British researchers have, for the first time, experimentally confirmed that heading a football causes pressure waves inside the skull. They discovered that different types of balls exert varying levels of stress on the brain—with differences reaching up to 55 times. Notably, classic leather balls become the most dangerous when wet.

Long-term Health Risks for Football Players

In recent years, the scientific community and football organizations have paid special attention to the potential long-term health consequences for professional players. Epidemiological studies conducted in the UK, Sweden, and France have shown that former footballers have a higher risk of developing neurodegenerative diseases. Previously, such conditions were more commonly associated with boxing and American football.

The main risk factor is considered to be repeated blows to the head with a ball throughout a player's career. Earlier research focused on analyzing head acceleration, impact force, and skull movement, which helped assess the risk of concussions or fractures. However, these studies did not explain the mechanism behind the development of micro-injuries in brain tissue that can lead to gradual brain degeneration over the years.

Experimental Methodology

Researchers from Loughborough University developed a special surrogate head to create the most realistic simulation possible. Instead of standard crash-test dummies used in automotive testing, they created a half-skull made of plastic with mechanical properties identical to human bone, produced using a 3D printer. The inner cavity of the skull was filled with a gel that mimics the density and sound propagation speed of brain tissue and cerebrospinal fluid.

At the center of the gel, five millimeters from the inner surface of the skull, they installed a highly sensitive sensor—a hydrophone—capable of detecting the slightest pressure fluctuations in the liquid medium. Footballs were launched at the surrogate head using a pneumatic cannon at speeds typical for real play—ranging from 13 to 23 meters per second.

Experimental Results

During 430 experiments with 20 different balls—from historical leather replicas from the mid-20th century to modern synthetic models used in major tournaments—it was found that each ball strike generated a pressure wave that reached the sensor inside the skull within the first 100 microseconds. This is thousands of times faster than the onset of head movement. All the energy from the shock wave was released in less than half a millisecond.

The maximum recorded pressure reached 31 kilopascals, comparable to the pressure inside a powerful hydraulic press used for metal forming in industry. This value exceeds the maximum permissible shock wave exposure levels for US military personnel during training, which are set at 27 kilopascals.

Impact of Ball Type and Playing Conditions

The effects of different balls varied significantly. The difference in peak pressure reached up to nine times, and the transmitted energy differed by almost 55 times. Modern synthetic and glued balls produced the strongest waves, while traditional leather balls in dry conditions showed the lowest pressure levels. However, after exposure to moisture—simulating play in the rain—the mass of leather balls increased, and the pressure they generated rose four to five times, approaching the levels of the toughest synthetic balls.