Niedfeldt, Mark W. MD
Soccer currently has more than 265 million male and female players around the world, with almost 25 million players in the United States alone (17). Soccer is the only contact sport that purposefully uses the head for controlling and advancing the ball, and heading is essential to the defensive and offensive strategies of the game (24). Contact to the head in soccer, either during tackling duels, where two players are competing for ball control, or purposely heading the ball, has the potential to cause traumatic brain injury. Head injuries related to individual incidents causing concussion or potentially as repeated concussive and subconcussive head trauma causing chronic brain injury can occur.
The issue of whether these repetitive subconcussive head injuries in soccer may lead to chronic brain injury was initially raised in a series of retrospective studies involving retired Scandinavian soccer players (41,43). In these studies, significant cognitive deficits were noted. However, significant methodological problems such as the lack of preinjury data, selection bias, failure to control for acute head injuries, lack of observer blinding, and inadequate control subjects flawed the results. Although the authors concluded that the deficits noted in the former soccer players were explained by repetitive trauma such as heading the ball, the pattern of deficits is equally consistent with alcohol-related brain impairment, a confounding variable that was not controlled for (15).
Researchers have previously implicated both concussive injury and heading as a cause of neuropsychological impairment in both amateur and professional soccer players (26-28). However, reanalysis of the data from these articles suggests that purposeful heading may not be a risk factor for cognitive impairment (23).
Helmets have been shown to reduce the risk of significant intercranial injury in other sports such as football and hockey. Boxing officials have required headgear at the amateur level as an attempt to lessen the force of impact of a punch to the head, potentially lowering the chance of injury (25). There are various types of headgear proposed to limit the effect of concussion in soccer athletes.
Head Injuries in Soccer
Head injuries account for 4% to 22% of all soccer injuries (2,4,34,36,38). This figure incorporates all types of head injuries, including facial fractures, lacerations, and eye injuries. The rate of brain injuries has been difficult to assess because of problems with reporting, defining, and grading concussions (24). It does appear that the higher the level of play and the more competitive the league, the higher the incidence of concussions (4,7,10,34). Studies based on player reports or reports by team medical personnel show that injuries mainly result from contact with other players (14,21,33). None of the reported concussions resulted from intentional heading of the ball (4).
Timing and cause of injuries.
Boden et al. (4) prospectively studied collegiate female and male players and found that approximately 70% of the concussions occurred during games and that head-to-head contact was the most frequent mechanism of injury, followed by head-to-ground and head-to-other-body-parts (foot, knee, elbow) contacts. Studies using the National Collegiate Athletic Association Injury Surveillance System found concussive injury to be 12 to 13 times more likely in collegiate games as compared to practice. In men, of the reported game concussions, 80.6% were from player contact, 8.0% were from ball contact, and 8.0% were from surface contact. In women, of all concussions, 67.7% were due to player contact, 18.3% were associated with ball contact, and 13.4% were associated with playing surface contact. Head injuries were 5.8% of total game injuries in men and 8.6% of total injuries in women. For severe head injuries (defined here as >10 d of activity time lost), player contact was the primary injury mechanism (1,13).
Player position and location at time of injury.
Fuller et al. (19) found that goalkeepers and forwards sustained more and midfielders fewer of all types of head or neck injury than would be predicted for a typical team positional alignment. The distribution of injuries as a function of playing position was markedly different between men and women. In men, head injuries predominantly occurred among the defenders (40%) followed by forwards (23%), midfielders (22%), and goalkeepers (15%). In women, injuries were fairly equal among defenders (34%), midfielders (29%), and forwards (29%), with goalkeepers (9%) being least often injured (19). Gessel et al. (20) found that goalkeepers had the highest risk of concussion, with 21.7% of injuries being concussions as compared to 11.1% among the other positions, using a younger-aged database. Others have not found playing position to influence the type of injury received (45).
