Evaluation of head impact exposure measured from youth football game plays

Abstract

OBJECTIVE

There is a growing body of literature informing efforts to improve the safety of football; however, research relating on-field activity to head impacts in youth football is limited. Therefore, the objective of this study was to compare head impact exposure (HIE) measured in game plays among 3 youth football teams.

METHODS

Head impact and video data were collected from athletes (ages 10–13 years) participating on 3 youth football teams. Video analysis was performed to verify head impacts and assign each to a specific play type. Each play was categorized as a down, punt, kickoff, field goal, or false start. Kickoffs and punts were classified as special teams. Downs were classified as running, passing, or other. HIE was quantified by play type in terms of mean, median, and 95th percentile linear and rotational acceleration. Mixed-effects models were used to assess differences in acceleration among play types. Contact occurring on special teams plays was evaluated using a standardized video abstraction form.

RESULTS

A total of 3003 head impacts over 27.5 games were analyzed and paired with detailed video coding of plays. Most head impacts were attributed to running (79.6%), followed by passing (14.0%), and special teams (6.4%) plays. The 95th percentile linear acceleration measured during each play type was 52.6g, 50.7g, and 65.5g, respectively. Special teams had significantly greater mean linear acceleration than running and passing plays (both p = 0.03). The most common kick result on special teams was a deep kick, of which 85% were attempted to be returned. No special teams plays resulted in a touchback, and one resulted in a fair catch. One-third of all special teams plays and 92% of all nonreturned kicks resulted in athletes diving toward the ball.

CONCLUSIONS

The results demonstrate a trend toward higher head impact magnitudes on special teams than for running and passing plays, but a greater number of impacts were measured during running plays. Deep kicks were most common on special teams, and many returned and nonreturned kicks resulted in athletes diving toward the ball. These results support policy changes to youth special teams plays, including modifying the yard line the ball is kicked from and coaching proper return technique. Further investigation into biomechanical exposure measured during game impact scenarios is needed to inform policy relevant to the youth level.

Concussions have received attention in recent years; however, subconcussive head impacts, those that do not result in signs and symptoms of concussion, are more common and may also have a negative effect on brain health. There is increasing evidence that subconcussive head impacts sustained in contact sports, such as football, may lead to brain alterations, even after a single season of play.^{1,2,9,14,15,28,36,37} Although prevention efforts have been implemented, enrollment in youth tackle football has trended downward over the last several years, possibly due to parental concerns for safety.³⁵ There is a growing body of literature informing efforts to improve the safety of football;^7,8,22,30,33 however, research relating on-field activity to head impacts in youth football is limited.

Head impact sensors are a valuable tool to measure and evaluate head impact exposure (HIE) during concussive and subconcussive events in a real-world setting.^{5,6,22,23,27,39} Previous research has highlighted the positional differences in HIE.^6,13 High school offensive schemes may also play a role in the resulting HIE with pass-first offenses exposed to higher-magnitude impacts and run-first offenses exposed to more frequent, lower-magnitude head impacts.²⁷ Previous youth football studies have demonstrated that youth athletes are exposed to a greater number of impacts per session in games than in practices; however, the majority of head impacts measured in a season are attributed to practices.²³ Additionally, older youth football players are exposed to high-magnitude impacts more frequently in games than their younger counterparts, who are exposed to high-magnitude impacts more frequently during practices.²³

As more studies have incorporated on-field video analysis into biomechanics data review, more information has been gleaned about the context in which HIE occurs. For example, earlier research has demonstrated that HIE varies by play type at the high school and collegiate levels of football.^30,33 Additionally, struck players have higher impact magnitudes than striking players involved in a tackle,^30,34,42 and contact resulting from greater closing distances results in higher-magnitude hits.^7,22,30,33 In youth football games, most high-magnitude head impacts occur in the open field.⁷ Full-speed tackling drills that resemble this mechanism of contact result in significantly greater linear and rotational acceleration compared with lower-speed drills focused on the development of blocking and tackling techniques;²² however, how variations in play type affect HIE at this level of play is unknown.

