The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body Source slideshow at OnHealth The heart is the hardest working muscle in the body Heart Disease: Symptoms, Signs and Causes The heart is the hardest working muscle in the body The heart is the hardest working muscle in the body