Tldr; I felt this book raised an interesting point that over exercising can be as bad as under exercising, however I feel it was overly alarmist given the limited amount of scientific studies available for evidence and presented a largely anecdotal based view. Worth a quick read with a pinch of salt. 3.5/5.  The Haywire Heart examines the idea that too much exercise can put undue stress on the heart and lead to some serious heart conditions later in life. It is an important read for anyone undertaking endurance training, especially those who have done it for 5-10 years or longer.  How Does My Heart Work? The book begins with an overview of how the heart works and the different systems that it uses to keep us alive. The authors do a nice job of explaining the workings of the heart and the different functions of its areas. It turns out that the heart is effectively two systems, plumbing and electricity as they put it.  The plumbing is the main part of the heart, keeping the blood enclosed and flowing to the right areas. The electricity part is what keeps the heart beating. A complex series of chemical interactions in