Subconcussive head impact exposure between drill intensities in U.S. high school football

Kyle Kercher; Jesse A Steinfeldt; Jonathan T Macy; Keisuke Ejima; Keisuke Kawata

doi:10.1371/journal.pone.0237800

. 2020 Aug 14;15(8):e0237800. doi: 10.1371/journal.pone.0237800

Subconcussive head impact exposure between drill intensities in U.S. high school football

Kyle Kercher ¹, Jesse A Steinfeldt ², Jonathan T Macy ¹, Keisuke Ejima ³, Keisuke Kawata ^4,^5,^*

Editor: Markus Geßlein⁶

PMCID: PMC7428124 PMID: 32797073

Abstract

USA Football established five levels-of-contact to guide the intensity of high school football practices. The objective of this study was to examine head impact frequency and magnitude by levels-of-contact to determine which drills had the greatest head impact exposure. Our primary hypothesis was that there would be an incremental increase in season-long head impact exposure between levels-of-contact: air<bags<control<thud<live. This observational study included 24 high-school football players during all 46 practices, 1 scrimmage, 9 junior varsity and 10 varsity games in the 2019 season. Players wore a sensor-installed mouthguard that monitored head impact frequency, peak linear acceleration (PLA), and rotational acceleration (PRA). Practice/game drills were filmed and categorized into five levels-of-contact (air, bags, control, thud, live), and head impact data were assigned into one of five levels-of-contact. Player position was categorized into lineman, hybrid, and skill. A total of 6016 head impacts were recorded during 5 levels-of-contact throughout the season. In the overall sample, total number of impacts, sum of PLA, and PRA per player increased in a near incremental manner (air<bags<control = thud<live), where live drills had significantly higher cumulative frequency (113.7±17.8 hits/player) and magnitude [2,657.6±432.0 g (PLA), and 233.9 ± 40.1 krad/s² (PRA)] than any other levels-of-contact, whereas air drills showed the lowest cumulative frequency (7.7±1.9 hits/player) and magnitude [176.9±42.5 g (PLA), PRA 16.7±4.2 krad/s² (PRA)]. There was no significant position group difference in cumulative head impact frequency and magnitude in a season. Although there was no difference in average head impact magnitude across five levels-of-contact and by position group PLA (18.2–23.2g) and PRA (1.6–2.3krad/s²) per impact], high magnitude (60-100g and >100g) head impacts were more frequently observed during live and thud drills. Level-of-contact influences cumulative head impact frequency and magnitude in high-school football, with players incurring frequent, high magnitude head impacts during live, thud, and control. It is important to consider level-of-contact to refine clinical exposure guidelines to minimize head impact burden in high-school football.

Introduction

The long-term consequence of sport-related head injury is a complex public health issue with no concrete solution [1, 2]. Despite inherent risk of head injury in contact sports (e.g., American football, hockey, soccer), participating in these team sports, especially during developmental age, provides well-documented benefits, including higher levels of physical activity, improved mental health, and lower likelihood of smoking cigarettes and using illegal substances [3]. In 2017, in an attempt to promote a safer football environment, USA Football (the national governing body over amateur football) developed a modified version of tackle football to introduce kids to the sport by reducing the field size and number of players, as well as rule changes to increase activity, game play, and learning [4]. These types of safety modifications are further substantiated in high school, college, and professional football. For example, in 2018 kickoff rules were adjusted to reduce injuries due to high concussion incidence during kickoff plays [5], players must wear helmets that meet certain laboratory safety standards, and a hit to the head or neck area and blindside blocking are prohibited [6, 7]. Consistent with these adjustments, concussion and catastrophic injury rates have been reduced [8, 9]; however, subconcussive head impact exposure has proven more complex.

Subconcussive head impact is defined as a hit to the head that does not induce overt concussion symptoms [10]. These head impacts are most common in American football, where athletes can experience several hundred impacts with some exceeding 1,000 head impacts in a single season [11]. Evidence has emerged to indicate that both high school and college football players with frequent experience of subconcussive head impacts exhibit neuronal microstructural damage [12, 13], abnormal brain activation [14, 15], ocular-motor impairment [16], and elevation in brain-injury blood biomarkers [17]. One line of research suggests that long-term exposure to these hits is a key factor in developing neurodegenerative disorders later in life [18, 19]. Although USA Football (for high school) and the NCAA (for college) have eliminated practicing two times in the same day (two-a-days) to minimize head impact frequency, one study found that total head impact frequency during a summer camp increased by 26% [20]. Similarly, a policy change to reduce the number of preseason practices from 29 to 25 failed to reduce head impact frequency in college football players, with one team’s cumulative head impacts increasing up to 35% [21]. These mixed results highlight the need to dissect football practices to increase our understanding of what type of contact drills and intensities cause the greatest frequency and magnitude of subconcussive head impacts.

USA Football has identified five levels-of-contact (air, bag, control, thud, live) that define the intensities and structure of football practices nationwide. The levels-of-contact were designed to guide effective practice schedules through a step-by-step approach to teach fundamental football skills [22]. The National Federation of State High School Associations implemented the levels-of-contact in their high school football practice guidelines beginning in 2014. However, it remains unknown whether, and to what extent, different levels-of-contact influence head impact frequency and magnitude in high school football players across an entire season and between position groups.

Therefore, we conducted a longitudinal observational study to examine cumulative head impact frequency and magnitude across different drill intensities in high school football players over the course of a single season. Our primary hypothesis was that there would be an incremental increase in season-long head impact frequency and magnitude between levels-of-contact, with live recording the greatest head impact exposure and air recording the lowest: air < bags < control < thud < live. Since the proximity to opponents and nature of contact during nearly every play for linemen [23], we also tested our secondary hypothesis that there would be a group difference in head impact frequency, in which linemen would have greater head impact frequency in most levels-of-contact, compared to the hybrid and skill positions. Our exploratory aim was to identify the average head impact magnitudes by levels-of-contact and position group, as well as to identify frequency of head impacts that were within 25–60 g, 60–100 g, or >100 g in each level-of-contact.

Methods

Participants

This single-site, observational study included 24 male high school football players at Bloomington High School-North. The study was conducted during the 2019 football season including practices and games during the pre-season, in-season, and playoffs. None of the 24 players was diagnosed with a concussion during the study period as confirmed by team athletic trainer and physician. Inclusion criterion was being an active football team member which was defined as any player, freshmen through seniors, planning to participate in the 2019 season. Exclusion criteria included a history of head neck injury (including concussion) in the previous year or any neurological disorders, although no participant met any exclusionary criteria. The Indiana University Institutional Review Board and the Monroe County Community School Corporation Research Review Board approved the study, and all participants and their legal guardians gave written informed consent.

