This trend can be partially explained by the fact that the number of sessions (practices and games) increased as the level of play increased, with the younger players in the 2012 study involved in 9.4 practices and 4.7 games for a total of 14.1 sessions, while players in the current study participated in an average of 14.4 contact practices and 7.4 games, for a total of 21.8 sessions, but fewer practices and games, and fewer impacts per session, than high school players, who experienced on average 15.9 impacts per session compared to the 10.6 impacts per session experienced by the 9-12 year old players in the current study.  The age-related differences reported among these three age groups are most likely due to increased size, athleticism, and aggression in older players, said Rowson.

Comparison of head impact exposure across various level of play^[2,5-7]

		Linear acceleration	(g)	Rotational acceleration	(rad/s2)
Level of Play	Number of impacts per season	Median (50%)	95%	Median (50%)	95%
Youth (7-8 years)	107	15	40	672	2347
Youth (9-12 years)	240	18	43	856	2034
High school (14-18 years)	565	21	56	903	2527
College (19-23 years)	1000	18	63	981	2975

Impact frequencies and magnitudes: games versus practice

Players experienced slightly greater impact frequencies and acceleration magnitudes in games than in practice, similar to findings of high school and college football studies. For example, a group of high school players, experienced a mean linear acceleration magnitude of 23 g in practices and 25 g in games while the players in this study had a mean linear acceleration magnitude of 22 g in practices and 23 g in games.

With regard to impact frequency, players in this study experienced a similar number of impacts per practice as per game. The rate of impact in practice was similar to the 9.2 impacts per practice that were  reported for high school football players. However, the high school players sustained 24.5 impacts per game. These data suggest that high school players experience fewer impacts in practice than in games, while the 9-12 year old players in this study had roughly equal numbers of impacts per session for the two session types.

Pop Warner rule changes helped

The players on team A who adopted the new Pop Warner rule^[3] changes - which now permit no more than a third of practice time, and no more than 40 minutes per session to contact drills and require coaches to give each player a certain minimum amount of playing time -  absorbed an average of 37 percent to 46 percent fewer hits than players on teams B and C during practices and games over the course of season. 

In addition to the Pop Warner rules, the authors attributed the reduction in impacts to several other factors:

1. special teams plays, including kickoffs and punts, were live plays for teams B and C, similar to high school, college, and professional football. By contrast, team A's special teams plays were controlled situations where no contact was allowed. Data from previous studies suggest that players on special teams are more susceptible to large magnitude head accelerations, which may lead to higher incidence of concussion on these plays;
2. all three teams played approximately the same number of games during the season, but teams B and C played 11 and 12 week seasons while team A had a 9 week season. With more time between games, teams generally practice at a higher frequency and intensity;
3. player skill, athleticism, and maturity could have implications on the level of exposure. Even within teams, variability among players is apparent, with some players experiencing substantially more impacts than the team average. No significant differences were found in game acceleration magnitudes or impact frequency, suggesting practice differences were not due to player; and differences among teams. Instrumented players ranged from experiencing 72 to 585 head impacts; and
4. coaching style has major influence on factors such as the types of drills used in practice and the plays called in games. These coaching variations would likely contribute to the differences in the head impact exposure that players experienced

Implications for youth football

"The data collected in this study may have applications towards improving the safety of youth football through rule changes, coach training, and equipment design. Prior to the 2012 season, many youth football organizations, including the league in which team A competed, modified rules, and provided coaches with practice guidelines to reduce head impacts in practice," Rowson noted.

"The data collected in this study suggest that total head impact exposure over the course of a season can be reduced significantly by limiting contact in practices to levels below those experienced in games. In addition to guiding future rules for youth football, this study can be used to aid designers in developing youth-specific football helmets that may be able to better reduce head accelerations due to head impacts for young football players. Impact location, frequency, and head acceleration magnitudes can be used to optimize helmet padding to maximize protection while keeping factors such as helmet size and mass to age appropriate levels."

The vast majority of head impacts recorded in both games and practices were below acceleration magnitudes generally associated with concussions; though, some high magnitude impacts, similar to those seen among older players, did occur.

"It is striking that you can cut head impacts for a player in half just by modifying practice, and it does not seem to change the game," said Alexander Powers, M.D., assistant professor of neurosurgery at Wake Forest Baptist and co-author of the study. "This may be very important in kids where brains are developing."

"We hope that the findings will help improve the safety of youth football through rule changes to limit contact in practices, coach training and equipment design, especially in developing youth-specific helmets to better reduce accelerations from head impacts," Rowson said.

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Source:  WakeHealth

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2. Daniel R, Rowson S, Duma S. Head Impact Exposure in Youth Football. Ann. Biomed Eng 2012;40(4):976-981. 

3. http://www.popwarner.com/About_Us/Pop_Warner_News/Rule_Changes_Regarding... (accessed May 17, 2012)

4. Kontos P, Fazio V, Burkart S, Swindell H, Marron J, Collins M. Incidence of Sport-Related Concussion among Youth Football Players Aged 8-12 Years. J Pediatrics 2013. DOI 10.1016/j.jpeds.2013.04.011 

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review. Ann. Biomed. Eng. 2012;40(1):37-46.

6. Rowson, S., and S. M. Duma. Development of the star evaluation system for football helmets: integrating player
head impact exposure and risk of concussion. Ann. Biomed. Eng. 2011;39(8):2130-2140.

7. Rowson, S., S. M. Duma, J. G. Beckwith, J. J. Chu, R. M. Greenwald, J. J. Crisco, P. G. Brolinson, A. C. Duhaime,
T. W. McAllister, and A. C. Maerlender. Rotational head kinematics in football impacts: an injury risk function for
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Posted August 15, 2013