Prior research has revealed important evidence informing rule changes in football. Special teams plays, often involving player-to-player contact at larger closing distances, have been a particular focus of prevention efforts with rule changes at the collegiate and professional levels of football.^10,17,30,31 Recent efforts have been made at the youth level to eliminate or modify (e.g., moving the line at which the ball is kicked) full-speed kickoff and punt returns; however, these rule changes are not ubiquitous at the youth level and are sometimes not informed by age or context-specific evidence about HIE. Therefore, the objective of this study is to evaluate differences in HIE measured among different play types during youth football games under the guidelines of American Youth Football (AYF) in which special teams plays have not been eliminated or modified.

Methods

Athletes participating on one of 3 local age- and weight-based youth football teams (level A: 10 years or younger, maximum weight 124 lbs, or age 11 years, maximum weight 104 lbs; level B: 11 years or younger, maximum weight 139 lbs, or age 12 years, maximum weight 119 lbs; and level C: 12 years or younger, maximum weight 159 lbs, or age 13 years, maximum weight 139 lbs) during the 2015 season were evaluated in this study approved by the Wake Forest School of Medicine institutional review board (Table 1). All athletes provided written assent and parental consent to participate in this study. All teams participated under the guidelines of AYF with mandatory play rules, meaning that athletes must play a minimum number of plays depending on how many athletes are present, excluding special teams plays. All players were fitted with youth Riddell Speed football helmets containing Head Impact Telemetry System (HITS) MX Encoders. The HITS is a spring-mounted system intended to maintain contact with the head.⁴ Therefore, to ensure proper fit of the helmet with the sensor, athletes were fit to each helmet by research assistants trained in proper helmet fitting. Research assistants monitored the HITS and collected on-field video data during each game. On-field video was time-synced with data from the HITS, and all impacts that did not occur during the game or did not result from the helmet being worn on the athlete’s head (i.e., dropped helmet) were removed from the final data set. Data processing algorithm and data collection methodology used in this study have been previously described.^12,23

TABLE 1.

Summary of athletes and games evaluated in this study

Level*	Age Requirement (yrs)	Season Max Weight (lbs)†		No. of Athletes	Mean Age (yrs)	Mean Mass (kg)	No. of Games
		Younger Age	Older Age
A	≤10 & 11	124	104	9	11.2 ± 0.6	44.9 ± 4.1	10.5
B	≤11 & 12	139	119	12	12.3 ± 0.6	43.5 ± 6.9	7
C	≤12 & 13	159	139	8	12.9 ± 0.6	56.8 ± 14.5	10

Max = maximum.

Mean values are mean ± SD.

^*Half of 1 level A game and 2 level B games were excluded from the analysis.

^†Five pounds is allowed at precompetition weigh-in for all equipment.

Two independent reviewers analyzed the on-field video data using Kinovea (experimental version 0.8.26) video analysis software independent of the biomechanics. For each game, the precise start and stop time of each play was recorded. The start time was recorded when the ball was snapped. The stop time was recorded when contact associated with the play had ceased. Each play was labeled as an offensive or defensive play and was further categorized as a down, punt, kickoff, field goal, or false start (Fig. 1). Kickoffs and punts were classified as special teams plays and were further categorized as returned and nonreturned kicks. Returned kicks included all kickoffs and punts in which the receiving team attempted to return the ball back up the field. Nonreturned kicks included fair catches, onside kicks, kicks that struck the ground before traveling 10 yards, and when a player or group of players dove on the ball without returning the ball. A description of the kick types is provided in Table 2. Field goals and extra point attempts were not included in the categorization of special teams plays as they did not involve opposing teams approaching one another from either end of the field.³⁰ Offensive and defensive downs were classified as running, passing, or other. Running plays were coded for quarter-back (QB) handoffs, QB-designed runs, and short shuffle passes. Passing plays were coded when the ball was thrown over the line of scrimmage. Other plays include sacks and trick plays. One evaluator reviewed all coded materials to ensure proper labeling of game plays and precise start and stop times. Finally, the time of each play classification was paired with the time-stamped biomechanical data collected from the HITS, resulting in an assigned play for each head impact collected during each coded play.

