Think of a bike accident as a chain of events transferring kinetic energy that starts with a cyclist on a bike and ends with the possible microscopic jostling of brain cells. Wearing a bike helmet is important in minimizing the damage. Here's how:

The bike stops suddenly because it hits a solid object, such as a car, or more gradually in a sliding fall. The first milliseconds of the crash are important because some of the energy is dissipated when the frame or wheel crumples, or by the friction of the slide.

The rider is still in motion, even though the bike has stopped. The rider might take a dive over the handlebars or simply tumble sideways to the ground. An unprotected human skull can withstand modest impacts—for example, if you fell while running and hit your head on soft ground or a tree. But the force involved in a bicycle accident is much greater. Many factors affect the outcome of a fall. But in general, depending on the rider’s height, in a free fall from a bike the head could hit at anywhere from 9.5 to 13.4 mph. The Consumer Product Safety Commission helmet test simulates impact speeds of 10.7 and 13.9 mph.

The bike helmet goes through several changes on impact that can save your life. The smooth, thin outer shell helps prevent minor punctures from sharp objects and allows your head to glide as it moves, which reduces the wrenching of your head and neck. The dense expanded polystyrene foam core compresses or breaks, spreading the force over a wider area. That lengthens the time it takes for your head to come to a complete stop, usually dissipating enough energy to prevent a skull fracture or damage to a major blood vessel, which can crush the brain as the sealed skull fills with blood.

The brain is suspended in a bath of cerebrospinal fluid, and when the acceleration of the head comes to a stop, it might continue to move. Even if the helmet did its job and prevented a skull fracture, the rider might still suffer a concussion. If the impact is largely linear—right between the eyes or across the head from one ear to the other—the brain sloshes back and forth inside the skull until the energy dissipates. If the impact is off-center, rotational forces come into play. The brain no longer simply moves back and forth—it also rotates inside the skull, increasing the chance of damage to the delicate tissues.

The brain cells might twist, stretch, or rupture, causing chemical changes that prevent them from transmitting information the way they are meant to. This metabolic crisis going on within a cell—as head-injury specialist Robert Cantu, M.D., a clinical professor of neurology and neurosurgery at Boston University Medical School describes what happens at a granular level inside the brain during a concussion—results in confusion, memory problems, trouble concentrating and other cognitive deficits. Those are the signs that doctors check for during a neurological exam.