Athletic Training and Sports Health Care

Original Research 

In-Game Head Impact Exposure of Male and Female High School Soccer Players

Derek Nevins, MS; Kasee Hildenbrand, PhD; Anita Vasavada, PhD; Jeff Kensrud, MS; Lloyd Smith, PhD

Abstract

Purpose:

To quantify and compare head impact exposure in male and female high school soccer players.

Methods:

The comparison was accomplished using analysis of game video and a small form factor head impact sensor consisting of a triaxial accelerometer and a triaxial gyroscope to measure head kinematics during play. Eight male and 15 female high school soccer players were monitored during eight and nine games, respectively, over the season.

Results:

Female participants were observed to experience head impact events more frequently on average than male participants (1.57 vs 1.30 impacts per player per game). Male players were observed to have significantly larger median peak linear accelerations (21.2 vs 15.8 g) and peak angular accelerations (4,482 vs 2,545 rad/s2) compared to females.

Conclusions:

These results support the hypothesis that sex differences in play contribute to sex differences in the distribution of impact magnitudes and frequency of head accelerations arising from ball-to-head and player-to-player contact. However, further study is needed to determine whether head impact exposure differences contribute to sex differences in concussion risk associated with participation in soccer.

[Athletic Training & Sports Health Care. 2019;11(4):174–182.]

Abstract

Purpose:

To quantify and compare head impact exposure in male and female high school soccer players.

Methods:

The comparison was accomplished using analysis of game video and a small form factor head impact sensor consisting of a triaxial accelerometer and a triaxial gyroscope to measure head kinematics during play. Eight male and 15 female high school soccer players were monitored during eight and nine games, respectively, over the season.

Results:

Female participants were observed to experience head impact events more frequently on average than male participants (1.57 vs 1.30 impacts per player per game). Male players were observed to have significantly larger median peak linear accelerations (21.2 vs 15.8 g) and peak angular accelerations (4,482 vs 2,545 rad/s2) compared to females.

Conclusions:

These results support the hypothesis that sex differences in play contribute to sex differences in the distribution of impact magnitudes and frequency of head accelerations arising from ball-to-head and player-to-player contact. However, further study is needed to determine whether head impact exposure differences contribute to sex differences in concussion risk associated with participation in soccer.

[Athletic Training & Sports Health Care. 2019;11(4):174–182.]

Soccer, or association football, is a popular sport with more than 265 million participants worldwide.1 In the United States alone, more than 800,000 high school-aged students participated in organized soccer during the 2014–2015 season.2 The popularity of soccer, routine nature of ball-to-head contact during play, and growing awareness of concussive injury has motivated a large number of epidemiological studies on players' brain health and cognitive performance and research on the biomechanics of head impact in soccer.

Negative health outcomes associated with repeated concussive injuries have been established,3 with indications of tauopathy observed in autopsy samples of soccer players.4,5 Although soccer has lower reported concussion rates than other contact sports (eg, American football, hockey, and wrestling),6–8 the risk of concussion injury persists. Several investigations have suggested that extended participation in soccer is associated with neurocognitive deficits later in life.9–15 In a prospective study of middle school girls, 14.5% reported concussion symptoms following head impact during soccer.16 In a retrospective study, more than 60% of collegiate soccer players reported concussion symptoms.17 Although it is not clear whether concussive injuries, repeated or otherwise, result in long-term negative health outcomes, participation in soccer is associated with the risk of concussion.

Concerns also have been raised regarding potentially deleterious effects of repeated subconcussive impacts,18 which is of particular concern in soccer because heading the ball is a normal part of play.19 Some investigations of the influence of heading on player health suggest that the activity is benign and report that heading produces relatively low accelerations, does not directly result in concussion injuries, and does not elicit measurable changes in postural control or cognitive function.20–25 However, other studies have found deleterious health outcomes associated with heading,26–30 and several retrospective studies have observed substantive correlations between the estimated number of headers and the magnitude of neurocognitive deficits.9,10,13 Studies of collegiate-aged professional and adult amateur soccer players also have found differences in white matter integrity compared to controls, even in the absence of a concussion history.9,31

