Startling new evidence from medical investigators in New York suggests that “heading” a ball in soccer – in which players propel it with their heads – can lead to cognitive impairment in members of the sport who are endowed with a dementia-associated gene.
The findings are preliminary and require replication by other geneticists. But the emerging data build on a growing body of evidence that suggests ball heading increases the risk for adverse cognitive outcomes, particularly for certain members of the sport.
Soccer is played by more than 265 million people worldwide, and heading the ball is a key component of the game. Although usually not strong enough to cause a concussion, medical investigators are theorizing that heading has a cumulative effect on the brain.
A new medical investigation involving players who had been in the game at least five years, revealed that verbal recall declined among those who frequently headed the ball and met a specific genetic profile. All participants in the investigation were tested for memory and recall.
Evidence of cognitive impairment was strikingly apparent among study participants who headed the ball frequently, and for whom a dementia risk factor was indelibly inscribed in their DNA.
“Soccer is the most popular sport worldwide, and this research is part of a large, ongoing study conducted by our collaborators, namely the Einstein Soccer Study, which has already enrolled hundreds of participants over many years,” said Dr. Yun Freudenberg-Hua, an assistant professor at the Feinstein Institutes for Medical Research on Long Island.
The research division is part of the Northwell Health system, which has headquarters in New York.
“The goal of this study is to investigate whether the effects of repeated and long-term, routine heading interacts with a well-known genetic risk factor for dementia,” Freudenberg-Hua told Medical Xpress.
That risk factor is the genotype, Apolipoprotein E ε4, usually written in scientific shorthand as APOE ε4. The research is a joint project of Albert Einstein College of Medicine in the Bronx, New York, and the Feinstein Institutes.
“Previous studies have shown that long-term heading is associated with abnormal white matter microstructure in the brain and with poorer neurocognitive performance,” added Freudenberg-Hua, who is also an assistant professor of psychiatry and molecular medicine at the Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, also on Long Island.
“This study only looked at an association between heading and verbal learning performance, together with the specific genetic risk factor for dementia, APOE ε4,” she said.
Reporting in the JAMA Neurology, Freudenberg-Hua and her colleagues defined their study as one that focused on a group as soccer players who ranged between the ages of 18 and 55.
All told, there were 379 participants who were allowed to take part in the research if they played for more than five years and were active in games more than six months a year.
Each player was genotyped for the APOE ε4 allele. Players also were asked to report heading exposure and to undergo a verbal memory assessment test.
“Participants were presented with a 12-item grocery shopping list at the beginning of the session and asked to recall that list at the end of the session, approximately 20 minutes later. The number of correct responses was recorded,” Freudenberg-Hua said.
Carriers of the APOE ε4 allele were more likely to have forgotten many of the items on the shopping list compared with fellow players without the genetic marker.
Study participants who carry the gene performed four times worse on the memory and recall task than players without the gene who rarely headed the ball.
“The APOE gene is a major risk gene for Alzheimer’s disease and the APOE ε4 allele is associated with a three-fold increased risk for Alzheimer’s disease,” Freudenberg-Hua explained.
What remains to be fully elucidated, she added, is precisely how ball heading exacerbates cognitive impairment with respect to the APOE ε4 allele.
“The way APOE ε4 allele confers this risk is not entirely clear yet, but likely involves multiple biological mechanisms, such as abnormal protein degradation, lipid metabolism, synaptic plasticity, spine integrity and neuroinflammation,” she added.
“This study shows that people who carry the APOE ε4 allele and who had a high level of heading exposure may have greater risk of cognitive dysfunction.”
As compelling as the research may seem, it has yet to be validated by other investigators, Freudenberg-Hua said, noting that reproducible findings define the gold-standard of scientific research.
Nevertheless, she and her team advise players who know that they carry the gene to avoid aspects of the game that involve head impacts with the ball.
“Genetic association results generally require replication in an independently collected group of participants,” Freudenberg-Hua said.
“The larger the number of participants, the more confidence we have for our results.
“However, conducting any such study requires many years of labor-intensive community outreach, enrollment, data collection and data analysis.
As geneticists, we always look to evaluate the effects of genetic variants across many traits,” she said.
Soccer is the most popular sport in the world and is unique, in comparison to other sports, as it is the only sport in which participants purposely use their head to hit the ball.
Heading is considered an offensive or defensive move whereby the player’s unprotected head is used to deliberately impact the ball and direct it during play.
A soccer player can be subjected to an average of 6–12 headings per competitive game, where the ball reaches high velocities (1).
Moreover, heading training is common in practice sessions. Although the scientific community has focused on the effects of concussion in contact sports, the possible consequences of subconcussive impacts, as it can occur during heading, have recently gained attention.
The term “subconcussive” was proposed to describe the impact to the head that may cause neuronal dysfunction in the absence of concussive symptoms (2).
