What arm should a lefty use for Tachanun? What arm should a lefty use when striking the heart during the Amidah and Vidui? Thank you.

Tachanun - During Shacharit when one wears Tefilin, one should use the arm without Tefilin. During Mincha Tachanun - a lefty should use the right hand. Rav Moshe Feinstein writes that a lefty should use both hands. Striking ones heart should be done with the hand that is most comfortable. Rabbi Yitzchak Grinblat