St. Ambrose wrote several books about the Patriarchs of the Old Testament, showing off their examples and extolling their virtues.  In his book on Jacob, he discusses the episode with his estranged brother Esau, in which Jacob resolved to make a present to him of much of his belongings.  This represented his detachment from the goods of this world.  St. Ambrose speaks here of the night before Jacob set out to meet his brother, when he wrestled with God:

"Jacob, who had cleansed his heart from all falsehood and bore a peaceful disposition after he had rejected all his possessions, remained alone and wrestled with God.  For, whoever renounces worldly things approaches near to the image and likeness of God.  Now, 'wrestling' with God means to undertake a struggle, to be engaged in a contest with something greater than oneself, and to be made a more capable imitator of God than anyone else.  And because his faith and devotion were invincible, The Lord revealed hidden mysteries to him when he touched the sinew of his thigh, that it was of his lineage that The Lord Jesus would be born of a Virgin, and he would not be dissimilar to, nor unequal with, God.  His Cross was signified by the injured sinew to show that when the Cross would be spread throughout the world, it would grant the resurrection of the dead to all 'slow' or 'dull' of body, by the forgiveness of their sins.  It is also not unjustly said that 'the sun rose' (cf. Genesis 32, 31) upon holy Jacob, on whose race the Cross of The Lord shone the light of salvation.  And, at the same time, the Sun of Justice arises on him who knows The Lord, for The Lord himself is the eternal Light.

"Jacob limped because of his thigh: 'Therefore, to this day the children of Israel do not eat the sinew.'  Would that they ate, and believed!  But they did not eat because they would not do the will of God."

Note how an odd detail that does not apparently add anything to the account, becomes, for St. Ambrose, essential to its meaning: the wound given by The Lord to the sinew of Jacob's thigh is seen as a sign of the Holy Cross.  The reference to the "slow" and "dull" means that just as Jacob's body was slowed by his limp, so sin causes the body to die, but the application of the Cross of Christ makes all to live again.  In the final paragraph translated here, St. Ambrose exclaims, "Would that they ate, and believed!"  He means that if the descendants of Jacob had obeyed the will of God (represented by the sinew / Cross), they would have received Jesus as their Savior, when he came.