FIG. 1.

Flowchart demonstrating the play classification scheme.

TABLE 2.

Video abstraction form completed for each special teams play

Question	Response
Is the instrumented team the receiving or kicking team?	○ Kicking ○ Receiving
What was the result of the kick?	□ Player(s) dives on ball □ Fumble; kicked out of bounds □ Onside kick: short kick resulting in ball bouncing/rolling 10 yards □ Downed ball/blocked □ Other team touches the ball □ Does not go 10 yards, no contact □ Does not go 10 yards, contact □ Player takes a knee □ Fair catch: receiving player waves hand in air before catching the ball. If caught, the ball may not be returned. If fumbled, the ball is considered “live.” □ Sky kick: high, short kick with longer hang time, targeting the middle returners □ Let the ball roll □ Touchback: ball caught in the endzone □ Squib: low trajectory, high speed kick □ Deep kick: ball is kicked deep into the returner’s territory □ Unknown
Check all possible contact that occurs during the play	□ Tackling □ Blocking □ Diving for the ball □ Tripping/falling to the ground □ Aggressive blocking/unnecessary rough contact □ Illegal contact
Speed at which the receiving player is moving prior to contact	○ Stationary: no visible foot movement ○ Slow: walking ○ Moderate: jogging (non-purposeful running with low knee lift) ○ Fast: running/sprinting (purposeful running with maximal effort, high knee lift) ○ Inconclusive ○ NA, no ball carrier
Was the receiving player aware of impending contact situation?	○ Appeared unprepared ○ Appeared to anticipate contact ○ Neither of the above ○ Inconclusive ○ NA, no ball carrier
Was the receiving player involved in a single player or multiplayer tackle?	○ Single ○ Multiplayer ○ Inconclusive ○ NA, no ball carrier
Number of players in tackle of the ball carrier	○ 1 ○ 2 ○ 3+ ○ Inconclusive ○ NA, no ball carrier
Was the closing distance for players involved in the tackle >5 yards or ≤5 yards?	○ >5 yards ○ ≤5 yards ○ NA, no ball carrier

NA = not applicable; ○ = single-option answer; □ = multioption answer.

To further evaluate contact that occurred on special teams plays, a standardized video abstraction form was completed for each play, including questions derived from previously published literature (Table 2).^{11,16,19,24–26} Game status (i.e., winning or losing) and quarter at the time of the play were recorded, when possible. Each special teams play was separately reviewed by 2 independent reviewers with prior experience playing football (reviewer 1, 10 years; and reviewer 2, 1 year). The video abstraction results were compared between the 2 independent reviewers, and discrepancies in coding were reevaluated until a consensus was reached. If a consensus was not reached, the discrepancy in coding was noted and the final coding was defaulted to the first reviewer’s selection who was deemed a subject-matter expert. Interrater relatability (K) was calculated for each question to compare agreement between raters and is presented in Table 3.

TABLE 3.

K statistics for interrater reliability for special teams plays

Video Abstraction Question	K
Result of kick	0.62
Contact during play	0.75
Speed of receiving player	0.92
Level of anticipation	0.83
Single-/multiplayer tackle	0.95
No. of players in tackle	0.92
Closing distance	0.85

Summary statistics (mean, median, 95th percentile) of the head impact data were computed from the distribution of peak resultant linear and rotational acceleration. Mixed-effects models were used to assess differences in acceleration among offensive and defensive plays as well as running, passing, and special teams plays while adjusting for impact date and age/weight level. Within special teams, differences in returned and nonreturned kicks were evaluated. Statistical analysis was performed using SAS (version 9.4, SAS Institute Inc.).

