During this year’s World Cup, one very peculiar image caught the attention of fans and commentators: Portuguese forward Pedro Neto played a match with the back of his cleats cut off, right above the heel. It wasn’t an accident. The modification was intended to relieve pressure on a particularly sensitive area of his foot.

Although the image went viral, it’s not new. For years, some professional soccer players have been modifying their cleats in the same way to reduce friction between the shoe and the heel. In a sport where cleats are fitted to the millimeter and every sprint requires sudden changes in direction, any discomfort in the heel can affect a player’s comfort and, consequently, their performance on the pitch.

The causes of discomfort in the heel area are varied. They range from simple, such as chafing and blisters, to soreness caused by training and exertion, such as tendon inflammation, to chronic conditions like bone deformities that simply cause someone to have a “larger heel” than others.

Among the most frequently cited explanations for heel discomfort during this year’s World Cup is Haglund’s syndrome. According to the American Academy of Orthopaedic Surgeons, this condition is a bony growth or prominence on the back of the heel bone that can rub against the Achilles tendon and cause pain and inflammation. Although technically a “deformity,” the symptoms associated with it—such as insertional tendinopathy and discomfort when wearing closed-toe shoes—make it a condition or syndrome.

Treatment depends on the severity of the symptoms or pain. The first strategy is to modify footwear. It is recommended to wear cushioned shoes and avoid tight-fitting ones, such as those used for sports, as well as to use protective heel pads. Since soccer cleats are required by regulation, they cannot be replaced, but they can be modified. It is a “loophole” similar to the one regarding the cut of soccer socks.

The syndrome gained attention following Portugal’s match against Spain. Player Pedro Neto wore two pairs of cleats with the heel section cut off. There is no public information explaining why the forward decided to modify his cleats for that match. Nor is there a statement from Neto or his team confirming a diagnosis of Haglund’s syndrome.

In professional soccer, there is no one-size-fits-all soccer shoe. Although the models used in the World Cup are the same as those available on the market, players customize them to fit the shape of their feet or to alleviate specific discomforts. The soccer shoe that made headlines for its cut-off heel is a Nike Mercurial Vapor 16 Elite.

This article originally appeared on WIRED en Español and has been translated from Spanish.