Although heading is commonly discussed within the scientific literature and popular press in the context of soccer and brain injury, other contacts during play have the potential to contribute to concussion risk and subconcussive impact exposure. In a study of female youth soccer players using an array of linear accelerometers affixed to custom made headgear, Hanlon and Bir26 reported 17 of 67 impacts measured above a 10 g threshold resulted from contact with the ground, other players, or the goal post. In laboratory reconstructions of head impacts in professional soccer based on video recordings of play, Withnall et al.32 reported player-to-player contact had the potential, in a worst-case scenario, to produce peak linear head accelerations ranging from 44.4 to 87.0 g. These results suggest player-to-player contact may contribute to both the concussion risk associated with soccer and subconcussive head impact burden.

Furthermore, observed sex differences in concussion incidence in soccer and other contact sports provides motivation for filling research gaps in head impact exposure during participation in soccer.33,34 Laboratory studies of heading activities indicated that female athletes experienced larger head accelerations during heading than their male counterparts,35,36 although a difference in head acceleration was not observed in a study where a relatively lower ball speed was considered.37 Most studies quantifying head impact exposure in soccer have considered collegiate women's soccer38–40 and female high school or youth play,26,39,41 whereas male players of all ages are relatively understudied. Few studies examining head impact exposure for female soccer players used video review to validate and obtain context for recorded impacts.40,42 Investigations of head impact sensor accuracy suggest that video review is necessary when using devices designed for use in unhelmeted sport.40,43 In addition to validating sensor impact identification, contextualization of head impacts during play describes differences in play style between male and female participants. Concussion epidemiological data indicate that player-to-player contact is the primary mechanism of injury for high school soccer players, suggesting that play resulting in a greater frequency of player-to-player contact increases risk.21

Because both head impact magnitudes and frequencies were linked to negative health outcomes, the purpose of this study was to quantify the frequency, magnitude, and context of head impacts during female and male varsity high school soccer games using video analysis and small form factor impact sensors. Because female soccer players have been observed to be at higher risk of concussion than their male counterparts and exhibit larger head accelerations in some laboratory studies of heading, we hypothesized that female players would experience head impacts more frequently and of larger magnitude than male players.

Methods

Participants

Head impact exposure was investigated in a prospective cohort study of male and female high school soccer players. All students playing on the male and female varsity team (16 male; 17 female) were invited to participate in the study, which resulted in a convenience sample that included 8 high school aged males (age: 16.75 ± 1.09 years; height: 1.76 ± 0.08 m; weight: 65.3 ± 5.2 kg) and 15 high school aged females (age: 15.33 ± 1.01 years; height: 1.65 ± 0.06 m; weight: 60.1 ± 10.3 kg) choosing to participate. Approval for this study was obtained from Washington State University's institutional review board; assent and informed permission was obtained from the participants and their legal guardians prior to the start of the study.

Sensor Application

Head impact exposure was measured using the xPatch head impact sensors (X2 Biosystems, Seattle, WA), where head impact exposure was quantified by the magnitude (peak resultant linear [PLA] and peak resultant angular acceleration [PAA]) and frequency of head impacts. The xPatch device used a combination of a triaxial accelerometer and a triaxial gyroscope to obtain linear and angular acceleration in a small form factor, which allowed the device to be worn during soccer games. Each sensor was mounted behind the right ear, just above the mastoid process, using a double-sided adhesive provided by the manufacturer.44 Before the warm-up for each varsity game, the mastoid area was cleaned with alcohol and the sensor was adhered to the same location for each player and game. At the end of each game, sensors were removed and placed on the charging board provided by X2 Biosystems, transported back to the laboratory, and recorded impacts were uploaded to a cloud-based data storage system maintained by the manufacturer. Impacts were processed using proprietary algorithms that removed spurious impacts and calculated peak linear and peak angular accelerations.

Video Analysis

A total of 8 male and 9 female varsity soccer games were recorded for each of their seasons (according to the number of home games for each sex). The male players' season was in the spring, and the female players' season was in the fall. The play field had adjacent elevations enabling video observation. Only games were observed with video because practice sessions were conducted at a different location where video observation of impact events was not feasible. Each game was recorded by four video cameras recording at 60 frames per second and at 1,080 pixel resolution. The cameras were located behind and above one goal, and arranged such that each camera observed approximately one-quarter of the field. Video recordings were synchronized with the head impact sensors prior to the start of each game. This was accomplished by mounting each xPatch sensor worn by a participant and an additional reference sensor onto a board prior to the start of each game. The board was struck, providing a synchronized event for all the sensors. The reference sensor was struck within view of the cameras, enabling the synchronization of video recordings from each camera with the sensors.