It has been suggested that the most likely sites on the field for a head injury would be within the penalty area (24). Fuller et al. (19) did not find this to be the case. In their study, the majority of injuries occurred as a consequence of aerial challenges for a high, free ball that took place in the defensive and attacking outfields, with the blow coming from the side or front. The majority of injuries occurred during play that was deemed by officials to be within the rules of the game.
Mechanism of injury.
Soccer-related head injuries may occur in at least four ways: head contact with the ball (heading), contact with another player (head, foot, arm), contact with the ground, and contact with stationary object (goal posts). Andersen et al. (3) prospectively looked at videotapes and injury information for 313 professional matches and found that, of the 297 acute injuries reported, 17 (6%) were head injuries, which corresponds to an incidence of 1.7 per 1,000 player hours (concussion incidence was 0.5 per 1,000 player hours). The most common playing action was a heading duel with 112 cases (58%). The body part that hit the injured player's head was the elbow, arm, or hand in 79 cases (41%), the head in 62 cases (32%), and the foot in 25 cases (13%). In 67 of the elbow, arm, or hand impacts, the upper arm of the player causing the incident was at or above shoulder level, and the arm use was considered to be active in 61 incidents (77%) and intentional in 16 incidents (20%). This video analysis showed that the primary mechanism of head injury during matches in elite soccer is a contact mechanism between two opposing players occurring in a heading duel rather than purposeful headings. Another study following athletes for 3 years found that the most common site of impact in a concussion-causing collision was the temporal region of the head (11).
When examining sports-related concussion, female athletes have been shown to have markedly greater declines in simple and complex reaction times relative to preseason baseline levels on standardized cognitive tests, compared with age-matched male athletes. Female athletes were cognitively impaired approximately 1.7 times more frequently than male athletes following concussions and they reported more postconcussion symptoms (6).
To look at the frequency of concussion between genders, Fuller et al. (19) examined all head and neck injuries from 6 years of Fédération Internationale de Football Association competitions. The most commonly diagnosed injury was a soft tissue contusion (57% for men and 41% for women). The next most common injury in men was a laceration (21%), but for women, the next most common head injury was a concussion (22%). Concussion was the fourth most frequent injury in men (8%). The incidence rate of concussion was 1.1 per 1,000 player hours for men and 2.6 per 1,000 player hours for women, a 2.4-fold higher rate. Delaney et al. (10) reported a similar, 2.5-fold greater, rate in women. Gessel et al. (20) reviewed data from two injury surveillance systems, namely, the High School Reporting Information Online and the National Collegiate Athletic Association Injury Surveillance System, which were analyzed to calculate rates, describe patterns, and evaluate potential risk factors for sport-related concussion, and found that girls had a higher rate of concussions (21.5%) than boys (15.4%) did and that concussions represented a greater proportion of total injuries among girls. The more frequent mechanism for boys was contact with another person, while girls were more likely to have contact with the ground or with the soccer ball.
Male and female players may have a different style of play, which accounts for some of the differences in head injury patterns. The point of contact to the head in male players tended to be the upper extremity, while head-to-head impact was more common in female players. Match referees deemed that 30% of the incidents analyzed that led to a head or neck injury were foul challenges with no significant differences noted by gender (19).
Data from experimental studies suggest that there are differences between the responses of male and female head-neck segment kinematic and neuromuscular control variables to an external force, which also may explain some of the differences noted in injury patterns (39). Female players showed considerably greater head-neck segment peak angular acceleration and displacement than male players did, despite initiating muscle activity much earlier and using a greater percentage of their maximum head-neck segment muscle activity. Differences in head-neck segment angular acceleration may be due to markedly lower isometric strength, neck girth, and head mass, resulting in lower levels of head-neck segment stiffness in female players. These data suggest that there are intrinsic differences in the ability of female and male athletes to withstand equivalent blows to the head-neck complex (16).