Study procedures

At the preseason data collection, self-reported demographic information (age, race/ethnicity, height, weight, number of previously diagnosed concussions, and years of experience in various contact sports, including tackle football) were obtained. Participants were custom-fitted with the Vector mouthguard (Athlete Intelligence, Inc.) that measured the number of hits and magnitude of head linear and rotational acceleration. Participants wore the mouthguard for all practices (n = 46), scrimmage (n = 1), and all games (n = 9 junior varsity, n = 10 varsity) from pre-season training camp (August 13, 2019) to the end of the season (November 1, 2019). The mean (SD) practice duration was 105 (20.5) minutes in duration. Video data were collected using Hudl (Agile Sports Technologies, Inc.) during the same timeframe as subconcussive head impact data collection. Participants’ playing positions were verified by team coaches and categorized into three groups as follows: 11 linemen athletes (defensive lineman, offensive lineman), 7 hybrid athletes (tight end, linebacker, running back), and 6 skill athletes (wide receiver, defensive back), which is in line with prior literature [24, 25]. No quarterbacks participated in this study. In accordance with USA Football guidelines [26], head impacts were categorized by levels-of-contact: air, bags, control, thud, and live. See Levels-of-Contact and Film Review section for more details and supplemental file A for example video for each level-of-contact.

Head impact measurement

This study used an instrumented Vector mouthguard for measuring frequency of head impacts as well as linear and rotational head accelerations during impacts, as previously described [27]. The mouthguard employs a triaxial accelerometer (ADXL377, Analog Devices) with 200 g maximum per axis to sense linear acceleration. For rotational acceleration, a triaxial gyroscope (L3GD20H, ST Microelectrics) was employed. An impact is detected when a linear acceleration magnitude exceeds 10.0 g for three consecutive samples (sampling every 0.2 milliseconds). All impact with a standard hit duration of 96 milliseconds were transmitted wirelessly through the antenna transmitter to the sideline antenna and computer, then stored on a secure internet database. The Vector mouthguard is installed with an in-mouth sensor to ensure that data acquisition occurs only when the mouthguard is securely fitted in one’s mouth. Linear acceleration data were transformed within the Athlete Intelligence software to the head’s center of gravity based on the 50^th percentile male. From raw impact data extracted from the server, the number of hits, peak linear acceleration (PLA) of each hit, and peak rotational acceleration (PRA) of each hit were used for analyses. Kinematic accuracy of the prototype of Vector mouthguard [16] showed an excellent correlation with the matched data from an anthropomorphic testing device (crash test dummy) [28, 29]. When the mouthguard, headgear-mounted, and skin-patch sensors were compared to high speed video during soccer headings, the mouthguard showed superior skull coupling (displacement < 1 mm) compared to headgear (< 13mm) and skin patch (< 4mm) for the ear canal reference point [30]. A researcher was present during all practices to track when the practice shifted between levels-of-contact, and head impact data were categorized into each level-of-contact by corresponding timestamps of head impact to timeframes of each level-of-contact.

Levels-of-contact and film review

The five levels-of-contact are air, bags, control, thud, and live with air being estimated to have the lowest intensity and live being the highest [26]. Air is defined as drills being run unopposed and without contact. Bags is defined as drills being run against a bag or soft-contact surface. Control is defined as drills being run at an assigned speed until the moment of contact. It does not involve tackling, rather contact is above the waist and players stay on their feet. Thud is defined as drills being run at a competitive, fast speed through the moment of contact. It does not involve full tackling, rather contact is above the waist and players stay on their feet and a quick whistle ends the drills. Live is defined as drills being run in game-like conditions that include live-drill during practice as well as real games. Live should be the only time players are allowed to fully tackle another player to the ground. All head impacts in air, bags, control, and thud were during practices, whereas live occurred in both practices and games.

Statistical analysis

Our primary aim was to examine whether cumulative head impact frequency, PLA, and PRA from a single season differ between 5 levels-of-contact (air, bags, control, thud, and live). Our secondary aim further examined the difference in these head impact measures between 3 position groups (lineman, hybrid, and skill). Three-way repeated measures ANOVA models were used to compare outcome variables (season-long cumulative head impact frequency, PLA, and PRA normalized per player) on 5 levels-of-contact and 3 groups. The assumption of sphericity was assessed with Mauchly’s test and resulted in violation of sphericity (p<0.01), thus the Greenhouse-Geisser correction was used to report within-subject outputs, followed by the effect size (Partial Eta Squared). When there was a significant effect for levels-of-contact and/or group, then Bonferroni post-hoc tests were used to determine where the difference in head impact outcome occurred. For the exploratory aim, we similarly assessed whether average head impact magnitude differed by levels-of-contact and group using repeated measures ANOVA. Lastly, a total number of head impacts within ranges of 25–60 g, 60–100 g, or >100 g in each level-of-contact was descriptively assessed in the overall sample. These exploratory thresholds are modeled based on the published papers that suggested < 25 g as minimal magnitude [11, 31], 60 g being previously thought to be a cut-off threshold to induce concussion [32], and 100 g (precisely, 102.5 + 33.8 g) being an average magnitude leading to concussion diagnosis [24, 33]. All the data were analyzed using SPSS Statistics Version 25, and the level of statistical significance was set to p<0.05. Data are presented per player.

Results

Demographics and overall head impact exposure

A total of 6016 head impacts were recorded during 5 levels-of-contact in 24 high school football players throughout the season, resulting in a median of 203 hits, 4310.5 g, and 415.5 krad/s². Consistent with previous reports [13, 34], the distribution of head impact count was strongly right skewed with a median PLA of 19.7 g (interquartile range: 15.3–27.8 g) and PRA of 1.8 krad/s² (interquartile range: 1.2–2.6 krad/s²) per impact (Fig 1). These data are not reflective of head impacts that occurred outside the 5 levels-of-contact, such as walk-through and pre-practice/game conditioning. For comparison purposes, a driver can experience 30 g to 40 g of head and chest acceleration when a car collides into a fixed wall at 30 mph [35, 36]. Demographics and head impact data in the overall sample are detailed in Table 1.

Fig 1 — A representative data from a defensive lineman demonstrates the estimated locations of hits based on the data from the Vector mouthguard (A, front right; B, front left; C, back). Histogram of distribution of peak linear acceleration (D) and peak rotational acceleration (E) for all hits occurred during 5 levels-of-contact.

Table 1. Group demographics and head impact kinematics.