Each impact recorded by the xPatch devices was identified in the video recordings. Impacts were reviewed and categorized by type of play, struck body part, and striking body part (Table 1). In cases where impacts of two types occurred simultaneously, categorization was based on the contact that appeared to affect subsequent head motion. For the purposes of this analysis, an “impact” was defined as any event that resulted in linear acceleration in excess of a 10 g threshold as measured by the xPatch device.45 In some cases, impacts were recorded that were not associated with contact with other players, the ball, or the ground. In these cases, impacts were considered to be caused by player motions if they corresponded with sudden changes in direction, speed, or hard ball strikes. In all cases where a precipitating event for the recorded impact was not apparent, the impact was considered a false-positive result and excluded from analysis.43 Impacts were also categorized by the type of play taking place at the time of the impact. Impacts during corner kicks, goal kicks, penalty kicks, or immediately preceding or following a shot on goal were categorized accordingly; impacts occurring at all other times were considered to have taken place during general play.

Impacts Category Definitions

Table 1:

Impacts Category Definitions

Data Analysis

The total number of impacts and impact frequency (impacts per player per game) were obtained to evaluate differences in head impact exposure between male and female cohorts. Impact magnitudes were compared across sexes using the Mann–Whitney U test, a non-parametric analog of the t test. Comparison across sexes of impact frequency, PLA, and PAA were conducted for all impacts (combined), head impacts (combined), and body impacts (combined); other subcategories (ie, ball-to-head) were not considered due to their small cell size. Sex differences in the distribution of impact magnitudes were assessed using cumulative distribution curves and the two-sample Kolmogorov–Smirnov test. Differences in the distribution of impacts across categories were assessed using a 2-sample chi-square test.

Results

A total of 79 and 206 head impacts were recorded over the course of the male and female soccer seasons, respectively (Table 2). Because all players did not participate in all games, the 8 male participants played in 7.62 games on average and the 15 female participants played in 8.73 games on average.

Number of Impacts and Impact Frequencies (Impacts per Athlete per Game) in Each Category Observed Over All Games

Table 2:

Number of Impacts and Impact Frequencies (Impacts per Athlete per Game) in Each Category Observed Over All Games

Maximum impact magnitudes observed over the seasons (ie, the largest impact across all participants and games) were comparable between the groups, and ball-to-head impacts resulted in the largest median and maximum impact magnitudes (PLA and PAA) for both male and female participants (Table 3). The distribution of PLA and PAA was skewed toward lower magnitude impacts for both male and female participant groups, but the skew was larger for female participants (Figure 1). Comparison of impact magnitudes using the two-sample Kolmogorov–Smirnov test indicated a statistically significant difference in the distribution of impact magnitudes between males and females for both PLA (D = 0.26, P < .01) and PAA (D = 0.28, P < .01), indicating that the distribution of impacts for male participants included a larger proportion of high magnitude impacts. Due to the significant difference in impact magnitude distributions, Mann–Whitney U tests were used to compare median impact magnitudes across sexes for all impacts to the head, body, and combined. When considering all impacts, median impact magnitudes experienced by males were significantly larger for both PLA (21.2 vs 15.9 g, U = 5,691, P < .01) and PAA (4,482 vs 2,581 rad/s2, U = 5,217, P < .01). Median PLA for body impacts was also observed to be significantly larger for male participants compared to female participants (15.7 vs 13.6 g, U = 791, P < .01). Differences in impact magnitude were insignificant for impacts to the head (both PLA and PAA) and for PAA for the impacts to the body.

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

Table 3:

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

Empirical cumulative distribution functions for data collected from male (dashed) and female (solid) participants. Vertical axes show the normalized cumulative frequency associated with (A) peak linear and (B) peak angular accelerations.

Figure 1.

