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Text word count: 2997Number of references: 30 "We're a machine built to run -and the machine never wears out…" a Abstract Background: Global physical inactivity pandemic is responsible for more than 5 million deaths annually through its effects on non-communicable diseases. This requires urgent intervention. Objective: To investigate associations of physical activity with cardiovascular fitness in a cross-sectional retrospective observational study. Data were collected for 21 years from 2530 healthy volunteers and athletes representing the entire spectrum of physical activity from the totally inactive sedentary persons to the highly trained national athletes. Methods: Simple echocardiographic parameters of cardiovascular fitness were analyzed. Cardiac fitness was characterized by reduced resting heart rate, increased relative left ventricular muscular mass, improved left ventricular diastolic function and peak exercise oxygen consumption. Results: We found that even moderate exercise is associated with improved cardiac fitness. The largest improvement of fitness was observed between the inactive and the least active group, whereas fitness decreased in the highly trained national athletes enduring up to 20 training hours per week.