Variables	Overall	Linemen	Hybrid	Skill
N (%)	24 (100)	11 (46)	7 (29)	6 (25)
Age, y	15.7 ± 1.1	16.1 ± 0.9	15.1 ± 1.2	15.7 ± 1.0
BMI, kg/m²	27.3 ± 6.3	31.6 ± 7.0	24.4 ± 2.5	23.0 ± 0.8
No. of previous concussion
0, n (%)	16 (66.7)	6 (54.5)	0 (0)	3 (50.0)
1, n (%)	6 (25.0)	4 (36.4)	0 (0)	2 (33.3)
2, n (%)	2 (8.3)	1 (9.1)	0 (0)	1 (16.7)
Tackle football experience, y	4.9 ± 2.7	5.8 ± 2.7	3.6 ± 2.5	4.8 ± 3.4
Race, n (%)
White	21 (88)	9 (82)	7 (100)	5 (83)
Black/African American	0 (0)	0 (0)	0 (0)	0 (0)
Asian	0 (0)	0 (0)	0 (0)	0 (0)
American Indian/Alaska	1 (4)	0 (0)	0 (0)	1 (17)
Multiracial	2 (8)	2 (18)	0 (0)	0 (0)
Ethnicity, n (%)
Not Latino/Hispanic	20 (83)	9 (82)	5 (71)	6 (100)
Latino/Hispanic	4 (17)	2 (18)	2 (29)	0 (0)
Impact Kinematics for season, median (IQR)
Median cumulative impact count	203 (118.0–350.0)	213 (139.0–478.0)	204 (153.5–314.5)	131.5 (51.3–263.5)
Median cumulative peak linear acceleration, g	4310.5 (2686.3–8616.8)	4289.5 (3178.6–11143.9)	5262.7 (3445.1–7203.1)	2837.7 (1117.2–6269.6)
Median cumulative peak rotational acceleration, krad/s²	415.5 (179.2–828.0)	358.7 (203.2–919.3)	438.6 (293.0–655.9)	296.5 (132.6–569.6)

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Data are reported as either mean (SD) or n (%), except for head impact data using median (IQR). BMI, body mass index. IQR, interquartile range. OL, offensive lineman. DL, defensive lineman. TE, tight end. LB, linebacker. RB, running back. WR, wide receiver. DB, defensive back.

Level-of-contact-dependent cumulative head impact exposure

Levels-of-contact displayed an influence on cumulative head impact frequency and sum of PLA and PRA sustained during a season, as illustrated by a statistically significant main effect and medium effect size on levels-of-contact in the overall sample [Frequency, F(1.95, 40.88) = 22.44, p<0.001, η_p² = 0.517; PLA, F(1.77, 37.08) = 21.70, p<0.001, η_p² = 0.508; PRA, F(1.62, 34.15) = 20.56, p<0.001, η_p² = 0.494]. Bonferroni post-hoc tests revealed incremental increases in head impact frequency and sum of PLA and PRA as levels-of-contact intensify, except for between control and thud (air < bags < control = thud < live). For example, a football player experienced an average of 113.7±17.8 hits, 2,657.6±432.0 g (PLA), and 233.9 ± 40.1 krad/s² (PRA) during live drills throughout a season, whereas 7.7±1.9 hits, 176.9±42.5 g (PLA), PRA 16.7±4.2 krad/s² (PRA) during air drills throughout a season. See Fig 2A–2C for the visual trend of the outcomes and S1 Table for Bonferroni post-hoc results.

Fig 2 — Cumulative (A) head impact count, (B) peak linear acceleration, and (C) peak rotational acceleration per player was influenced by the level-of-contact in an incremental manner, with *live* being the highest and *air* being the lowest. Data are presented as mean ± SD. Bonferroni post-hoc results are listed below: please refer to S1 Table for exact p-values of all possible comparisons. Please refer to S2 Table for median (IQR) values of head impact kinematics. @ Live is greater than thud (p = 0.045), control (p<0.001), bags (p<0.001) and air (p<0.001). # Thud is lesser than live (p = 0.045), no difference from control (p = 0.095), and greater than bags (p = 0.013) and air (p = 0.008). & Control is lesser than live (p<0.001), no difference from control (p = 0.095), and greater than bags (p = 0.008) and air (p = 0.003). $ Bags is lesser than live (p<0.001), thud (p<0.01), and control (p = 0.008), and greater than air (p = 0.041). * Air is lesser than live (p<0.001), thud (p = 0.007), control (p = 0.003), and bags (p = 0.041).

While all 3 position groups exhibited similar incremental patterns of head impact frequency and magnitude (air < bags < control < thud < live), there was no significant group difference in cumulative frequency as well as sums of PLA and PRA (Fig 3A–3C). Head impact kinematics for each level-of-contact are detailed in S2 Table.

Fig 3 — All 3 groups shared similar incremental pattern in cumulative (A) head impact count, (B) peak linear acceleration, and (C) peak rotational acceleration per player, with *live* being the highest and *air* being the lowest. There was no group difference in the cumulative head impact frequency or magnitude. Data are presented as mean ± SD. Please refer to S2 Table for median (IQR) values of head impact kinematics.

Head impact magnitudes by levels-of-contact and position group

Median head impact magnitudes (PLA and PRA) were similar across all levels-of-contact and all groups, ranging between 18.2 and 23.2 g for PLA (Fig 4A) and 1.6 and 2.3 krad/s² for PRA (Fig 4B) per head impact. See S3 Table for median PLA and PRA in each level-of-contact. Consistent with published papers [13, 23, 34], it was evident that a large number of head impacts across all levels-of-contact fell within 10 to 30 g, which might have diluted the minority of high magnitude head impacts. Our exploratory descriptive analysis identified that levels-of-contact also influence the number of high magnitude head impacts, whereby 60–100 g and >100 g were most prevalent in live, followed by thud and control, whereas very few hits were observed in bags and air (Fig 5A–5C).

Fig 5 — A total number of head impacts from the overall sample throughout a season was categorized into peak linear acceleration ranging (A) 20–60 g, (B) 60–100 g, and (C) > 100 g. *Live* and *thud* consistently showed frequency head impacts in high impact magnitudes.

Discussion

Subconcussion research is still at its infancy, but it is a rapidly growing area of concern in sport injury prevention. To contribute to this emerging field, this study examined whether cumulative subconcussive head impact frequency and magnitude in a single season differed across levels-of-contact and between player position groups. There were four key findings in this study. First, cumulative head impact frequency and magnitude increased as the level-of-contact increased, with the greatest head impact burden observed during live, followed by thud and control drills, and minimal head impacts during bags and air drills. Second, there were notable position group differences in head impact measures, where median values of head impacts in the linemen and hybrid players were greater in all levels-of-contact than those of the skill players. However, this was not supported by statistically significant group differences likely due to the lack of sample size in each group. Third, the mean head impact magnitude was similar (18 to 23 g) across all levels-of-contact. Lastly, very high impact magnitudes (>100 g) were small in number overall but were more frequent in thud and live than other levels-of-contact. Taken together, our data, for the first time, empirically support the USA football’s categorization of levels-of-contact while calling for a need to dissect football practice guidelines to make more specific recommendations for practice and games to minimize cumulative head impact burden on adolescents’ brain health.