Results

Head impact data (n = 3772) were collected across 30 games during the season. Half of 1 level A and 2 level B games were excluded from the analysis due to insufficient film quality. Therefore, for this study, video footage was evaluated for 3003 head impacts (79.6%) from 29 athletes (ages 10–13 years) participating on 3 age- and weight-restricted youth football teams over 27.5 games and 2134 plays. Excluding plays halted by false-start penalties, there was an average of 80.2 ± 19.0 (level A), 66.7 ± 9.3 (level B), and 73.8 ± 18.8 (level C) plays per game. Among all plays, impacts were measured on 69.5% (level A), 66.6% (level B), and 65.0% (level C) of the plays. The proportion of impacts attributed to offensive plays for each age/weight level was 52.9% for level A, 49.9% for level B, and 45.8% for level C. For the entire sample, a total of 1626 impacts (54.1%) were measured on defensive plays, and 1377 impacts (45.9%) were measured on offensive plays.

The number of game impacts per season per athlete ranged from 4 to 533 with a median (95th percentile) value of 103 (401) game impacts per season and an average of 14.3 ± 11.3 impacts per game. The 95th percentile linear and rotational accelerations were 54.9g and 2621.5 rad/sec², respectively, for defensive plays and 52.4g and 2428.3 rad/sec², respectively, for offensive plays. The mean (95% CI) linear acceleration measured during defensive and offensive plays was 21.7g (20.9g–22.5g) and 21.0g (20.2g–21.8g), respectively. The mean (95% CI) rotational acceleration was 934.0 (891.9–978.5) rad/sec² and 897.3 (854.8–941.8) rad/sec², respectively. There was no significant difference in the mean linear or rotational acceleration measured between offense and defense.

HIE by Play Type

Offensive and defensive plays were categorized as passing, running, and special teams plays. Of the 3003 head impacts evaluated, 209 impacts were classified as other (e.g., sacks and trick plays) and were excluded from the analysis, resulting in 2794 impacts attributed to running (n = 2223, 79.6%), passing (n = 391, 14.0%), and special teams (n = 180, 6.4%) plays. The mean linear acceleration measured for special teams plays was significantly greater than that for passing and running plays (p = 0.03 and p = 0.03, respectively). There was no significant difference in the mean rotational acceleration among play types. Among defensive plays, there was no significant difference in mean acceleration among play types. Among offensive plays, special teams plays resulted in significantly greater mean linear acceleration compared with passing and running plays (p = 0.04 and p = 0.003, respectively). Age/weight level did not have a significant effect on the mean acceleration by play type. A summary of the HIE measured among play types is provided in Table 4.

TABLE 4.

Summary of HIE for each play type

Type	Variable	Overall		Defense		Offense
		Linear Acceleration (g)	Rotational Acceleration (rad/sec2)	Linear Acceleration (g)	Rotational Acceleration (rad/sec2)	Linear Acceleration (g)	Rotational Acceleration (rad/sec2)
Pass	No. of impacts	391		242		149
	Mean (95% CI)	21.1 (19.9–22.3)	935.6 (864.8–1012.1)	20.8 (19.4–22.4)	865.8 (779.8–961.4)	21.6 (19.8–23.5)	1011.3 (893.0–1145.3)
	95%ile (95% CI)	50.7 (44.0–64.3)	2640.5 (2240.0–3164.0)	49.3 (42.1–69.9)	2308.5 (2144.4–3553.2)	57.2 (44.0–77.7)	3100.2 (2409.6–4331.1)
Run	No. of impacts	2223		1170		1053
	Mean (95% CI)	21.3 (20.6–22.0)	910.4 (873.9–948.4)	21.9 (21.0–22.9)	930.6 (878.6–985.7)	20.8 (19.9–21.8)	891.1 (845.6–939.1)
	95%ile (95%CI)	52.6 (50.2–56.5)	2470.8 (2392.5–2619.6)	55.6 (52.1–61.8)	2625.7 (2500.3–2774.5)	50.4 (46.9–53.9)	2266.6 (2118.4–2450.8)
Special teams	No. of impacts	180		101		79
	Mean (95% CI)	23.2 (21.5–25.1)	950.7 (849.5–1063.9)	21.9 (19.7–24.3)	926.3 (795.7–1075.3)	24.9 (22.2–28.0)	975.6 (822.2–1157.8)
	95%ile (95%CI)	65.5 (56.8–84.1	3435.0 (2898.9–4234.0)	70.2 (46.5–93.0)	3420.0 (2786.4–5394.1)	65.1 (55.5–83.0)	3525.4 (2714.3–3886.1)

