That also explains why the heart becomes "more" vulnerable when it is physically strained by sports activities:# The increase in heart rate (exercise tachycardia) may double the probability above (e.g., with 120 beats per minute the cardiac cycle shortens to 500 milliseconds without fundamentally altering the window-of-vulnerability's size);# Relative exercise-induced hypoxia and acceleration of the excito-conductive system of the heart make it more susceptible to stretch-induced ventricular fibrillation.