Several suggestions to decrease head injuries and concussions in soccer have been proposed. The most promising strategy to reduce head injury risk may be to discourage elbow and arm use in heading duels. One analysis showed that 20% of the elbow to head incidents involved what appeared to be intentional strikes with the arm or elbow, although nearly 70% of the incidents did not lead to a referee calling a foul (3). Urging referees to ensure that reckless and potentially harmful actions to other players are not permitted during the game may be helpful. In addition, rule changes to decrease elbow or arm contact during heading duels, such as banning the use of elbows at or above shoulder level, and more focus on stricter enforcement of the laws of the game in relation to elbow use when challenging for ball possession are ideal. Some have called for rule changes to limit heading (29).
Other strategies have included the following: using appropriately sized balls for younger players; proper maintenance and avoiding hyperinflation of balls; teaching of heading by a qualified individual with good knowledge of the different methods of heading a soccer ball to lessen the forces applied to the cranium during heading; limiting the amount of heading in youth soccer; strengthening neck musculature, which may help dissipate the energy applied to the head after contact; padding soccer goalposts; and, finally, using mouth guards (9,10,12,43). However, one anonymous survey of 278 youth soccer players aged 12 to 17 years found no significant relationship between mouth guard use and the rate of concussion (12).
Younger soccer players should use the smallest ball size (size 3) up to the age of 9 years, at which point a size 4 ball can be used. Once the athlete has turned 14 years old, use of the largest size, a size 5 ball, can begin (23). The smaller ball size not only protects the athlete from impact with a heavier object and prolonged head contact due to increased size but also affords greater control of the ball for the player who is still developing physically.
Heading in Soccer
Early retrospective research on the effects of soccer heading was done by Tysvaer and Lochen (42) in Norway. They reported higher rates of electroencephalographic abnormalities and cerebral atrophy among former professional soccer players. It was known that these players head the ball, on average, 6 times per match or 2,000 times or more in their careers (43), and it was postulated that heading the ball contributed to a chronic brain injury similar to dementia pugilistica in boxers. Others disagreed and stated that other injuries, concussions sustained in falls, or collisions with other players were responsible for decreased scores on cognitive tests (22). A study from the Netherlands again revisited the question of heading causing brain damage. In their study, swimmers and track athletes were compared with soccer players. The soccer players scored significantly lower on tests of planning and memory. These authors found that participation in soccer, in general and concussion specifically, were associated with the measured changes (26).
Prospective, controlled studies have failed to find any evidence of cognitive impairment in players using clinical examination, neuroimaging, or neuropsychological testing (22,35). No evidence of neuropsychological impairment due to heading exposure or previous concussion was revealed by computerized neuropsychological testing in the examined group of soccer players (15). These results support the contention that heading a modern soccer ball represents a much lower risk than head-to-head contact (24,30) due to the move away from the older, leather, water-absorbing, and heavier ball to the modern, synthetic, water-resistant, and lighter ball. Ball-to-head contact is usually not of sufficient force to cause a concussion, either with or without headgear (44), and the deficits reported in previous studies are more likely the result of accidental head impacts that occur during the course of the matches (3).
Proper heading technique may be the athlete's greatest defense against injury from heading a ball. Many coaching books avoid soccer heading until 12 years of age, and some physicians recommend waiting until 14 years (24). The younger soccer athlete who performs head balls may be at greater risk for injury because of smaller size, less muscular development, and a less skillful heading technique. Once soccer athletes begin heading, coaches should instruct them to strike the ball just below the hairline on the thickest part of the cranium, the frontal bone, while simultaneously isometrically contracting the neck musculature. To counteract the force of the impacting ball, the athlete should apply a counterforce generated by moving the trunk into flexion. By performing the maneuver as described, the body of the athlete becomes a single, rigid unit that lowers the risk of injury by decreasing the linear and rotational accelerations on the head as forces generated by the ball are dispersed across the athlete's body (24).