Owing to the sensor-installed helmets, mouthguards, headbands, and skin patches, our knowledge of head impact exposure in American football has drastically improved in the past 15 years. These technological advancements allowed researchers to evaluate head impact frequency and magnitude in various position groups, practice types (e.g., shell-only, full-gear), play types (e.g., running, passing, special teams), and time-based hit rates [37–43]. Previous research has suggested that overall head impact exposure elevates in relation to increased practice duration, contact intensity, and time spent in high risk drills [21, 39]. However, these variables differ greatly between players and position groups. For example, previous literature demonstrated that the differences between individual players accounted for 48% of the variance in head impact exposure during practice [44]. Additionally, recent research suggests different types of plays (e.g., running, passing, special teams) also have different average head impact magnitudes which will influence cumulative head impact exposure [41]. Despite this high degree of variance between players and play types, previous studies reported that the linemen and hybrid position groups consistently have higher head impact exposure compared to skill positions such as receivers, defensive backs, and quarterbacks [37, 38]. In our sample, we were able to observe a similar trend in the context of five different levels-of-contact, where the linemen and hybrid positions sustained a minimum of 27% more head impacts in live drills, with upwards of 3 to 4 fold higher head impact exposure in air, bags, and control than those of the skill position (see S2 Table). However, these group differences did not reach statistical significance likely due to the heterogeneous head impact exposure within each group (e.g., some skill players sustain many hits while several linemen experienced less hits), pointing to the issue of lack of sample size.

Another important finding from this study was that the average head impact magnitude (PLA and PRA) was similar (18 to 23 g) across 5 levels-of-contact. However, the frequency of strong magnitudes of head impacts (60–100 g and >100 g) were greater during live and thud drills than other drills. These observations illustrate the fact that the majority of head impacts in high school football are considered mild, but the minority high magnitude impacts are evident in contact-prone drills. This evidence fills the critical gap in knowledge that the restriction of practice frequency and shortening of a season may not be effective unless considering the intensity of practice drills. Nevertheless, there have been attempts by various football governing bodies to restrict the amount of full contact in practices [20, 21, 26, 38, 45]. For example, USA Football’s National Practice Guidelines for Youth Tackle Football suggest that full-contact should not be done for more than 30 minutes per day and no more than 90 minutes per week during the regular season [26]. By limiting full contact practices from 3 days to 2 days per week, there was an average decline of 42% in head impact frequency (~250 hits per player) in high school football players [38]. In the same study, although the overall average head impact magnitude remained unchanged before and after reduction of contact practice, researchers found that a frequency of high magnitude head impacts were elevated in linemen and hybrid positions [38], suggesting that the team might have implemented higher intensity drills (i.e., live, thud, control) to compensate for the reduction of practice frequency. This compensatory trend was more conspicuous in the college setting. Even though the NCAA eliminated two-a-day practices and reduced practice frequency from 29 to 25 practices during summer camp, to compensate for the loss of practice times, football teams tended to incorporate more high-intensity, contact-prone practice drills, leading to increased head impact frequency [20, 21]. These data further substantiate the importance of regulating the duration of specific drill types, rather than restricting practice type and frequency.

Several studies suggest that football players who go on to sustain concussion tend to experience frequent subconcussive head impacts [24, 33]. Since none of the players in the current study were diagnosed with concussion, we were unable to suggest the potential preventive effect of levels-of-contact on concussion incidence. However, it is noteworthy that our exploratory analysis revealed no correlation (r = 0.07, p = 0.71) between previous number of concussion and subconcussive head impacts sustained during this season. This finding suggests that previous concussion history has almost no influence in how players perform, in the context of head impact exposure, during the season.

Clinical implications

Pending confirmation by a larger-scale study, these results may have important implications for the clinical management and guidelines regulating subconcussive head impact exposure in high school football. If a restriction of contact-prone drills (i.e., live, thud) during practice is indeed effective in reducing head impact frequency and associated magnitude, establishing a policy or guideline to minimize head impact burden would be a logical next step. In addition to the consideration of level-of-contact, the most effective method for informing clinical guidelines may be a blend of the different strategies. For example, it is important to implement guidelines informed by head impact data derived not only from entire teams or position groups, but also from more targeted and specific practice structural variables such as drill type and hit per player per minute rates. Additionally, individual-level variables such as tackling technique, starting status, or count of repetitions players participate in would be imperative to determine who might be at risk for sustaining many head impacts in a short window. Fortunately, coaches across the nation have the ability to implement the USA Football levels-of-contact within their practice structures with relatively minimal effort. This strategy may allow coaches to directly influence the total subconcussive head impact exposure in their athletes.

Limitations

There are several limitations to this study. Our examination of head impacts in high school football is limited in that it was conducted on a single high school football team in the Midwest composed of primarily white males. Because of lack of racial and ethnical diversity in our sample, we were unable to conduct any analysis to identify whether race/ethnicity played a role in subconcussive head impact exposure. This should be addressed in a future study along with whether race/ethnical background influence one’s neurologic resiliency and susceptibility to subconcussive head impacts. Therefore, the results from the current study are not generalizable to the broader U.S. population of high school football teams. A second limitation of the study is that the USA Football levels-of-contact guidelines are just that, guidelines. They are not legislation, and it is unknown what percentage of high schools currently utilize the USA Football level-of-contact system; thus, this further limits the generalizability. Implementation of the levels-of-contact is not a perfectly reliable variable in that situations occur in practice that may have overlapping levels-of-contact. This overlap made cumulative calculations of time spent in each level-of-contact difficult during certain drills and the time spent in each level is likely to influence cumulative head impact exposure. Coaches in this high school attempted to implement the levels-of-contact consistently, but to control up to 22 athletes participating in a single drill at the same time is not always feasible. An additional limitation is that the Vector mouthguard has not been validated for true positive versus false positive data. These limitations portend a follow-up research question as to whether regulating the number of repetitions in each level-of-contact is a more effective approach than teamwide contact restrictions for minimizing the frequency and magnitude of subconcussive head impacts in individual players (or position groups). Hence, for future research we suggest examining subject-specific head impact data and playing style (run-first vs. pass-first offense) [46] that incorporate multiple predictor variables such as time spent in each level-of-contact, the repetitions per player per drill, levels-of-contact, position group, and impacts per event (i.e., drill, practice/game, or week) for head impact outcomes.

Conclusion

Levels-of-contact may influence cumulative head impact frequency and magnitude in high school football players, with players incurring frequent head impacts during live, thud, and control. Strong magnitudes of head impacts (> 60 g) were frequently observed especially during live and thud. It is important to consider levels-of-contact to refine football practice guidelines/policies to minimize cumulative head impact burden in high school football players.

Supporting information

S1 Table. Bonferroni post-hoc results on 5 levels of contact.

(DOCX)

Click here for additional data file.^{(17.8KB, docx)}

S2 Table. Cumulative head impact frequency and magnitude for the entire season.

