It does not appear like you have an answer as of yet, so I will take a non-qualified stab at providing my interpretation of your question.
You only feel if the nerve ending is stimulated.
I am guessing that there is no nerve ending in the vacinity of the heart movement, during normal load, that can pick up this movement, hence, you normally do not feel the heart beating.
On the other hand, if you run the Boston marathon in olympic time, and sprint the final 800 m in 96 seconds, there will be a good chance you will feel your heart beat since it now has to work much harder, hence, the effects can be felt further from the heart, and more nerve endings will have a chance of picking up the movements.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?