Within special teams plays, differences in measured accelerations among returned and nonreturned kicks were evaluated. The mean (95% CI) linear acceleration measured on returned and nonreturned kicks was 25.1g (22.8g–27.6g) and 22.3 (19.1g–26.0g), respectively. The mean (95% CI) rotational acceleration measured was 1105.1 (943.1–1295.0) rad/sec² and 853.0 (658.5–1096.1) rad/sec², respectively. No significant difference in mean acceleration was observed between returned and nonreturned kicks; however, nonreturned kicks trended toward higher 95th percentile impact magnitudes (returned: 62.4g and 3295 rad/sec²; nonreturned: 78.3g and 3828.7 rad/sec²).

Video Abstraction of Special Teams Plays

On average, 90.4% of the coded responses from each special teams play were initially agreed on. The most common discrepancies were in coding the result of the kick (K = 0.62) and type of contact observed during the play (K = 0.75) (Table 3). Of the 27.5 games evaluated, there were 191 special teams plays, 85 (44.5%) of which resulted in measured head impacts from the athletes followed in this study. Overall, the most common event resulting from special teams plays was a deep kick (36.6%), followed by an onside kick (28.3%) and sky kick (13.6%). Of the special teams plays in which instrumented players received head impacts, the kick most commonly classified was a deep kick (46%), followed by a sky kick (15%) and onside kick (13%). Attempts were made to return 85% of deep kicks. No special teams plays resulted in a touchback, and only one resulted in a fair catch. One-third of all plays and 92% of all nonreturned kicks resulted in athletes diving toward the ball. The most common source of contact observed during kickoffs and punts was blocking, followed by tackling (Fig. 2). There were 10 plays in which aggressive/unnecessary contact was observed, and one play was noted as having illegal contact. Lastly, for all special teams plays in which a ball return attempt was made, the characteristics of contact involving the ball carrier were evaluated to better understand the contact involved with the ball carrier in youth-level special teams plays (Fig. 3).

FIG. 2.

Type of contact observed on special teams plays.

FIG. 3.

Characteristics of contact involving the ball carrier on special teams plays in which the ball was attempted to be returned.

Note: the result was deemed inconclusive if the response could not be determined from the video.

Discussion

HIE was compared between the age/weight levels and play types, revealing significantly greater linear acceleration measured during special teams plays than running and passing plays. Age/weight level did not have a significant effect on head impact magnitude among play types. Among special teams plays, the most common kick result was a deep kick, and only one kick resulted in a fair catch. On nonreturned kicks, players often dove toward the ground to capture the ball, possibly leading to more head-to-ground impacts.

Approximately two-thirds of the plays resulted in measured head impacts, which were nearly evenly split among offensive and defensive plays. The mean linear and rotational accelerations among offensive and defensive plays were similar, resulting in a 0.7g and 37-rad/sec² difference, and were comparatively lower than the mean values previously reported at the high school and collegiate levels.^30,33 Most of the plays (63.1%) conducted by the teams were running plays, resulting in 74% of the measured head impacts. A previous study found differences in HIE measured among offensive schemes at the high school level, reporting that a pass-first offense resulted in significantly higher-magnitude impacts than a run-first offense.²⁷ In contrast, a run-first offense was associated with more frequent, lower-magnitude head impacts. A previous study found that 59.4% of high-magnitude impacts occurred in the open field during competitions, a scenario that often occurs on passing play types. While the present study did not have a substantial number of passing plays for comparison, similarities in the HIE measured among running and passing plays were observed, with a nonsignificant 1.9g and 169.7-rad/sec² difference in 95th percentile accelerations measured among the 2 play types.