(DOCX)

Click here for additional data file.^{(19.3KB, docx)}

S3 Table. Average peak linear acceleration and peak rotation acceleration.

(DOCX)

Click here for additional data file.^{(17.2KB, docx)}

S1 Video

(MP4)

Click here for additional data file.^{(17.4MB, mp4)}

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

Indiana University Office of Vice President for Research (to K. Kawata) https://research.iu.edu/about/leadership/index.html, the Spinal Cord & Brain Injury Research Fund from the Indiana State Department of Health (to K. Kawata: ISCBIRF 0019939) https://www.in.gov/isdh/, and Indiana University Women’s Philanthropy Council (to K. Kawata) https://iufoundation.iu.edu/leadership-giving/womens-philanthropy/index.html. Sponsors had no role in the design or execution of the study; collection, management, analysis, or interpretation of the data; preparation, review, or approval of the manuscript; or decision to submit the manuscript for publication.

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PLoS One. doi: 10.1371/journal.pone.0237800.r001

Decision Letter 0

Markus Geßlein

3 Jul 2020

PONE-D-20-13120

Subconcussive head impact exposure between drill intensities in U.S. high school football

PLOS ONE

Dear Dr. Kawata,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Due to the importance of your submitted manuscript for the sports community there has been a rigorous peer-review by different specialists in neuro-science and / or concussion researchers.

I am very pleased to tell you that all peer-reviewers found your manuscript valuable and methodological sound.

Nevertheless there are some minor revisions needed before I can consider the manuscript suitable for publication.

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: I Don't Know

Reviewer #5: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #5: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #5: Yes

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5. Review Comments to the Author

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Reviewer #1: Dear Editor,

Dear Authors,

Thank you very much for the opportunity to review the manuscript "Subconcussive head impact exposure between drill intensities in U.S. high school football."

The manuscript by Kercher et al. suggests that level-of-contact in high school football practices influences cumulative head impact frequency and magnitude. With the use of a sensor-installed mouthguard, head impact frequency, peak linear acceleration and rotational acceleration were investigated and categorized into five levels-of-contact (air, bags, control, thud, live) as well as three player positions (lineman, hybrid, skill). The total number of impacts per player increased in a near incremental manner, where live drills had higher cumulative frequency than the other levels-of-contact. High magnitude (60-100 g and >100 g) head impacts could be more frequently observed during live and thud drills compared with air, bag and control. The manuscript is well written and the main issue that authors discuss would be important for prevention or minimization of head impact burden in high school football by refining the clinical exposure guidelines.

This is an interesting and potentially valuable study but the manuscript could be improved in some ways. A number of minor critical points are described below:

Abstract

[P2, L29] The authors should add the number of days of all practices and games in the 2019 season.

Introduction

[P3, L51] A brief description of the long-term consequences of a sport-related head injury, especially in the context of (high school) football, would be useful.

Methods

[P5, L109] How was an active football team member defined? The authors should describe the inclusion criteria in more detail.

[P6, L118] What role does the determination of race/ethnicity play in the study and what conclusions can be drawn from the results? If these variables are not informative, they should be excluded.

[P6, L122] The observation period from pre-season training camp to the end of the season should be stated with the exact number of days and average minute duration on which the practice was performed.

[P6, L128] Why were no quarterbacks included in the study? The authors should state reasons for the exclusion.

[P8, L178] Why was the total number of head impacts split into groups of 25-60 g, 60-100 g or >100 g? The authors should add a short explanation why this categorization was chosen.

Results

[Table 1] The variables “Playing position, n (%)” seem to have the same meaning as the first variable "n". Perhaps the authors could simply put the percentage in brackets after the variable "n" to make the table clearer.

The authors might also consider referring to the following articles:

Crisco JJ, Wilcox BJ, Beckwith JG, et al. Head impact exposure in collegiate football players. J Biomech. 2011;44(15):2673‐2678.

Martini D, Eckner J, Kutcher J, Broglio SP. Subconcussive head impact biomechanics: comparing differing offensive schemes. Med Sci Sports Exerc. 2013;45(4):755‐761.

Reviewer #2: Thank you for the opportunity to review the manuscript titled “Subconcussive head impact exposure between drill intensities in U.S. high school football”. This study examined head impact frequency and magnitude by different levels of contact and player position in a small cohort of high school football players. The study is well conceived and conducted. The manuscript is well structured and written. I only offer a few minor comments that will hopefully assist the authors with improving the presentation of their findings.

1. For the subsection about ‘Head impact measurement’ (p. 6-7), please add some pertinent information about the reliability and validity of the measurement tool (i.e. Vector mouthguard).

2. For Table 1, the row with sex can be omitted. The proportions by position group can be moved to the row with sample sizes. It is unnecessary to report age, BMI, experience, PLA, and PRA using two decimal places. It gives a false sense of precision of the measurements. I think one decimal place will suffice.

3. For the subsection on ‘Average head impact magnitudes by levels-of-contact and position group’ (p. 11), the word ‘average’ is ambiguous. I suggest making is clearer that all the values are medians.

4. For Figure 2, please make it clear that the plots show median number of impacts per player. Presumably the error bars indicate the IQR. In the footnotes, please ensure that all p-values greater than 0.001 are reported with and equal symbol (i.e. not inequality symbol).

5. For Figure 3, please make it clear that the plots show median number of impacts per player. Presumably the error bars indicate the IQR.

6. For Figure 3, please make it clear that the plots show median number PLA and PRA. Presumably the error bars indicate the IQR.

Reviewer #3: Summary:

The authors provide an observational study of subconcussive head impact frequency and magnitude in a high-school-football team during the 2019 season depending on level-of-contact and player position during training and game sessions. They followed-up on a high-school football team in the mid-west of the USA including 24 male teenagers (age 15,7 +/- 1,08 y), with player positions lineman, hybrid and skill, during the whole 2019 season, including all training sessions. Written informed consent of players and legal guardians was given. The players wore sensor-installed mouthguards monitoring impact frequency, peak linear and rotational acceleration to every training session and game. Via video-analyses of all training sessions and games, activity was categorized into five levels of contact with increasing intensity, as established by USA Football (air, bags, control, thud, live).

The research question deals with whether and to what extent, different levels-of-contact and different player position influence head impact frequency and magnitude. The primary hypothesis suggests an incremental increase in head impact frequency and magnitude over the season between levels of contact (air<bags<control<thud<live). and="" differences="" frequency="" group="" head="" hypothesis="" impact="" magnitude="" regarding="" secondary="" suggests="" the="">

The authors give a clear objective and clear research questions, and these are up to date and relevantly embedded into the current literature. They address an important subject, as repetitive head injuries lead to structural damage and neurologic deterioration in athletes. Especially in children and young adults, grave consequences can arise as their brains are still developing. Moreover, the impact of subconcussive head injuries on the development of structural and neurobehavioral pathologies is not clear. Therefore, the systematic measurement of head impact frequency and magnitude in a high-school team during one season, as conducted in this study, is of relevance.

