my granny used to say that while eating our heart rate increases a bit, i don't know whether its correct or not and by keeping elbow on table makes the heart to beat faster,since hand is almost equal in height with heart and if this happens daily we will be in risk of blocking the arteries with in 20-25 years, so if anybody says this to children they wont listen. so everyone says that its bad manners to keep elbow on table,especially left elbow because heart is located on left side.. . i don't know whether its correct or not.