Does heart muscle get "tired" like say leg muscle? I'm a 52 year old male and have been bicycling for pleasure and exercise for many years. I have noticed a pattern that would like some comment on. I have noticed that if on a hour to two hour ride that if I push too hard for too long that short term I will recover but the next day or two my energy level is low, I'm tired and lack energy. My thought is that like a leg muscle, if the heart muscle is pushed too hard it will take several days to recover completely, and with my heart not at peak performance I feel tired. So last Saturday I went for a two hour bike ride and pushed quite hard. Sunday I'm a zombie and later that night when I go for a shorter ride I can tell my power is way off. I'm still a bit off today and don't feel quite as sharp as I do on a good day. It seems there is a fine line between a good workout and over doing it? Thanks for any ideas or suggestions.