Empirical cumulative distribution functions for data collected from male (dashed) and female (solid) participants. Vertical axes show the normalized cumulative frequency associated with (A) peak linear and (B) peak angular accelerations.

The distribution of recorded impacts across head, body, and other events were found to be significantly different between male and female participant groups (chi-square = 22.1, degrees of freedom = 3, P < .01), with male players experiencing a higher proportion of impacts due to direct contact with the head (68% vs 38% for females), whereas female players were observed to experience a larger proportion of body impacts (55% vs 30% for males) (Table 2).

The distribution of head impacts across play types was not found to be significantly different across sexes (chi-square = 7, degrees of freedom = 4, P = .14) (Table 4). Relatively few impacts occurred during corner or goal kicks. Median impact magnitudes for these play conditions were larger than for general play, but they were not evaluated statistically due to low occurrence.

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

Table 4:

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

Discussion

Soccer players face a substantive risk of concussive injury at all levels of play, and the exposure to repeated subconcussive head impacts may pose further risks. Moreover, female soccer players have a higher occurrence of concussive injury than their male counterparts. This study used small form factor head impact sensors and synchronized video review to examine head impact exposure in male and female high school soccer games. We hypothesized that female players would experience head impacts more frequently and of larger magnitudes than male players. Although our hypothesis was not directly supported, we found differences in the distribution of impact magnitudes and type of impacts between female and male soccer players.

Female participants were found to have a 22% higher frequency of all impacts than males and experienced body impacts at more than twice the frequency of males. Males experienced direct head impacts 48% more often than females, due to more frequent heading of the ball. Males were also more likely to experience higher impact acceleration magnitudes than females (on average 34% higher PLA and 76% higher PAA), but maximum impact magnitudes were comparable for males and females overall (1% difference for PLA and 16% difference for PAA) (Table 3).

Impact frequencies observed in this study are slightly lower than those observed in prior investigations of head impact exposure for female soccer players using the xPatch. Using a threshold of 20 g, McCuen et al.39 observed impact frequencies of 1.69 and 2.85 impacts per player per session for practices and games, respectively, for female high school soccer players. In a study of impact exposure in collegiate soccer, Lynall et al.38 used a threshold of 10 g and observed an overall impact frequency of 7.18 impacts per player per 90-minute session. At the youth level, Chrisman et al.41 observed an impact frequency of 1.01 impacts per player per game using a threshold of 15 g. Because each study used a different threshold to identify impacts, direct comparison is difficult. Furthermore, none of the studies used game video to review impacts recorded by the xPatch, which is a process that would be expected to reduce impact frequencies. In the current study, 105 female and 20 male impacts were excluded from analysis based on video review. This is supported by the work of Press and Rowson,40 who conducted a video review of impacts and reported a head impact frequency of 2.16 impacts per player per game for female collegiate soccer players using the xPatch, a value 70% lower than that reported by Lynall et al.38 and Cortes et al.,46 who reported the xPatch overestimated the number of impacts while playing soccer by approximately 50% based on video review. Considering the substantial influence of video review in lowering impact frequency metrics, the relatively low impact frequencies observed in this study are expected.

Similarly, median PLA and PAA of 15.8 g and 2,545 rad/s2, respectively, observed for female participants in this study agreed with multiple reports of female soccer players. Median PLAs ranging from 12.5 to 30.5 g have been reported for youth to collegiate players with median PAAs ranging from 2,093 to 6,530 rad/s2 reported in the same studies.38,39,41,47 The acceleration values observed in this study are lower than those reported by McCuen et al.39 for high school soccer players, but that study used a 20 g threshold, which would be expected to increase median accelerations. Considering the influence of magnitude threshold, level of play examined in this study, and video review of impacts, our results are consistent with prior characterizations of head impact magnitude in women's soccer.

The findings of this study suggest that impact exposure rates and impact type may contribute to observed sex differences in concussion risk for high school soccer players. Male participants were observed to have larger median impact magnitudes and head the ball more often than females, but female players had a greater overall impact frequency and a higher rate of player-to-player contact resulting in head acceleration. Studies across levels of play ranging from youth to adult have reported that male soccer players head the ball more frequently than their female counterparts.41,48,49 The observation that males have greater impact magnitudes is consistent with findings reported by Reynolds et al.50 but, in that study of collegiate soccer players, male athletes were observed to have a greater overall impact frequency than female athletes.

