From the early Church Fathers to now.
“My knees are weakened through fasting: and my flesh is changed for oil.”
“"I became also a reproach unto them" [Psalm 109:24]: through the death of the Cross. "For Christ has redeemed us from the curse of the law, being made a curse for us." [Galatians 3:13] "They looked upon Me, and shaked their heads." Because they beheld His crucifixion, without beholding His resurrection: they saw when His knees were weakened, they saw not when His flesh was changed.”
“And thus also thou canst shew that all the righteous men and Prophets, either by their own will, or by God's command unto them, endured always tribulations and labours, even as the blessed David maketh known that from the severity of his fasting the very limbs of his body had become enfeebled, saying, "My knees have become sick through fasting, and my flesh faileth of fatness."”
The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.