ASSOCIATION BETWEEN TACKLING TECHNIQUE AND HEAD ACCELERATION MAGNITUDE IN YOUTH FOOTBALL PLAYERS

Abstract

In order to address concerns about head injury in youth sports, a number of youth football organizations have developed rules and recommendations surrounding the tackling form which should be used in order to reduce unnecessary head impact exposure. Reduction in injury has been suggested with these programs, but association between tackling form and head acceleration magnitude has not been studied previously. To address this knowledge gap, grading criteria were developed from multiple youth organizations’ recommendations for a collision. A total of 142 tackles from a youth football team were graded. Head acceleration data were collected from helmet-mounted accelerometer arrays. An association was found between poor form and resultant head acceleration being greater than 40 g for both the tackler and the ball carrier. This study demonstrates the potential usefulness of tackling technique coaching programs in youth football.

INTRODUCTION

Concussions continue to be a major health concern in American football. With a large majority of players of this contact sport at the youth level, the accumulation of head impact exposure over a lifetime has begun to be extensively studied as a potential risk factor for impairment later in life [1–5]. Specifically, Alosco et al. [4] found that exposure to football before age 12 resulted in a twofold increase in odds of having clinically impaired scores on self-reported measures of executive function and behavioral regulation, depression, and apathy in former amateur and professional football players. Montenigro et al. [5] suggested that this increase in odds may be more strongly related to repetitive head impact exposure than other metrics, including concussion history. Associations such as these have led a number of organizations to seek methods of reducing head impact exposure in athletes, rather than only addressing injuries. The three best strategies today are thought to be development of better equipment, rule changes prohibiting head contact, and teaching better technique when contact occurs [6, 7].

Recently, multiple organizations have created or prioritized rules which prohibit certain tackling techniques, and some have even started programs teaching what the organization considers to be proper tackling technique [8–11]. Previous studies have shown these types of programs have resulted in less injury overall. Kerr et al. [12] found that injury rates for all types of injuries in games were lower among teams implementing USA Football’s Heads Up Football program. Concussions were only found to be reduced in practice if the Heads Up Football program was implemented and Pop Warner’s practice rules were also followed, which limited time allowed for contact in practices and eliminated high-speed, head-on tackling drills. These findings scaled with age, with stronger effects from these tackling recommendations and rule changes seen in players aged 11–15 rather than those 5–10 years old. There has been disagreement as to the degree of effectiveness these programs truly have, as reported concussion reduction may have been skewed when initially reported [12, 13]. To date, tackling technique programs have only been studied in terms of concussion incidence numbers, but none have attempted to determine if individual impacts with proper technique actually result in lower head accelerations for the athletes involved.

The objectives of this study were to discriminate between good and poor impact form for both a tackler and ball carrier in distinct impacts occurring in youth football practices via a generalized grading scale and to determine if poor technique was associated with increased head acceleration magnitude.

METHODS

Head impact data were collected from 18 youth football players (age 11.9 ± 0.6 years, weight 51.4 ± 11.8 kg) on a local youth recreation league team. Players had not been formally trained on any tackling technique outside of conventional coaching. Linear acceleration data were collected using previously validated helmet mounted accelerometer arrays which incorporate six single-axis linear accelerometers (Head Impact Telemetry System, Simbex, Lebanon, NH) [14]. The devices use an algorithm which calculates linear acceleration using the signals from all six accelerometers. The pre-set trigger for the device to record and download an impact was 10 g. Any impacts below this threshold were not recorded. This study was approved by the Virginia Tech Institutional Review Board (IRB # 15–517) on May 5, 2017 in order to involve human subjects. Standard procedures were followed using de-identified subject ID numbers, requiring parental and coach consent as well as minor subject assent; all wording was screened and cleared for coersive consent language.

A tackling technique grading scale was developed using criteria from a variety of modern youth football tackling programs which recommend keeping the head out of the impact (Table 1). These modern criteria are at odds with some previous criteria in that previous criteria recommended contacting the ball carrier with the helmet [15]. New criteria were selected based on their priority given to avoiding head impacts. Some information from USA Hockey’s checking guidelines were also used, as these guidelines include useful recommendations about absorbing an impact, whereas most tackling programs only address criteria for a tackler [16].