Since repetitive subconcussive impacts are substantial in soccer, their clinical significance as a potential cause of traumatic brain injury and a risk of later development of neurodegenerative disease may represent a public health problem.
Research on the effects of soccer heading on brain structure and function has produced mixed results (3).
A small number of studies have investigated the impact of heading on brain structure by using neuroimaging techniques. While some of these studies have found evidence of abnormalities (4–6), others have failed to show any evidence of brain damage in soccer players (7–9).
Also, another line of research has specifically investigated biochemical markers of brain injury and their relationship with head impacts in soccer players. However, while some authors have demonstrated associations between heading and biochemical signs of brain damage (10–15), others have not corroborated these findings (16, 17).
Similarly to the research addressing the effects of soccer heading on brain structure, investigations on the effects of subconcussive impacts on brain function has produced unclear results.
A recent study demonstrated an association between soccer heading and immediate and transient changes in electrophysiological measures of brain function (18). There has been a growing number of studies focusing on cognitive functioning in soccer players, but their results are also ambiguous, with some suggesting an association between heading practice and cognitive impairment (5, 6, 18–27) and others finding no evidence of association (8, 9, 15, 28–36).
However, it is important to emphasize that these studies vary considerably in sample size, age of subjects, soccer practice level, estimation of heading frequency, cognitive assessment methods, presence and composition of control groups, and study design.
Only two cross-sectional studies conducted cognitive assessment of soccer groups composed entirely by active professional soccer players, with contradictory results. An investigation conducted by Matser et al. (20) compared male professional soccer players with a group of elite non-contact sport athletes, by using an extensive neuropsychological test battery, and showed poorer performance on verbal and visual memory, planning, and visuoperceptual processing tasks in the soccer group.
Moreover, an increasing number of headings incurred during soccer participation was associated negatively with cognitive functioning. Field position also influenced performance on neuropsychological testing, as forward and defensive players performed significantly poorer than midfield players and goalkeepers on some tasks.
On the other hand, a study conducted by Straume-Naesheim et al. (32) examined performance of male professional soccer players on computerized neuropsychological tests, measuring motor function, decision-making, simple, divided and complex attention, working memory, and learning and memory, and showed no association between estimated match or lifetime heading exposure and cognitive performance.
Only 1.5% of the players qualified as outliers for one or more subtasks when compared with the normal range.
As described above, the research about the effects of heading on brain has produced intriguing results, but the findings are still inconclusive. It has been estimated that professional soccer players head the ball more than 2,000 times during their careers (28).
However, as mentioned, few studies focused on active professional players (7, 8, 11–14, 20, 32), and only one involved computerized cognitive assessment (32). To our knowledge, there is currently no study about the possible deleterious effects of heading on brain functioning involving concomitantly a sample of active professional soccer players and a combination of computerized and conventional cognitive assessment.
Also, only two studies (20, 32) evaluating active professional players mentioned the educational level of the participants, a variable that can significantly influence cognitive performance (37).
Furthermore, it is noteworthy the absence of research about the effects of soccer heading on brain in developing countries, as is the case of Brazil, where the schooling level of the population tends to be lower when compared to developed countries.
Given the role of education in improving cognitive reserve (38), it is particularly important to investigate individuals who, besides being subjected to repetitive head impacts in their professional activity, may be more cognitively vulnerable, due to low educational level.
In this cross-sectional study, we aimed to investigate the effects of soccer heading on cognitive functioning, by comparing the performance of active professional soccer players and control subjects on computerized and conventional neuropsychological tests.
Discussion
To our knowledge, this is the first study investigating the effects of soccer heading on cognitive functioning involving concomitantly a sample of active professional soccer players and a combination of computerized and conventional cognitive assessment.
Our data demonstrated better performance of soccer players, compared to controls, on computerized neuropsychological tests, namely reaction time measures in general motor coordination, executive functioning and memory tests, and accuracy measures in executive functioning tests. No significant difference between groups emerged on Neupsilin battery.
Interestingly, from the seven variables showing significant differences between groups on computerized cognitive assessment, five variables refer to reaction time and only two refer to accuracy. The advantage of soccer players on reaction time measures, when compared to controls, could be at least partially explained by better general motor coordination, since athletes had shorter reaction times also in the simple reaction time test.
There is evidence that participation in sports relates to faster processing speed on measures of simple reaction time (46). The better performance of soccer players on accuracy measures in executive functioning tests, specifically assessing task switching and working memory, may be associated to team sport practice, which demands strategic decision-making in complex and quickly changing contexts.
Previous research, although not focusing on the effects of soccer heading on brain function, has demonstrated better executive functioning in soccer players in comparison to general population (47).
It is also noteworthy that while computerized cognitive assessment revealed some differences between soccer players and controls, conventional neuropsychological tests did not show any difference between groups.
This is in line with studies investigating the reliability of computerized cognitive assessment, which have suggested that measures of response speed are more sensitive in detecting neuropsychological changes than measures of response accuracy in healthy young adults (48).
