Albert Barnes Commentary Psalms 109:24

"My knees are weak through fasting; And my flesh faileth of fatness." — Psalms 109:24 (ASV)

My knees are weak through fasting - Hunger; lack 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 to 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.