Epidemiological analysis indicates concussion rates are higher for female than male soccer players at the high school level, and the most common mechanism of reported injuries is player-to-player contact.21 The larger frequency of both head impacts and player-to-player contact observed in this study for females compared to males may contribute to these differences. A higher impact frequency for female athletes would result in both a larger number of head impacts and higher time density of head impacts compared to males over a period of exposure, which may contribute to higher concussion rates for females.51 Similarly, if a larger proportion of impacts experienced by females are due to player-to-player contact, the type of contact most likely to produce a concussion injury, it would likewise increase the expected number of concussions and, thus, the concussion rate.21 It may be possible that one or both of these factors offset, if only in part, the differences in head impact magnitudes observed between female and male players and contributed to the overall observed sex differences in concussion rates.

Reasons for sex differences in overall and player-to-player impact frequencies are unclear, but may result from more aggressive or uncontrolled play by female participants. In either case, it may be possible to decrease concussion risk through coaching interventions and rules alterations that decrease player-to-player contact, as in other contact sports.52 Additionally, further research is needed to determine the relative significance of the impact frequency and magnitude in relation to concussion injuries in soccer, as well as the clinical significance of subconcussive head impact exposure. Relative differences in impact frequency between male and female athletes observed in this study were substantive (22%), but the absolute differences in impact rates were small (0.27 impacts per game) with respect to the variation in impact rates reported in the literature (1.01 to 7.18 impacts per game). As discussed previously, significant methodological variation exists within the literature. If future research supports the clinical relevance of differences in impact rates between male and female athletes, with regard to either concussion injury or subconcussive impact exposure, more studies will be required to develop standardized methodologies for measuring this impact exposure parameter. Moreover, further efforts are needed to determine whether these trends are present at younger and older levels of play to inform training practices and rules of play.

Limitations

Evaluations of xPatch performance have indicated its accuracy may be limited by the mounting interface and sampling frequency.43,44 Wu et al.44 suggested the skin–skull interface results in a non-flat frequency response that amplifies low frequency signal components and dampens high frequency components. This may cause the sensor to overestimate head accelerations for long duration contacts. In laboratory tests of xPatch performance, Nevins et al.43 reported good agreement for long duration impacts but poor performance in short duration contacts, and the authors suggested the sampling rate of the device contributed to the observed differences in performance across test conditions. Due to the comparative nature of our study, accuracy of the xPatch in absolute terms is less important than in other work, such as the development of concussion risk curves from head acceleration data. Nonetheless, in the context of the analysis in this study, it should be noted that noise in head acceleration results due to xPatch accuracy limitations may obscure differences between male and female soccer player head impact exposure characteristics related to head impact magnitudes.

Implications for Clinical Practice

This study provides a novel comparison of head impact exposure in male and female high school soccer involving both wireless head sensors and a video review of recorded impacts. Median head impact magnitudes were significantly larger for male athletes compared to female athletes, but females were observed to have higher overall and player-to-player impact frequencies during play. These results suggest that sex differences in concussion risk for soccer players may be due to a greater rate of head impacts (number of impacts per season) and higher frequency of player-to-player contact for female participants. Female players were observed to experience body impacts more frequently during play than male players, suggesting that a difference in play style may contribute to higher overall impact frequencies for female players. Therefore, female players may benefit more from training practices or rules adjustments that reduce impact frequency than interventions designed to reduce impact magnitudes.

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Impacts Category Definitions

CategoryDescription
Head impact with
  BallIntentional or unintentional contact between the player's head and the ball (header)
  BodyPlayer wearing the xPatch device contacted another player's body (eg, shoulder) with his or her head
  GroundPlayer fell, landing on his or her head
  HeadHead-to-head contact between 2 players
Body impacts
  BallPlayer wearing the xPatch device contacted the ball with his or her body (eg, chest)
  BodyContact between 2 players that did not directly involve the head (eg, shoulder-to-shoulder)
  GroundPlayer fell, landing on another part of his or her body beside their head
  HeadContact between the body of the player wearing the xPatch device and another player's head
Other events
  Player motionPlayer executed a sudden change in direction, speed, or struck the ball with force at the time of impact

