Therefore the children of Israel eat not of the sinew which shrank, which is upon the hollow of the thigh, unto this day: because he touched the hollow of Jacob's thigh in the sinew that shrank.
Read Chapter 32
Ambrose of Milan
But Jacob limped because of his thigh. “On account of this the children of Israel do not eat the sinew even to the present day.” Would that they had eaten it and had believed! But because they were not about to do the will of God, therefore they did not eat. There are those too who take the passage in the following sense, that Jacob limped from one thigh. Two peoples flowed from his lineage, and there was then being revealed the numbness that one of them would presently exhibit toward the grace of faith. And so it is the people itself that limped by reason of the numbness of its unbelief. ...
The sinew in beasts of any kind, corresponding with that part of Jacob's thigh. (Haydock)
Some refrain from the whole quarter, others extract the sinew. This they do, without any command, in memory of this transaction. (Calmet)
Just as the truth is quite plain in this case, however, so with the good man a kind of apparition occurred that had the effect of convincing the good man of the degree of care he enjoyed from God and the fact that Jacob would be unvanquished by all those plotting against him. Then, in order that the vision he had would not be forgotten by anyone in the future, “his thigh made him limp. For this reason the sons of Israel do not eat the nerve that stiffened on the flat of his thigh, because he lay hold of the flat of Jacob’s thigh, and it stiffened.” You see, since the good man had completed his lifespan and was about to leave this life, and since the providence accorded him by God and the marvelous considerateness should be known to the whole human race, accordingly it says, “The sons of Israel do not eat the nerve that stiffened on the flat of his thigh.” Aware of their ingratitude and the way, they forgot God’s kindnesses; he therefore devised a constant reminder for them to have of t...