The figures and tables included in the study display the results comprehensively. The supplementary material helps visualizing head impact frequency and magnitude and highlights the study design. All statistic findings are displayed in the supplementary material comprehensively.

The authors collected 6016 head impacts throughout the season. Total number of impacts, magnitude and acceleration increased with level-of-contact (air<bags<control=thud<live). and="" difference.="" frequently="" group="" head="" high="" impacts="" in="" live="" magnitude="" more="" no="" position="" significant="" there="" thud="" was="" were="">

The results are comprehensively displayed, visualized, and supported by the figures and tables in the manuscript and the supplementary material. They are set into context to the recent relevant literature. The methods are appropriate and reproducible, in the supplementary material even video examples for analysis are included.

The authors conclude with the suggestion to include the 5 level-of-contact recommendations to refine clinical exposure guidelines to minimize head impact burden in high-school football. The conclusion matches the research question and the results and is clinically relevant.

Overall impression:

The authors provide a relevant, interestingly designed observational study with 24 teenage athletes over a whole high-school football season. They combine the technical measurement of head impact magnitude and acceleration data via sensor-installed mouthguards with video-analyzed levels-of contact and different player position. The research question is relevant and up to date, the hypotheses are well-chosen and the results and conclusion match. The study contributes to the steadily evolving body of evidence that further and stricter regulations in recreational and professional sport are necessary to reduce the risk of repetitive head impacts and their potentially grave consequences. We recommend publishing this work and give only minor issues, as mentioned below.

Major Issues:

None.

Minor Issues:

1. We would recommend including the research question and hypotheses in the Abstract.

2. We would also recommend giving the dates the mentioned rule-changes and recommendations for reducing head trauma frequency in high-school / college football were established (levels-of-contact, tackle football, kickoffs rules) for a better overview on the subject and current stand (lines 56-65 and 74-79).

3. The authors give the number of previous concussions in Table 1 (line 193), however, they do not describe in the manuscript the relevance of their presence or absence; we would recommend to also meet this topic of risk potentiation in repetitive head impacts.

4. The authors also give in Table 1 (line 193) in detail the race and ethnicity of the players, although 88% of the players are white and only 4% American Indian and 8% multiracial. Moreover, they do not explain in the manuscript potential interracial differences in concussion risk. We would recommend to either add this information or to shorten the table regarding this aspect.</bags<control=thud<live).></bags<control<thud<live).>

Reviewer #4: Dear authors,

thank you very much for this interesting study. I have some comments below.

Title

Abstract

Introduction

well written, good introduction to the topic, clear research question

Material and Methods

line 123: this is a short season from mid August 2019 to end of October 2019 for preseason training to the end of season?

Results

line 197: the results are clearly describing the level of contact and head impact exposure. However, what is missing, is how much time the athletes were performing activities of the different contact levels. e.g. if most time was spent doing live contact, it is no wonder that most head impact was generated during this typ of impact. it would be optimal to also include exposure time...

Discussion

line 231: also dependent on exposure time, see comment above...

line 233: the skill group received much less cumulative impacts 130 vs over 200. Although no statistically significant finding, don't you think that receiving 30% less impacts does not count? please discuss! otherwise please add power analysis and sample size calculation etc... please see also line 254 where you say so, that skill positions have leas head impact

line 299ff: another limitation is that exposure time of the different levels of contact is not reported...

line 318: usually references are not cited in the conclusion, since the conclusions are your main findings and not the findings of other studies (since you published an original study)

References

Tables

Table 1:please remove psychiatric conditions, since there are none. makes the table smaller and easier to read... also remove the lines "playing position, n (%)", since this information is redundant. the cumulative impact count is actually "average cumulative impact count"? also for the peak linear and rotational acceleration?

Figures

Reviewer #5: For me as a neurologist it is very important that efforts are made to reduce the cumulative exposure of subconcussional cerebral microtrauma in adolescent high school football players. To verify the validity of the concept of different levels-of-contact which has been establishes by USA Football, the authors present an interesting study that compares the real exposure to mild cranial trauma at different levels-of-contact and different positions of football players. In this respect, the authors prove and validate the intuitive assumption that the different increasing levels-of-contact reflect a hierarchy of cumulative exposure to hits to the head. I have no objections to or suggestions for improvement of the manuscript.

**********

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Reviewer #1: Yes: Juergen Taxis

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

Reviewer #5: No

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PLoS One. 2020 Aug 14;15(8):e0237800. doi: 10.1371/journal.pone.0237800.r002

Author response to Decision Letter 0

12 Jul 2020

We thank the reviewers and editor for their appreciation of the potential clinical significance of the current manuscript and their detailed and constructive comments, which have strengthened and clarified the methodology and deliverable of this manuscript. As the editor and all reviewers acknowledged, this study will play a significant catalyst to improve policy/guideline to ensure players’ safety.

Reviewer #1:

Abstract

The authors should add the number of days of all practices and games in the 2019 season.

RESPONSE: Thank you for the suggestion. We added the number of practices (46), scrimmage (1), junior varsity (9), and varsity games (10) to the abstract, as well as in the study procedure of the method section.

Introduction

[P3, L51] A brief description of the long-term consequences of a sport-related head injury, especially in the context of (high school) football, would be useful.

RESPONSE: Excellent point. We have had a long discussion about this point prior to the initial submission whether to mention the potential long-term consequences (e.g., CTE, dementia). Although many data are beginning to point to the adverse long-term outcome, we are also aware of several opposing lines of research. This manuscript does not assess neurologic consequences. Instead, our objective was to examine head impact exposure by levels-of-contact; therefore, we made a conscious decision not to dive into long-term neurologic affects. We instead included some of the acute/subacute (e.g., season-long head impact) neurologic outcomes in the 2nd paragraph of the Introduction. We would like to maintain this stance if that is acceptable. If you think we need to include the potential long-term consequence (e.g., CTE), please let us know.

Methods

[P5, L109] How was an active football team member defined? The authors should describe the inclusion criteria in more detail.

RESPONSE: Clarification of this was added to the first paragraph of the methods section.

[P6, L118] What role does the determination of race/ethnicity play in the study and what conclusions can be drawn from the results? If these variables are not informative, they should be excluded.

RESPONSE: This is an important point. We listed the lack of generalizability to the general population as the first weakness in our limitations section due in part to the small sample size and lack of racial and diversity (i.e., primarily white males). Because of lack of racial and ethnical diversity, we were unable to conduct any analysis to identify whether race/ethnicity played a role in head impact exposure (i.e., White players tend to sustain more/less head impacts than black and other race). However, we felt as though it is important to state that we were working with a primarily white sample because future research may have different findings/conclusions when working with different race/ethnicity samples. Especially because this is first of its kind to delineate level-of-contact influence on head impact exposure, we think it is important to report race/ethnicity data. We expanded this aspect in the limitation section.