Table 1:

Grading criteria used to evaluate individual impacts for the tackler and ball carrier in a one-on-one impact. Players performing any of the failing criteria listed in the given phase were given a fail for that phase. Phases were defined as approach – advancement toward opposing player up until moment of contact; impact – moment when two players collide; and drive – period following contact until drill is over, as signified by a coach’s signal.

	Approach	Impact	Drive
Tackler	Feet spread less than shoulder width	Does not lead with near shoulder	Does not wrap with both hands
	Only lowers torso, bends at hip rather than knees	Does not initiate contact, shies away	Stops attempting to move legs
	Face directed downward, head not up	Aims shoulder too high (head) or too low (knees) into ball carrier
		Does not keep head up
Ball Carrier	Feet spread less than shoulder width	Does not lead with near shoulder	Stops attempting forward progress
	Only lowers torso, bends at hip rather than knees	Does not initiate contact, shies away
	Face directed downward, head not up	Does not tuck ball and elbow
		Does not keep head up

These grading criteria were used to evaluate 142 impacts from video of one-on-one tackles from a variety of drill types in practices at different points throughout the season. Three distinct phases of a tackle were identified and graded individually: approach, impact, and drive. If a player failed to meet any of the criteria for a given phase, a fail (0) was given. If all criteria were met for a given phase, a pass (1) was awarded. Total scores across the three phases were used to dichotomize tackles into good (summed score ≥ 2) and poor (summed score ≤ 1). All impacts were evaluated by a single grader to determine if significant association existed between tackling grade and head acceleration. The grader was familiar with tackling form evaluation and all players and coaches in this study.

Based on previous analyses, a threshold of 40 g was used to classify high magnitude impacts for this population [7]. This threshold resulted in high magnitude impacts accounting for approximately the top ten percent of data collected over the course of the season. A chi-square test for independence was used to determine the association of tackling form (good/poor) with presence of high magnitude acceleration (Y/N). Results were deemed significant for p ≤ 0.05, and all statistical calculations were completed in R (R Foundation for Statistical Computing, Vienna, Austria).

RESULTS

A total of 142 tackles were graded; three cases could not be graded because they involved players not enrolled in the study. This resulted in 281 total grades, one for the tackler and the ball carrier involved in each tackle. Of the graded impacts, 92 interactions resulted in high magnitude accelerations and 189 resulted in low magnitude accelerations. Tacklers saw 48 of the 92 high magnitude accelerations, while the ball carriers received the other 44. A total of 146 tackles passed overall, 61 for the tacklers and 85 for the ball carriers. Acceleration distributions by grade show a generally decreasing trend with increasing grade (Figure 1).

Figure 1:

Head acceleration magnitude showed a generally decreasing trend with overall tackling grade for both positions in this drill. Tackles receiving a summed grade of 2 or greater were considered good form, while tackles receiving 1 or less were considered poor form. All tackles in which a linear acceleration was not recorded were assumed to have not reached the system’s trigger threshold of 10 g. These tackles were presumed to be between 0 and 10 g, and were therefore coded as the average of 5 g.

When combining all grades from tacklers and ball carriers, significant association between high magnitude accelerations and poor overall grade was identified (p = 0.009). By position, tackles showed significant association between high magnitude accelerations and poor overall grade for the tackler (p = 0.005), but not for the ball carrier (p = 0.542).By phase, approach and impact for the tackler were associated (p = 0.013 and p = 0.001, respectively), and the impact phase for the ball carrier displayed good evidence of association with head acceleration (p = 0.072). No other phases showed association between high magnitude accelerations and poor impact grade (Figure 2).

Figure 2:

Grade was shown to be associated with high magnitude accelerations in the Approach (p=0.013) and Impact (p = 0.001) phases for the tackler and to display good evidence of association with outcome in the Impact phase (p = 0.072) for the ball carrier.