Considering the literature investigating the effects of soccer heading on cognition (5, 6, 8, 9, 15, 18–36), as pointed out earlier, only two cross-sectional studies involved cognitive assessment exclusively of active professional soccer players. One study (20) demonstrated an association between soccer heading and cognitive impairment, but the other (32) did not corroborate this finding.
However, it is important to emphasize a distinction between these two studies with regard to the control groups. While Matser et al. (20) included elite noncontact sport athletes as controls, Straume-Naesheim et al. (32) compared the performance of soccer players with normative values for the general population.
Taking into account the evidence of positive effects of physical exercise practice on cognition (49), a comparison involving a control group composed by non-athletes, as is also the case of our study, could difficult to some extent the detection of possible deficits on cognitive performance of soccer players when compared to controls.
Moreover, another relevant observation is that our control group was composed by doormen and guards, professions that do not normally involve high cognitive demand, which may also have influenced the results.
A limitation of our study refers to the composition of the control group, which involved some subjects who practiced soccer, albeit at recreational level. It was not possible to measure the amount of irregular exposure to headings within the control group.
However, it is important to emphasize that soccer is an extremely popular sport in Brazil, among all age ranges and socioeconomic conditions. Therefore, it would not be surprising that the control group involved subjects who practiced soccer as a leisure activity, since that is part of the country’s culture.
We do not believe that such a limitation has significantly affected our results, considering the difference in sport practice level between professional soccer players and non-athletes who play soccer recreationally.
There were no significant differences on test variables between soccer players who reported larger or smaller number of headings per game. Similarly, no significant correlations were found between estimate of exposure to heading during professional soccer career and cognitive performance, in contrast with the study by Matser et al. (20) and in accordance with the results by Straume-Naesheim et al. (32).
However, another important distinction between these two studies refers to the estimate of heading exposure. In the first study, the authors estimated the total number of headings in only one season, whereas in the second one, as in our study, the estimate involved the entire professional soccer career.
In any case, some considerations must be taken with respect to the estimate of exposure to heading. In most studies, as in our investigation, such an estimate is based on self-reported information provided by soccer players, a method with unknown validity.
The self-report on the number of headings may be quite variable and potentially biased, which may explain the high standard deviation of the variable regarding long-term heading exposure. In addition, even assuming a correspondence between the self-reported and the actual values, the number of headings may, in fact, vary across matches and sport seasons, an aspect that was not addressed in our estimate.
Moreover, when estimating the exposure to heading during professional soccer career, we considered only the number of competitive matches played by each athlete, excluding the training sessions over time, which may considerably underestimate heading exposure.
Also, heading technique may vary among soccer players (5), resulting in situations of greater or lesser risk for head injury, a variable that was not controlled for in our study. Even though few soccer players in our sample reported a lifetime history of concussion, the results showed a relative homogeneity of cognitive performance in the soccer group, since only one test variable showed a significant difference between athletes with and without history of concussion.
Although no evidence of brain function impairment in soccer players has been found, we cannot draw conclusions about the issue involving soccer heading and brain injury from cross-sectional data. Moreover, the selection of soccer players from only two soccer teams may also limit the generalizability of the findings.
Further longitudinal studies involving larger samples will be crucial for better understanding of this matter, which remains controversial. The first longitudinal study assessing brain structure and function in active professional soccer players did not show evidence of abnormalities or deficits over a period of 5 years (8), but these results need corroboration.
It could be argued that soccer heading does not involve enough force to represent a subconcussive impact and, therefore, does not affect cognitive function. In order to investigate this hypothesis, future research should also combine heading exposure, cognitive and biomechanical data, and their potential relationships.
As emphasized by some authors (50), although a spectrum of chronic neurological injuries associated with sports-related concussive and subconcussive brain trauma may occur, a primary concern involves chronic traumatic encephalopathy, which has already been reported in studies involving soccer players (51, 52), as well as other contact sport athletes (53, 54).
The prevalence of chronic traumatic encephalopathy is still unknown and in vivo diagnosis is not currently possible, which may enhance the concern about potential consequences of soccer heading.
In summary, our data demonstrated no evidence of cognitive dysfunction in active professional soccer players compared to non-athletes. Conversely, soccer players even outperformed control subjects on some computerized neuropsychological tests.
Among soccer players, there were no significant differences between those who reported larger or smaller number of headings per game and no significant correlations between estimate of exposure to heading during professional soccer career and cognitive performance.
Longitudinal investigations will help to clarify whether heading may be associated with brain injury and cognitive dysfunction.
More information: Liane E. Hunter et al. Associations of Apolipoprotein E ε4 Genotype and Ball Heading With Verbal Memory in Amateur Soccer Players, JAMA Neurology (2020). DOI: 10.1001/jamaneurol.2019.4828
