesus Christ were exposed to many inconveniences.