Studies attempting to mimic the forces of heading a ball during a soccer match have been done. Naunheim et al. (32) estimated the severity of impact to be as high as 2,000 N for ball speeds of 36 m·s−1 and found that peak accelerations at the surface of the head were 160% to 180% greater from heading a soccer ball than from routine (noninjurious) impacts during hockey or football, respectively. However, this was measured with a ball speed of 35 to 40 mph, a situation that does not occur frequently in regulation soccer games. Others have shown that ball mass and pressure reductions can reduce head impact severity. These changes in ball characteristics can have an equal or a greater effect than structured heading training and coaching technique changes and make emphasis on ball size in younger players especially pertinent (37).
Summary of heading data.
While concussions continue to be a prominent concern in soccer, most have been shown to be caused by player-to-player contact (84%) during games and not by contact with the ball (8%), although these data did not specify purposeful heading versus other ball contacts (1). Fuller et al. (19) found no concussions and found that only one cervical strain among 248 head and neck injuries could be attributed to purposeful heading of the ball, and Andersen et al. (3) did not identify heading the ball as a mechanism for head injury at all. However, one study looking at two injury databases of younger players found that, in male and female soccer players, the activity most frequently reported to be associated with concussions was heading the ball with concussions, accounting for more than 64% of injuries sustained while heading the ball (20). Interestingly, this study used the same database as Agel et al. (1) and Dick et al. (13) did for one of the two databases but had strikingly different findings, although this may be explained by the fact that this study (20) did not specify concussions that occurred due to head-to-head contact while in a heading duel (which accounts for most concussions in other studies) versus inadvertent head-to-ball contact or purposeful heading. The difference may be in the method of reporting the concussions. Overall, these findings support research suggesting that repetitive purposeful heading does not produce concussions or acute neurocognitive impairments in mature (collegiate and higher) players (4,15,30). Younger players may possibly have more concussions as a result of heading the ball because of their lower skill level or technique flaws (20).
Headgear in Soccer
There are various types of headgear proposed to limit the effect of concussion in soccer players, and manufacturers of soccer headgear have designed them to decrease the forces associated with heading, assuming this will reduce the risk of head trauma. Although there have been nonrandomized studies of the effect of headgear on head injuries in soccer, there have been few analytic studies looking specifically at the role of headgear on concussion rate or severity, which is a potential area of interest (11,12). There are different designs, from a headband to full helmet, similar to what boxers wear. To better "absorb and dissipate energy" from soccer ball impact, the headgear should fit snugly across the scalp (5).
Impact between the head and a soccer ball is different from impact between a head and a rigid object due to the ability of the ball to deform when striking a harder surface. In the impact, the majority of deformation occurs in the most compliant (softest) link. The addition of a compliant headband would only change the duration of impact significantly if 1) it were at least as compliant as the ball and 2) it did not compress fully. A soft headband of reasonable thickness would be fully compressed before the ball (softest object) had completed deformation from impact, rendering it ineffective in reducing acceleration. A headband stiff enough not to compress fully would reduce peak forces and accelerations, but not by a great amount. Because of the differences between deformable soccer balls and hard surfaces, the traditional methods for testing helmets will not work for testing soccer headgear. Dropping a head form wearing a headband onto a hard surface would not reproduce the effect of a collision with a compliant ball (31). Thus, many studies on headgear are not pertinent to heading a soccer ball.
Naunheim et al. (31) studied peak acceleration of a head form model when struck by a soccer ball at variable speeds. They showed that headgear has little ability to reduce impact when heading, but they suggest that headbands may play a role in attenuating the impact for more forceful blows at the highest speeds.
