One of the best things we can do for our health is regular exercise. However there is a point where exercise can become detrimental. When tissue injury exceeds repair. For example, high intensity running may harm your heart. While a modest amount of running has shown to be beneficial, there is a point when it becomes damaging. Check out the latest on running and your heart:

www.womenshealth.gov/news/headlines/665307.cfm

The best suggestion, moderate exercise. And mix it up between cardio, strength training and flexibility.