I did a quick search on Google and found the following information in an address given by Father Cassian Folsom. Fr. Folsom is a Benedictine monk who is vice-rector of the Pontifical University of Sant’ Anselmo and pro-president of the Pontifical Liturgical Institute in Rome. The document is in the EWTN Library and can be viewed at:
http://www.ewtn.com/library/Liturgy/SIGNS.HTM
“Next we come to the Confiteor although unfortunately this option of the Penitential rite is not always used. During the words
mea culpa, mea culpa, mea maxima culpa, there is the gesture of striking the breast. This is a sign of repentance, of humility, like the parable of the Pharisee and publican in the Gospel: But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying: “0 God, be merciful to me a sinner!” (Lk 18:13)
“In the Missal of Pius V, the rubric for this gesture was very specific: "
He strikes his breast three times, saying: mea culpa, mea culpa, mea maxima culpa." The rubric in the Missal of Paul VI is less precise. It simply says:
“striking themselves on the breast they say mea culpa, mea culpa, mea maxima culpa.” To make matters more confusing, in the English translation, the words are not repeated three times, but only once:
"I confess to Almighty God, and to you my brothers and sisters, that I have sinned through my own fault, in my thoughts and in my words, in what I have done and in what I have failed to do…"
“The gesture is still there, although both words and gesture have been much reduced. The words express our repentance verbally. Striking the breast expresses our repentance physically, in body language.
“[Theologian Romano] Guardini has something to say about this gesture too. He asks the question: "
What is the significance of this striking the breast? All its meaning lies in its being rightly done. To brush one’s clothes with the tip of one’s fingers is not to strike the breast. We should beat upon our breasts with our closed fists. In the old picture of Saint Jerome in the desert he is kneeling on the ground and striking his breast with a stone. It is an honest blow, not an elegant gesture. To strike the breast is to beat against the gates of our inner world in order to shatter them. This is its significance."