The frequency in running plays observed in this study may be due to the athletes developing passing, catching, and coordination skills at this level of play;^3,41 however, it may also be due to coach preferences. This underscores the importance of an age-, weight-, or level of play–specific prevention strategy. Underdeveloped passing/catching skills on a team may lead to greater utilization of running plays, which may be associated with more frequent head impacts. Additional study at the youth level among different teams, organizations, and regions will help better elucidate the influence of coaching style and play type on HIE at the youth level. Knowing the level at which offensive and defensive strategies and exposure diverge will allow for a more effective and targeted approach to reduce HIE in youth football games.

Special teams plays resulted in significantly greater linear acceleration than running and passing plays (each p = 0.03). In the present study, 6.4% of head impacts were measured during special teams plays, which is lower than the rate at the collegiate level, where approximately one-quarter of head impacts have been attributed to special teams plays.³⁰ The overall mean linear acceleration measured during this play type was 23.2g, compared with 21.1g and 21.3g measured on passing and running plays, respectively. Schmidt et al. did not find a significant difference among play types when comparing offensive and defensive downs to high-contact (punts and kickoffs) and low-contact (field goals and extra point attempts) special teams plays at the high school level.³³ Overall, though, the mean linear accelerations among all play types were higher at the high school level (offensive down, 25.6g; defensive down, 26.0g; special teams high contact, 25.9g; special teams low contact, 23.9g) than those reported in the present study. The 95th percentile linear acceleration measured during special teams plays was 65.5g. Comparatively, the 95th percentile linear acceleration of special teams plays reported at the collegiate level was 81.6g in long closing distance plays and 52.7g at short closing distances.³⁰

Greater impact magnitudes in special teams plays compared with other play types have been reported at the high school and collegiate levels due to the full-speed nature of this play type, resulting in player-to-player contact at larger closing distances.^10,17,30,31 Additionally, kickoffs and punts may result in a greater risk of concussion than running and passing plays, despite more time spent on running and passing plays.^20,31 Injury rates have declined with recent rule changes aimed at reducing closing distances and increasing the number of touchbacks.³² At the youth level, some organizations have eliminated full-speed special teams plays, while others have simply modified the location of the kickoff yard line. While the present study demonstrates a trend toward higher head impact magnitudes on special teams plays as demonstrated at the high school and collegiate levels, there are limited data for comparison at this level of play. Further study is needed in youth football to adequately inform policy related to special teams plays at the youth level; however, the results of the present study support modification of special teams plays to reduce exposure to higher-magnitude hits. Given that only one kick in the present study resulted in a touchback, the yard line at which the rate of touchbacks increases at the youth level of play should be determined, and informed by data, for future policy.

Level of play has been previously shown to influence mean linear acceleration at the youth level of play;²³ however, it did not influence accelerations observed among play types. There are several factors that may be attributed to the lack of relationship, including wide variability in HIE observed within individuals participating at each level of play in youth football.^23,40 The individual factors that may influence HIE include prior tackle football experience, attitudes and behaviors, and tackling technique.^18,34,38 A team’s practice structure may also influence a player’s skill and potential exposure to contact in games.^8,21,22 Lastly, situational factors, such as playing position,^7,13 the player’s role in the tackle (i.e., striking vs struck),^7,30 and closing distance,^7,22,30 may play a greater role in the resulting head acceleration than the size or age of the player alone.

Characterizing the result and type of contact involving the ball carrier observed on special teams plays revealed that kickoffs and punts most often resulted in a deep kick, for which 85% of the time a return was attempted. One-third of all plays and 92% of all nonreturned kicks resulted in athletes diving toward the ball. The tendency of athletes in this study to dive toward the ball may have increased the number of head-to-ground impacts, although the contact surface of each head impact was not recorded in this study. If so, this may have contributed to the increased 95th percentile linear and rotational acceleration values observed among nonreturned kicks. Youth coaches should address this tendency to dive toward the ball when teaching proper return technique on special teams plays, including identifying when and how to call a fair catch in an effort not to advance the ball. This will help protect the ball carrier, reduce contact, and ultimately reduce HIE.

