My knees are become feeble. Though David had the necessaries of life, yet he emaciated himself by voluntary abstinence, to which, as well as to prayer, he gave himself, and therefore we may regard this verse as expressive of his sorrow and sadness. We may also understand it as expressive of his having no relish for meat or drink, knowing, as we do, that persons who are in sorrow and sadness have no appetite for food; even life itself is burdensome to them. Should any one prefer restricting the interpretation to David's being in want of the necessaries of life, when he hid himself in the dens of wild beasts, to escape the fury of his enemies, and was then subjected to hunger and thirst, he may do so. It appears to me, however, that by this language he intends to point out the extreme anguish which he felt, because, with death staring him in the face, he loathed all food; and this is in accordance with the next clause, in which he says, my flesh faileth of fatness; because "a sorrowful spirit drieth up the bones," (Proverbs 17:22) By the term, fatness, some understand delicacies; meaning that he was deprived of all that food which is pleasing to the palate. The more natural way is to consider it as denoting his becoming emaciated by reason of grief and fasting, inasmuch as the natural moisture was wasted. Another proof of his sad situation arises from this, that, according to what he states in Psalm 22:7, he was held in scorn by all. It is, indeed, a sad and bitter thing which God's children endure, when they are made to feel that the curse which he denounces against the transgressors of his law is directed against themselves; for the law says to the despisers of it, "Thou shalt become an astonishment, a proverb, and laughing-stock," (Deuteronomy 28:37) With this species of temptation David was assailed; and he declares that he was not only regarded as a condemned person, but also cruelly derided; God at the same time coming in for a share of it; for it is usual with the ungodly to conduct themselves with insolence and pride towards us when they see us oppressed under afflictions, and, at the same time, to rail at our faith and piety, because God renders us no help in our miseries.

My knees are weak through fasting - Hunger; want of food. Strength to stand is connected with firmness in the knee-joints, and hence, weakness and feebleness are denoted by the giving way of the knees. Compare Hebrews 12:12.
And my flesh faileth of fatness - I am lean and weak. There is not the proper supply for my strength. The idea seems to have been that fatness (Hebrew, oil) was necessary to strength.

My knees are weak through fasting - That hunger is as soon felt in weakening the knees, as in producing an uneasy sensation in the stomach, is known by all who have ever felt it. Writers in all countries have referred to this effect of hunger. Thus Tryphioderus Il. Excid. ver 155:
Τειρομενου βαρυθειεν ατερπεΐ γουνατα λιμῳ.
"Their knees might fail, by hunger's force subdued;
And sink, unable to sustain their load."
Merrick.
So Plautus, Curcul, act. ii., scen. 3: -
Tenebrae oboriuntur, genua inedia succidunt.
"My eyes grow dim; my knees are weak with hunger."
And Lucretius, lib. 4: ver. 950: -
Brachia, palpebraeque cadunt, poplitesque procumbunt.
"The arms, the eyelids fall; the knees give way."
Both the knees and the sight are particularly affected by hunger.

My knees are weak through fasting; and my flesh (n) faileth of fatness.
(n) For hunger that came from sorrow, he was lean and his natural moisture failed him.

My knees are weak through fasting,..... Either voluntary or forced, through want of food or refreshment; this was verified in Christ, when he kneeled and prayed, and his sweat was as it were great drops of blood falling to the ground; see Psalm 69:10.
And my flesh faileth of fatness; or "for want of oil" (k); the radical moisture of his flesh being dried up like a potsherd, Psalm 22:15.
(k) , Sept. "propter oleum", V. L. "propter defectum olei", Eth. Arab.

Taunts and reproaches aggravate his afflicted and feeble state (Psalm 22:6-7).