Video quality varied by practice and drill type. Some phases of certain tackles were difficult to grade due to decreased video quality. Examples of good and poor grades for each phase of both the tackler and ball carrier can be seen in Figure 3. The most significant phase for both positions was the middle impact phase, which resulted in 39–45% of poor tackles being high magnitude accelerations and over 77% of good tackles being low magnitude (less than 40 g).

Figure 3:

Exemplary failing tackles for the tackler and ball carrier. First row from left: 1) the tackler puts his head down and in the path of the ball carrier; 2) the tackler leads with his chest and aims too high; 3) the tackler does not wrap the ball carrier. Second row from left: 4) the ball carrier directs his face downward, 5) the ball carrier does not initiate contact, and 6) the ball carrier (right) stops attempting forward progress after collision.

DISCUSSION

Although participants had not been trained on the tackling criteria used to evaluate their tackling form in this study, these players exhibited both good and poor form as defined by the established criteria during their practices. Tackling form did have an association with acceleration magnitude outcome, especially for the tackler, in this group of youth football players during one-on-one tackles. For both position groups, the impact phase was the most significant phase of the collision in determining acceleration level outcome. The drive phase was seen to have the least significant association for both positions.

The association between grade and outcome was thought to be high in the impact phase because this phase was graded at the moment of collision. Additionally, criteria for this phase explicitly included using anything other than the near shoulder (e.g. the head) to lead when striking the opposing player. This suggests the impact phase should be stressed most during tackling technique training, as this is where the most improvement can occur to avoid high magnitude head impact exposure. Interestingly, a logistic general linear model relating approach grade to impact grade for both positions showed that the chances of obtaining a passing impact phase score were 2.4 times higher when the player first passed in the approach phase. This suggests that the approach is also a very important piece in avoiding high magnitude accelerations.

The drive phase did not appear to have a meaningful effect on the head acceleration magnitude in these impacts. This phase occurs after the impact has already taken place and does not have a large effect on the head’s involvement in a tackle. Reanalyzing the data without the drive phase yielded very similar results: for overall scores, the tackler showed strong evidence for association between high acceleration and poor form, and the ball carrier showed similar trends but with less statistical evidence. As all the noted trends were similarly seen in both tackler and ball carrier results, it may be important to include sessions on absorbing a tackle in youth football programs in order to protect players on both sides of the ball.

Lower association among ball carriers with the tackle-absorbing criteria developed for this study suggest there may be other factors which were not addressed in this grading scheme that also influence acceleration outcome for these offensive players. This is likely due in part to the fact that ball carriers and tacklers have entirely different goals when entering a one-on-one collision scenario. The tackler is aimed only at bringing the ball carrier to the ground and impeding his progress. On the other hand, the ball carrier’s goal is to get himself and the football past the tackler in any way he desires. This can include making a last-second cut to avoid the tackler, driving his body directly into the tackler, or any combination of these types of moves. In other words, a ball carrier is less predictable and his criteria is therefore less generalizable. Regardless, a criteria for training ball carriers might be relevant for youth football training programs, as ball carriers still received 48% of all high magnitude impacts graded in this study.

This study was limited by video quality, as some impacts occurred closer to the camera than others. In addition, grading in this study was completed by a single reviewer on a single team’s tackling technique as a feasibility analysis and because of his familiarity with the subject population and tackling form requirements. The next step in this process will be to optimize the grading criteria to obtain sufficient inter-rater reliability with multiple reviewers.

CONCLUSIONS

Youth football players exhibit a number of different types of tackling form under conventional coaching. Youth sports organizations recommend shoulder tackling and keeping the head out of the tackle to avoid head injury and cumulative head impact exposure. This study reveals that these training techniques may have efficacy in that good tackling form did show significant difference from poor form in the acceleration magnitude outcome for this cohort of players. This study did not specifically investigate any one youth football organization’s tackling recommendations; further research should be done to confirm that good form as defined by each organization is truly associated with better head acceleration outcome. In addition, these or similar grading criteria could be used to evaluate information retention and translation of learnings to the field among youth athletes trained in proper tackling technique.