when the left ventricle is repolarizing, moving from systole to diastole (relaxation). The small window of vulnerability in the cardiac electrical cycle explains why it is a rare event. Considering that the total cardiac cycle has a duration of one second (for a base heart rate of 60 beats per minute), the probability of impact trauma within the window of vulnerability is 13 percent only. The cellular mechanisms of commotio cordis are not fully understood. However, it is widely recognized that it may be related to the mechanical impact, or stretch, on the myocardial tissue cell membranes. This impact is believed to activate the stretch-activated, pressuresensitive proteins called ion channels. Changes in ion channels lead to altered repolarization in the heart's electrical activity, which in some cases, if occurring right at the trailing edge of a previous electrical cycle, can induce ventricular fibrillation, also known as stretch-induced VF. Since the trailing edge of the preceding electrical cycle travels over the ventricular surface, the critical window for mechanical induction of ventricular fibrillation varies locally across the ventricle. Risk reduction For some sports participation in which hard balls are used, such as baseball or lacrosse, softer, more pliable balls may reduce the impact trauma causing commotio cordis. The shape of the impact object may be modified for some conditions, such as by using a flat object which did not induce ventricular fibrillation upon impact in preliminary research whereas spherical objects with smaller radii were more likely to induce ventricular fibrillation. Safety baseballs having lower degrees of hardness, e.g., softer, pliable and elastic balls used for Tee-ball or more pliable baseballs for older age groups, may reduce the risk of commortio cordis. The risk of impact may be reduced by improved coaching techniques, such as teaching young batters to turn away from the ball to avoid errant pitches in baseball. Defensive players in lacrosse and hockey may be coached to avoid using their chest to block the ball or puck. Starting in 2017, high school lacrosse players are penalized, and play is stopped immediately if they enter their own goal crease with the apparent intent of blocking shots or acting as goalkeeper. Chest protectors and vests are designed to reduce trauma from blunt bodily injury, but many commercially available chest protectors do not offer protection from commotio cordis and may offer a false sense of security. A 2010 study found that almost 20