Less contact during practice could mean a lot less exposure to head injuries among young football players and the kind of repetitive subconcussive blows that some researchers suggest can lead to long-term brain injury, but does not result in exposure to higher impact hits during games, says a new study. 
Researchers at Wake Forest Baptist Medical Center and Virginia Tech measured the number, force, location, and effect of blows to the head in 50 youth-league players ages 9 to 12. Their study followed a 2012 study  of small group of youth players, ages 7 to 8, in which they found that most high impact hits occurred during practice, not games. That study prompted Pop Warner to swiftly move to impose limits on the time spent in full-contact practice.
The new study involved one team that, while not affiliated with Pop Warner, chose to follow its new practice rules, and two others that did not. Significantly, the data showed that reducing the number of head hits in practice did not, as a later study by researchers at the University of Pittsburgh Medical Center  predicted, lead to higher force impacts during games.
This is precisely as the researchers in their earlier study had hoped. Their finding suggests that the heavy criticism of the Pop Warner practice limits by the UPMC researchers that the new rules would "not only have little effect on reducing on reducing concussions but may also actually increase the incidence of concussions in games via reduced time learning proper tackling in practice" may have been wide of the mark.
"The concern is if we don't teach kids how to hit in practice, they're going to get blown away in the games," said Stefan Duma of the School of Biomedical Engineering and Sciences and one of the co-authors of the study, in an interview with The New York Times. "This shows you can dramatically cut the amount of exposure in practice and have no more exposure during the games."
Numerous concussion and biomechanical studies have been conducted involving high school and college football players, but only few studies have focused on players under the age of 14, who represent more than 70 percent of those playing the sport. As a result, there has been, up to now, no clear, scientifically-based understanding of the number, force, and effect of blows to the head in young players, said Steven Rowson, Ph.D., assistant professor at the Virginia Tech-Wake Forest University School of Biomedical Engineering and Sciences and lead author of the study, published online in the journal Annals of Biomedical Engineering.
To quantify youth football players' exposure to head impacts in practices and games over the course of a single season, the researchers outfitted helmets of 50 players on three teams in two different leagues with the HIT (Head Impact Telemetry) system, an array of helmet-mounted accelerometers (i.e. hit sensors) installed on an elastic base inside the helmet. Data from the sensors were transmitted wirelessly to a computer on the sideline and processed to measure both the linear and rotational head acceleration resulting from each impact. All the data were then analyzed for each individual player basis and averaged to represent the exposure level of a typical 9- to12-year-old football player.
The data yielded the following results:
- Total impacts: 11,978 impacts were measured, ranging from 10 to 126 g's of linear accelerations, and 4 to 5838 rad/s2 of rotational acceleration;
- Impact distribution by location:
- front of the helmet (41%)
- back (25%)
- side (23%).
- top of the helmet (11%)(although these impacts resulted in the highest magnitude)
- Number of impacts
- Average over course of season: 240 ± 147
- Average per session (practice/games): 10.6
- Force of impacts:
- Most were lower-magnitude impacts:
- 961 impacts (8.0% of total) greater than 40 g of linear acceleration
- 160 impacts (1.3% of total) greater than 60 g
- 36 impacts (0.3% of total) greater than 80 g.
- Most were lower-magnitude impacts:
- Average impacts for player
- 19.2 greater than 40 g
- 3.2 greater than 60 g
- 0.7 greater than 80 g.
- Concussions: Among the three teams participating in this study, four instrumented players sustained concussions diagnosed by physicians: two on the B team (players B4 and B6), a pee wee (10-12-years-old) team, one (player A8) on the A team, a 9-11-years old junior team, and one on the C team (player C18), a junior pee wee team (9-11-years-old) team.
- Player A8's concussion was associated with an impact to the front of his helmet and had a linear acceleration of 58 ± 9 g and rotational acceleration of 4548 ± 1400 rad/s2.
- Player B4''s concussion was associated with an impact to the back of the helmet with linear and rotational acceleration magnitudes of 64 ± 10 g and 2830 ± 900 rad/s2.
- Player C18's concussion was linked to an impact to the side of the helmet with linear and rotational acceleration magnitudes of 26 ± 4 g and 1552 ± 500 rad/s2.
- No impacts were recorded for B6 on the day of his concussion due to a battery failure in the sensor array in his helmet.
Impact force increases with age, level of play
Previous studies over the last decade have investigated the frequency and magnitude of head impacts in other tackle football populations, including youth (7-8 years), high school (14-18 years),[5-8] and college (18-23 years). Data from these studies show a trend of increasing acceleration magnitude and impact frequency with increasing level of play.
Not surprisingly, the 9-12 year old players in the current study were found to experience linear acceleration magnitudes between those found in 7-8 year old players and high school players, although, for rotational acceleration, the 95th percentile magnitude found in this study was less than that found previously in younger players.
Rotational acceleration tends to correlate well with linear acceleration, though impact location can heavily influence the relationship. Players in this study experienced more impacts to the front of their helmets and fewer to the side than the 7- to 8-year-old players who were the subject of a 2012 study  by the same researchers.
In that study, impacts to the front of player's helmets were associated with lower rotational acceleration magnitudes, while those to the side were associated with higher magnitudes. As with magnitude, the impact frequency reported in the current study fell between those of 6- to 9-year-olds and high school athletes, with the average player experiencing 240 impacts over the course of a season compared to 107 impacts per season for 6- to 9-year-old players, and 565 for high school players.