Conditions involving the ball carrier on special teams plays in which a ball return was attempted were examined in this study. Eighty-three percent of all returns involved fast, purposeful running of the ball carrier. This player was most often involved in a single-player tackle for 52% of the plays. Contact often (47% of plays) occurred from 5 or more yards of separation, and most ball carriers appeared to anticipate contact from opposing athletes (55%). While head impact magnitude was not evaluated with respect to anticipation of contact in the present study, Schmidt et al. previously reported that level of anticipation did not influence head impact magnitude.³³ Schmidt et al. also noted that level of anticipation is not a binary variable and is often difficult to determine from on-field video captured while athletes are helmeted. Our experience was similar, with one-third of special teams plays coded inconclusive for anticipation of contact, and resulted in a lower interrater reliability (K = 0.83) than many of the other questions. More objective measures are needed to evaluate the relationship between visual perception and anticipation of contact and head impact magnitude, particularly for young athletes who are still developing proper skills in the sport.^29,43 Lastly, it is important to note that contact occurring on special teams plays was not isolated to the ball carrier, as blocking and tackling were prevalent among most players on the field in an effort to clear the path for the ball carrier to advance.

The results of this study provide important insights regarding HIE resulting from game plays in youth football; however, there are limitations that should be considered. Although data were collected from 3 different teams in North Carolina, this is a limited perspective of the larger US youth football population. The athletes participated in teams under the guidelines of AYF. Other youth football organizations or leagues may have rules, regulations, styles of play, and/or coaching styles that differ from the participants in the present study and may also differ by geographic region. These factors may influence the frequency of head impacts among the different play types, and possibly the magnitudes; therefore, the results may not be generalizable outside the study sample. The HITS used in this study has some measurement error (15.7%) associated with individual impacts but an average overall error of only 1%.⁴ To minimize the effect of this error, analysis was conducted on distributions of data rather than on individual acceleration measurements.¹⁰ It should also be noted that errors in acceleration measurements are within the range of acceptable error for other measurement devices and methods. Additionally, the HITS is the most widely used commercial head impact sensor.⁴ Additionally, several special teams’ video abstraction questions relied on subjective determination of what was happening on field, requiring prior knowledge of the sport. While the 2 reviewers had experience playing and watching the sport, one reviewer had far more experience (10 years) and, thus, discrepancies in coding were defaulting to his responses, possibly resulting in observer bias. Lastly, athlete plays-per-game were not evaluated in the present study; however, the focus of this study was on population-level HIE by play type rather than individual game impact exposure. Future studies evaluating individual variations among HIE in games may benefit from incorporating participation-based measures of HIE.

Conclusions

Differences in HIE were evaluated between play types during youth football games conducted by 3 youth football teams. The results of this study demonstrate that most head impacts measured during a game are attributed to running plays; however, a trend toward higher head impact magnitudes was observed on special teams plays. Age/weight level did not have a significant effect on head impact magnitude among play types. Within special teams plays, the most common kick type was a deep kick, with the majority attempted to be returned; however, many returned and nonreturned kicks resulted in athletes diving toward the ball. The results of this study encourage clinicians, parents, and athletes to be informed of the rules and regulations surrounding game play of local youth football leagues and highlight the opportunity for special teams rule changes, supporting modification of special teams plays to reduce exposure to higher-magnitude hits at the youth level of play. This may include establishing the yard line at which the rate of touchbacks increases at this level of play. Additionally, coaches at the youth level with teams participating in leagues with live special teams plays should incorporate instruction on proper return technique in practices. Lastly, this study highlights the need to further investigate biomechanical exposure measured during youth football game impact scenarios to adequately inform policy related to special teams plays.

ABBREVIATIONS

AYF

American Youth Football

HIE

head impact exposure

HITS

Head Impact Telemetry System