Incidence, time-loss, and injury burden of sport-related concussion in Japanese professional women’s football: a retrospective analysis of two-season injury surveillance data
This retrospective analysis of two seasons of Japan's top professional women's football league reveals that while sport-related concussion incidence is low and higher in matches than training, the overall injury burden is highly sensitive to extreme outliers, underscoring the need for comprehensive surveillance in both settings and cautious interpretation of burden metrics.
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In the high-stakes world of professional sports, a single moment of impact can alter an athlete's career. When a player's head strikes the ground or collides with another body, the result is often a sport-related concussion, a mild traumatic brain injury that requires careful management. Unlike a broken bone that heals with a cast, a concussion affects how the brain functions, and the path to recovery is rarely a straight line. Medical teams must balance the need for the player to rest with the pressure to return to the field, a decision that hinges on when symptoms truly disappear and the brain is ready for full activity. To understand how often these injuries happen and how long they keep athletes out of the game, researchers rely on injury surveillance. This is a systematic way of counting every injury and measuring the time players spend away from training and matches. By tracking these numbers, health experts can see patterns, compare risks between practice and competition, and calculate the total "burden" an injury places on a team—essentially, the sum of all the days lost to recovery.
A team of researchers in Japan recently turned their attention to this issue within the Women Empowerment League, the country's top professional women's football division. While data on concussions in men's football is abundant, and information exists for women in college or amateur leagues, there has been a significant gap in knowledge regarding professional women's football in Asia. To fill this void, the researchers conducted a retrospective analysis, looking back at injury records collected during the 2022 and 2023 seasons. They worked with four teams that volunteered to share their data, carefully recording every instance where a player sustained a suspected head injury and was subsequently diagnosed with a concussion by a team physician. The study focused on three key questions: how frequently these injuries occurred, how many days players missed as a result, and the total impact of these absences on the teams' availability.
The researchers gathered a massive amount of data on player time, known as exposure, which represents the total hours players spent on the field during matches and in training sessions. Over the two seasons, they tracked nearly 54,000 player-hours. Within this vast amount of time, they identified 13 confirmed concussion events involving 12 different players. One player experienced two separate incidents. When they calculated the rate of injury, they found that concussions occurred roughly once every 3,000 to 6,000 hours of play, a figure that remained relatively consistent between the two years. Interestingly, while the actual number of injuries was higher during matches, the vast majority of the time players spent on the field was actually during training. Because training accounted for about 90 percent of all the hours logged, most of the recorded concussions—eight out of the 13—happened during practice sessions rather than official games. This finding suggests that monitoring safety in training is just as critical as monitoring it during matches.
The time players spent away from the game offered another layer of insight. For most athletes, the recovery period was relatively short. The typical player missed about nine and a half days in 2022 and nine days in 2023 before being cleared to return to full, unrestricted participation. However, the data contained one extreme outlier that dramatically skewed the overall picture. In 2023, one player suffered a training-related concussion that kept her out of the sport for 554 days. This single case was so significant that it changed the entire statistical story for that year. When the researchers calculated the "injury burden"—a measure of the total days lost per unit of time played—the 2023 figure appeared to be twenty-one days lost per thousand hours, a number that seemed to suggest a much more severe problem than the other cases indicated.
To understand the true nature of the injury landscape, the researchers performed a sensitivity analysis, essentially asking what the numbers would look like if that one prolonged case were removed. When they excluded the 554-day absence, the injury burden for 2023 dropped sharply to just over two days lost per thousand hours, a figure nearly identical to the 2022 season. This exercise revealed that the high burden estimate was not a reflection of a general trend of long recoveries, but rather the result of a single, exceptional medical case. The study concluded that while the typical recovery time for a concussion in this league is around a week and a half, the total impact on a team can be heavily influenced by rare, prolonged cases. The researchers emphasized that because the number of events was small, the exact rates of injury are not precise enough to draw definitive comparisons between different settings or seasons. Instead, the study serves as a vital first step, demonstrating that effective surveillance must track both matches and training, and that understanding the full scope of an injury's impact requires looking at the average recovery time alongside the rare, long-term outliers that can dominate the statistics.
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