Number of Impacts and Impact Frequencies (Impacts per Athlete per Game) in Each Category Observed Over All Games

CategoryNumber of Impacts in SeasonImpact Frequency (Impacts/Athlete/Game)


Male (n = 7)Female (n = 15)Male (n = 7)Female (n = 15)
All impacts792061.301.57
Head impact with54780.890.60
  Ball49720.800.55
  Body240.030.03
  Ground120.020.02
  Head200.030.00
Body impact with241140.390.87
  Ball2200.030.15
  Body16540.260.41
  Ground6400.100.31
  Head200.030.00
Other events1140.020.11
  Player motion1140.020.11

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

CategoryPeak Linear Acceleration (g)Median (Range)Peak Angular Acceleration (rad/s2)Median (Range)


Male (n = 7)Female (n = 15)Male (n = 7)Female (n = 15)
All impacts21.2 (10.7 to 69.6)15.9 (10.1 to 70.6)4,482 (1,089 to 15,864)2,581 (467 to 18,042)
Head impact with29.0 (11.6 to 69.6)23.0 (10.1 to 70.6)6,738 (1,089 to 15,864)5,403 (635 to 18,042)
  Ball33.2 (11.6 to 69.6)23.2 (10.1 to 70.6)6,923 (1,089 to 15,864)5,515 (635 to 18,042)
  Body14.9 (14.6 to 15.2)18.0 (11.6 to 34.0)2,251 (1,966 to 2,536)5,044 (1,314 to 8,640)
  Ground12.613.7 (13.0 to 14.4)1,7323,792 (2,412 to 5,163)
  Head22.1 (12.8 to 31.3)2,379 (2,006 to 2,752)
Body impact with15.7 (10.7 to 52.5)13.6 (10.1 to 54.6)3,118 (1,352 to 12,583)1,852 (467 to 8,034)
  Ball22.2 (11.6 to 32.8)13.1 (10.3 to 23.4)4,404 (1,529 to 7,280)1,875 (755 to 5,001)
  Body15.0 (10.7 to 52.5)13.7 (10.3 to 54.6)3,286 (1,386 to 12,583)2,089 (467 to 7,923)
  Ground17.5 (10.9 to 23.7)13.6 (10.1 to 32.6)2,805 (1,352 to 4,379)1,554 (669 to 8,034)
  Head
Other events
  Player motion10.713.9 (10.5 to 27.0)2,5862,115 (548 to 6,137)

Peak Linear Acceleration and Peak Angular Accelerations for Each Impact Category

Play TypeNumber of Impacts Male/FemalePeak Linear Acceleration (g)Median (Range)Peak Angular Acceleration (rad/s2)Median (Range)


Male (n = 7)Female (n = 15)Male (n = 7)Female (n = 15)
Corner kick3/342.2 (13.7 to 45.8)19.8 (11.8 to 55.8)8,290 (2,917 to 8,708)3,219 (1,376 to 12,277)
Goal kick6/842.4 (14.2 to 63.2)53.7 (11.2 to 63.9)8,107 (1,457 to 15,816)6,753 (1,260 to 18,042)
Shot on goal2/3744.0 (21.7 to 66.3)13.5 (10.1 to 43.1)6,224 (3,025 to 9,422)1,814 (669 to 15,115)
Penalty kick0/0
General play68/15819.7 (10.7 to 69.6)16.1 (10.1 to 70.6)4,135 (1,089 to 15,864)2,739 (467 to 17,969)
Authors

From the School of Mechanical and Materials Engineering (DN, JK, LS), the Athletic Training Program, College of Education (KH), and the Voiland School of Chemical Engineering and Bioengineering (AV), Washington State University, Pullman, Washington.

The authors have no financial or proprietary interest in the materials presented herein.

The authors thank X2 Biosystems for providing the xPatch used within this research study.

Correspondence: Derek Nevins, MS, School of Mechanical and Materials Engineering, Washington State University, P.O. Box 642920, Pullman, WA 99164-2920. E-mail: dnevins@wsu.edu

Received: December 13, 2017
Accepted: April 18, 2018
Posted Online: August 24, 2018

10.3928/19425864-20180802-02

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