During And After Long Periods Of Physical Exertion The Muscles

During and after long periods of physical exertion the muscles in the center of my chest start to ache, while at the same time I have a rapid heart beat. It feels like they are sore. Which could very well be the case. I don't believe that the pain is heart related, because I have been to my doctor about it before and he didn't believe it was. He gave me an EKG test about 3 or 4 times and every time the results were normal. I am 40 years old and generally in good health. I think the problem could probably just be common muscle strain. But I want to make sure it isn't anything more serious. Any ideas?