RESPONSE: Thank you for pointing this out. We added the exact number of observed practices and games along with the mean (SD) minute duration to the line you suggested.

[P6, L128] Why were no quarterbacks included in the study? The authors should state reasons for the exclusion.

RESPONSE: Excellent point to clarify. There was only one quarterback in the entire high school program during the study enrollment period, and unfortunately, he declined to participate in the study due to his lack of interest and reluctancy to wear the Vector mouthguard. We have clarified this point in the manuscript per your request.

[P8, L178] Why was the total number of head impacts split into groups of 25-60 g, 60-100 g or >100 g? The authors should add a short explanation why this categorization was chosen.

RESPONSE: Thank you for this comment. We clarified these exploratory thresholds in the line suggested, by explaining that we modeled Barber-Foss et al. 2019 and Bailes et al. 2013 that suggested < 25 g as minimal or negligible magnitude; Guskiewicz et al. 2007 that introduced 60 g as a cut-off threshold to induce concussion, whose nuance was softened after many papers reporting that concussion can occur head impact below 60 g. Nonetheless, we thought this was a relevant threshold to descriptively present our data. Lastly, Beckwith et al. 2013 reported that average head impact magnitude that led to concussion diagnosis was 100 g (precisely, 102.5 + 33.8 g). These points are now summarized in the revised manuscript.

Results

RESPONSE: We have implemented this suggestion.

The authors might also consider referring to the following articles:

Crisco JJ, Wilcox BJ, Beckwith JG, et al. Head impact exposure in collegiate football players. J Biomech. 2011;44(15):2673‐2678.

Martini D, Eckner J, Kutcher J, Broglio SP. Subconcussive head impact biomechanics: comparing differing offensive schemes. Med Sci Sports Exerc. 2013;45(4):755‐761.

RESPONSE: We added the Crisco article as a reference as it pertains to suggesting significant variance in head impact exposure between position groups. We also included Martini et al paper to suggest the potential interaction between playing style (run-first vs. pass-first offense) and level-of-contact drills. Thanks for the suggestion.

Reviewer #2:

1. For the subsection about ‘Head impact measurement’ (p. 6-7), please add some pertinent information about the reliability and validity of the measurement tool (i.e. Vector mouthguard).

RESPONSE: Thank you for pointing this out. We added a comment in the limitations section about the lack of literature validating the Vector mouthguard for true positive versus false positive head impacts, although our unpublished internal validation from our previous study1,2 yielded 85-90% agreement between Vector mouthguard and film analysis. Additionally, we added a statement in the Head impact measurement section on the reliability of movement in sensor-installed mouthguards compared to skin patch and skull cap head impact devices.

1. Zonner S, Ejima K, Bevilacqua ZW, Huibregtse ME, Charleston C, Fulgar CC, Kawata K. Association of increased serum S100B levels with high school football subconcussive head impacts. Front Neurol. 2019;10:327.

2. Zonner SW, Ejima K, Fulgar CC, Charleston C, Huibregtse ME, Bevilacqua ZW, Kawata K. Oculomotor response to cumulative subconcussive head impacts in high school football players: a pilot longitudinal study. JAMA Ophthalmol. 2019;137(3):265-270.

RESPONSE: These are excellent points. We have made the requested revisions to Table 1.

RESPONSE: Thank you for identifying this ambiguous term, we have changed average to median in this section.

RESPONSE: Thank you for noticing this. We have made the suggested revisions in the legend and fixed the equal symbols.

5. For Figure 3, please make it clear that the plots show median number of impacts per player. Presumably the error bars indicate the IQR.

RESPONSE: Thank you for noticing this. We have made the suggested revisions in the legend.

6. For Figure 3, please make it clear that the plots show median number PLA and PRA. Presumably the error bars indicate the IQR.

RESPONSE: Thank you for noticing this. We have made the suggested revisions in the legend.

Reviewer #3:

Minor Issues:

1. We would recommend including the research question and hypotheses in the Abstract.

RESPONSE: In addition to the objective, we have revised the abstract to include the aspect of our research question aiming to determine which drills had the greatest head impact exposure, subsequently stating our primary hypothesis. Thank you for the suggestion.

RESPONSE: Thank you for these suggestions. We have added years to two of the initiatives mentioned in the introduction.

RESPONSE: Indeed, previous concussion history was not included in the analysis as a covariate, since our outcome measure is head impact exposure rather than neurologic outcome. Yet, this is an important point to discuss; therefore, we added a short paragraph elaborating the potential intersection between concussion and subconcussive head impacts right before the clinical implication paragraph in the discussion. Thank you for the suggestion.

RESPONSE: Excellent point. Reviewer #1 also pointed this out and we expanded the limitation section to acknowledge our lack of racial and ethnical diversity. We decided to keep this race/ethnicity in the table so that future research is encouraged to settle this unsolved question.

Reviewer #4:

Material and Methods

line 123: this is a short season from mid-August 2019 to end of October 2019 for preseason training to the end of season?

RESPONSE: This is a great question. It was shorter season than expected because they lost out of the season earlier than expected on Friday, November 1st. Had they won a playoff game, the season could have lasted more weeks. There first varsity game of the season was on Friday, August 23rd.

Results

RESPONSE: Thanks for pointing this out, we agree. We were sure to include this at the end of our limitations section and made an addition of “time spent in each level of contact.”

A future direction for this line of research is to examine subject specific head impact data that also incorporates time, and more importantly repetitions participated in. Part of the issue with team-wide contact guidelines/restrictions is that there is such large variance between players in both practice and game play participation.

For example, as you can imagine, in any given hour of certain drill one player may experience 50 repetitions of drills and associated head impacts, whereas another player may experience only 0 to 10 repetitions. This creates a significant flaw for team-wide restrictions that are based on contact time rather than repetitions.

Discussion

line 231: also dependent on exposure time, see comment above...

RESPONSE: Thank you again, this is an important limitation of the study. See comment above too. Time is an important variable to consider and we would like to see it added as part of future research designs exploring intensity levels. Despite this limitation, and our previous comment above, ultimately the outcome of cumulative head impact exposure per level-of-contact is still an important finding even without the amount of time involved. For example, policy makers and other stakeholders should still be interested in which levels have the most head impact exposure associated with them and could potentially utilize those levels to decrease exposure.

RESPONSE: Thank you for pointing this out. It is, indeed, a noticeable difference in head impact experiences between linemen/hybrid vs. skill. There are some heterogeneity in within-group head impact exposure where some skill players sustain many hits while several linemen experienced less hits, which points to the issue of lack of sample size. We elaborated this issue (clinical significance vs. statistical significance) in both the 1st and 2nd paragraphs of the discussion. This is a great point and we appreciate your suggestion.

line 299ff: another limitation is that exposure time of the different levels of contact is not reported...