Withnall et al. (44) evaluated three types of headgear and concluded that none of the three headgears tested appeared to be particularly effective at reducing the impact from ball contact, whether it is voluntary or incidental. In some cases, the results were slightly worse wearing the headgear than without. The reason for this is that the ball deformed more than 10 times the thickness of any headgear tested. Headgear causes a change in the radius of the head, which increases both the ball's moment arm and the head's moment of inertia. They did find that a headband is effective when inserted between two stiff objects and could mitigate head hits by up to 33%. Their conclusion was that headgear could be helpful in head-to-head or other non-ball-related impacts such as elbow-to-head involving noncompliant objects.
One retrospective anonymous online survey by Delaney et al. (12) of youth soccer players, aged 12 to 17 years, studied the role of headgear on concussion symptoms. This study asked respondents not only how many concussions they had experienced in the prior season but also how many times they had experienced specific symptoms associated with concussions in response to a collision. About 7.2% of the players reported having experienced at least one concussion, whereas 7.8% reported having experienced concussive symptoms at least once. Players self-selected on the decision to wear headgear, and those players who had suffered a previous concussion were more likely to wear headgear. Those wearing headgear were significantly less likely to receive a laceration to those areas of the scalp and face covered by the headgear. It was found that not wearing headgear was associated with a 2.65 relative risk of concussion. This finding might be explained in part by the fact that fewer athletes who wore headgear considered themselves to be headers, assuming they were at lower risk due to their style of play or underestimation of their usual number of game headers. Female players also were more likely to suffer a concussion, which is similar to findings in other studies (10,29). Although this study is promising, it was not ideal because headgear use was nonrandomized, and the retrospective study relied principally on information recollected by athletes at the end of the season.
Concerns with headgear.
Rule changes in football, hockey, and lacrosse have suggested that mandating headgear removes inhibitions to strike or risk strikes to the head because it reduces pain from scalp injuries and lacerations. If all players were to become accustomed to a playing style in which contact to the head was no longer off limits, the addition of headgear might result in an increase in the frequency of total collisions to the head and may increase the total number of concussions as well (8).
This idea is potentially supported by Tierney et al. (40), who examined differences in head acceleration while wearing headgear. They were able to identify sex differences in head acceleration response associated with wearing soccer headgear. The women in the study attacked balls more vigorously and added to impact energy when wearing headgear. Overall, women had higher head accelerations during impact compared with men, which had an apparent detrimental effect on women, possibly because of their lower head-neck segment stability qualities. This finding supports the idea presented previously that athletes feel the need to strike the ball harder or may feel safer when wearing the protective equipment (18) and that players may develop a false sense of security while wearing the headgear and become overly aggressive when heading, thus increasing their risk of injury (5). Therefore, headgear may not be beneficial for all athletes (40).
Head injuries are a significant problem in soccer. Concussions seem to be more common at higher levels of soccer and are mainly caused by contact between players during game play. Most injuries occur during heading duels and are ruled as legal plays by game officials. It is not clear if player position increases the risk of head injury. As in other sports, female soccer players seem to be more prone to concussion, possibly partly due to intrinsic differences between the sexes. Rule changes and tighter enforcement of current rules may help prevent head injuries. Although early retrospective research on the effects of soccer heading seemed to suggest that purposeful heading may contribute to long-term cognitive impairment, prospective controlled studies did not support this and, in fact, suggested that purposeful heading does not produce concussions in mature (collegiate and older) players. Younger players may possibly have more concussions as a result of heading the ball, most likely because of smaller size, less muscular development, or lower skill level and technique flaws. Therefore, efforts to have youth players use appropriately sized soccer balls and receive instruction from qualified individuals knowledgeable in methods of heading a soccer ball to reduce forces applied to the cranium may be helpful to reduce the incidence of youth concussions.
Headgear has been proposed to limit the forces to the head, thus decreasing risk of concussion. Headgear has not been shown to be effective in reducing ball impact but may be helpful in reducing the force of non-ball-related impacts. There are concerns that universal use of headgear may cause players to develop a false sense of security and cause more aggressive heading and head challenges, leading to increased risk of injury.
The author declares no conflict of interest and does not have any financial disclosures.
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