RESPONSE: We added this revision more clearly in two statements within the limitations section. Thanks for your emphasis of this point.

line 318: usually references are not cited in the conclusion, since the conclusions are your main findings and not the findings of other studies (since you published an original study)

RESPONSE: Thanks for identifying this irregularity. We thought about this and it is our hope that we could leave these references in the conclusions as a reminder to the audience that how our results may play a catalytic role to fill the current gap in our knowledge. We would really like to hammer home the point as part of our conclusion that greater dissection of football structure and guidelines may lead to measures that more effectively decrease subconcussive head impact exposure. To balance your perspective, the conclusion section within the abstract does not contain such referential statement. We hope that you will understand our rationale.

Tables

RESPONSE: We have made these revisions and significantly simplified Table 1.

RESPONSE: Thank you for reviewing our article, we appreciate your feedback and interest from neurologist standpoint. This is encouraging for us to hear.

Attachment

Submitted filename: Response to Reviewers.docx

Click here for additional data file.^{(25.8KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0237800.r003

Decision Letter 1

Markus Geßlein

29 Jul 2020

PONE-D-20-13120R1

Subconcussive head impact exposure between drill intensities in U.S. high school football

PLOS ONE

Dear Dr. Kawata,

==============================

ACADEMIC EDITOR:

- Please remove the references from the conclusion as this is uncommon. I also would advise to remove the lines 413 to 415 as it does not reflect the results of your study. Please also refer to the suggestions made in the review concerning the conclusion and adapt the abstract.

-I would ask you to add a short explanation for the physical denotations used (like "g" ..and krad/s2) so that interested readers (as non-concussion experts) get the channce to understand those values better. I think it is important to refer to the "normal/usual values" (you can give examples) of forces applied to the head/brain in a normal envirment and in American Football. This might be helpful to sharpen the awareness of the medical staff. If you have any further questions in this topic you can contact me directly through the editorial manager to speed up your publication.

Please ensure that your decision is justified on PLOS ONE’s publication criteria and not, for example, on novelty or perceived impact.

==============================

Please submit your revised manuscript by Sep 12 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: All comments have been addressed

Reviewer #4: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

Reviewer #4: Yes

**********

6. Review Comments to the Author

Reviewer #1: The authors have done well to comply with the requested revisions. There are no further points of criticism.

Reviewer #2: Thank you for the opportunity to review the revised manuscript titled “Subconcussive head impact exposure between drill intensities in U.S. high school football”. The authors have satisfactorily addressed the reviewer comments. I only have one final suggestion.

P19L403-405: Please remove this sentence; it is not appropriate to include findings from previous studies in the conclusion section in this manner. I also recommend removing the words ‘Our data suggest that the…’ from the first sentence of the conclusion (P19L400), and replacing the words ‘Our data indicate the importance of considering…’ with ‘It is important to consider…’.

Reviewer #3: (No Response)

Reviewer #4: dear authors,

thanks you very much for the performed revision, all my questions have been fully answered, i have no further issues!

**********

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Reviewer #1: Yes: Juergen Taxis

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

PLoS One. 2020 Aug 14;15(8):e0237800. doi: 10.1371/journal.pone.0237800.r004

Author response to Decision Letter 1

29 Jul 2020

Thank you for your thorough attention to important details. As pointed out, the conclusion section was revised to reflect reviewers’ phrasing as well as removal of references and its sentence all together, as suggested by the editor. Abstract’s conclusion section also reflects these changes.

Additionally, in the first paragraph of result section, we inserted some comparative information about g-force using an example of car crash, which is often used as a referential example in head trauma research. However, there are no good real-life examples about rotational acceleration (krad/s2), and this rotational acceleration is almost always correlated to g-force in concussion research (if player experience high, cumulative g-force, then their rotational acceleration (krad/s2) is high. Therefore, the car crash example for g-force should suffice to sharpen awareness of the medical personnel.

Thank you.

Attachment

Submitted filename: R2 Response to Reviewers.docx

Click here for additional data file.^{(15.8KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0237800.r005

Decision Letter 2

Markus Geßlein

4 Aug 2020

Subconcussive head impact exposure between drill intensities in U.S. high school football

PONE-D-20-13120R2

Dear Dr. Kawata,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Markus Geßlein

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Thank you very much for your thorough scientific work and your prompt cooperation.

Best regards

Reviewers' comments:

none

PLoS One. doi: 10.1371/journal.pone.0237800.r006

Acceptance letter

Markus Geßlein

6 Aug 2020

PONE-D-20-13120R2

Subconcussive head impact exposure between drill intensities in U.S. high school football

Dear Dr. Kawata:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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on behalf of

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

S1 Table. Bonferroni post-hoc results on 5 levels of contact.

(DOCX)

Click here for additional data file.^{(17.8KB, docx)}

S2 Table. Cumulative head impact frequency and magnitude for the entire season.

(DOCX)

Click here for additional data file.^{(19.3KB, docx)}

S3 Table. Average peak linear acceleration and peak rotation acceleration.

(DOCX)

Click here for additional data file.^{(17.2KB, docx)}

S1 Video

(MP4)

Click here for additional data file.^{(17.4MB, mp4)}

Attachment

Submitted filename: Response to Reviewers.docx

Click here for additional data file.^{(25.8KB, docx)}

Attachment

Submitted filename: R2 Response to Reviewers.docx

Click here for additional data file.^{(15.8KB, docx)}

Data Availability Statement

All relevant data are within the manuscript and its Supporting Information files.

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PERMALINK

Subconcussive head impact exposure between drill intensities in U.S. high school football

Kyle Kercher

Jesse A Steinfeldt

Jonathan T Macy

Keisuke Ejima

Keisuke Kawata

Roles

Abstract

Introduction

Methods

Participants

Study procedures

Head impact measurement

Levels-of-contact and film review

Statistical analysis

Results

Demographics and overall head impact exposure

Fig 1. Head impact distribution in 24 high school football players in a single season.

Table 1. Group demographics and head impact kinematics.

Level-of-contact-dependent cumulative head impact exposure

Fig 2. Cumulative head impact kinematics between levels-of-contact throughout a season.

Fig 3. Group-dependent cumulative head impact exposure between levels-of-contact.

Head impact magnitudes by levels-of-contact and position group

Fig 4. Average head impact magnitude per impact between levels-of-contact and group.

Fig 5. Frequency of head impacts within various magnitude range.

Discussion

Clinical implications

Limitations

Conclusion

Supporting information

Data Availability

Funding Statement

References

Decision Letter 0

Markus Geßlein

Roles

Author response to Decision Letter 0

Decision Letter 1

Markus Geßlein

Roles

Author response to Decision Letter 1

Decision Letter 2

Markus Geßlein

Roles

Acceptance letter

Markus Geßlein

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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