This investigation implemented a telemetry based acceleration monitoring system in adolescent football athletes in an attempt to identify the biomechanical threshold for concussive injuries. An analysis of the 13 concussive episodes captured by the Head Impact Telemetry System indicated that measures of rotational acceleration, linear acceleration, and impact location appear to be the most important variables in establishing injury prediction criteria. More specifically, our injuries occurred at levels (ie 5582.3 rad/s2
, 96.1 g, and front, side, or top impacts) that are consistent with previous reports from data collected at other levels of play (12
Our analysis demonstrates that rotational accelerations in excess of 5582.3 rad/s2
may be the bottom threshold in increasing the probability of concussion in the high school athlete. This value and our range of values (5582 to 9515 rad/s2
: ) that resulted in concussion is similar to findings generated from adult athletes (17
). Indeed, biomechanical data collected with the Head Impact Telemetry System on concussed collegiate athletes identified the mean rotational acceleration at 5311.6 rad/s2
). Likewise, video reconstruction of concussed professional athletes found the mean rotational acceleration to be only slightly higher at 6432 rad/s2
). It is unknown if these differences are clinically meaningful, but they all exceed previously proposed concussions thresholds (26
The evaluation of linear acceleration values collected in this investigation indicated that impacts generating acceleration in excess of 96.1 g increased the sensitivity of our decision making tree. This value is nearly identical to the mean linear acceleration of 98g reported in professional athletes (30
) and similar to 102.8 g mean reported in collegiate athletes (17
). The range of linear accelerations (77.8 to 146.0 g: ) also fell within the range of previous reports.
A complex interrelationship exists between impact location, linear and rotational acceleration and concussion. Temporal impacts, particularly when the athlete is unaware of the impending blow, are thought to generate the greatest concussion risk (16
). This may be in part to the higher rotational component seen here () and elsewhere (17
) that result from lateral forces and its effect on brain stem integrity and subsequent loss of consciousness (25
). Anatomically, brain stem fibers have an increased susceptibility to rotational loads due to their linear alignment (2
) and computer simulation models indicate higher brain shearing with temporal impacts (34
). Further, an analysis of Australian rules football athletes revealed the majority concussions (61 of 97) resulted from temporal region impacts that generated a high rotational component (24
). Others have reported that linear acceleration is the single best predictor of concussion (30
) and more tightly linked with severe injuries such as cerebral contusion and hemorrhaging (16
). Our data do not support the use of linear acceleration as the prime variable of interest as rotational acceleration was the chief predictor within our classification tree.
The combined interpretation of rotational acceleration, linear acceleration, and impact location significantly improved the sensitivity of biomechanical variables in making injury diagnoses over previous estimates. In particular, when the single variable concussion threshold proposed by the Pellman and colleagues (30
) was applied to our dataset, 259 impacts exceeded the 98g level, nine of which resulted in concussions, providing a 3.5% sensitivity. An alternative multivariable threshold estimate implementing linear acceleration, rotational acceleration, HIC, and impact location in excess of 63(12
) also identified nine concussive episodes, but with only 150 false positive results (6.0% sensitivity). Our CART analysis however, accurately identified ten concussions among the 47 impacts exceeding our injury tolerance level (21.3% sensitivity). The improved sensitivity may have resulted from a more sophisticated data analysis technique and/or the development of an injury profile specific to the high school athlete. Importantly, the analysis supports a multi-variable approach in determining the biomechanical components of the concussion mechanism.
An additional concern related to injury diagnostics is the tolerance athletes have in sustaining blows that do not result in concussion. That is, individual variability results in not all impacts exceeding the injury predictor levels to result in concussion. Thus, there is utility in determining the percent risk for injury given a pre-determined set of head acceleration criteria. As such, evaluation of concussion data collected in the NFL suggests that a 98g acceleration places the athlete at 75% injury risk (34
). These same data were later re-evaluated on the premise of exposure bias during the original data collection period (11
). The authors’ statistical evaluation suggested that similar impact values (ie 107g, 6619 rad/s2
, and HIC of 191) lowered the injury risk in the professional athletes to 1%. Further evaluation of collegiate level impacts indicated that a similar set of acceleration criteria (ie 109g, 6714rad/s2
, and HIC of 232) resulted in the same 1% injury risk level (11
). Application of these estimates to our high school dataset revealed 71 impacts exceeded the 1% risk estimate for the professional athlete, three of which were concussive impacts (4.3%). Where as application of the 1% risk estimates generated for collegiate athlete athletes generated 60 impacts exceeding the threshold with one reported concussion (1.7%). These findings suggest that the tolerance level in the high school football athlete is slightly lower than that of the collegiate and professional athlete, placing him at an increased risk for injury compared to his older counterparts given equivalent head kinematics following impact.
The clarification of concussion biomechanics and injury tolerance levels will ultimately yield better injury prevention equipment. American football helmets that were initially designed to reduce the risk of skull to skull contact resulting in fracture are now engineered to reduce concussion incidence. The Riddell (Elyria, OH) Revolution uses thickened padding in the temporal region that protects the athlete from lateral blows (8
). Revolution helmets were solely employed in this investigation and we found only 2 of 13 concussions resulted from side impacts. It may therefore be reasonable to believe that this innovation has been successful at reducing concussion incidence resulting from blows to the lateral helmet, but other impact areas should be evaluated more closely. For example, consistent with impact descriptives from high school football (7
), concussions resulting from top of head impacts were the result of the highest magnitude linear acceleration values (108.8 g: ), but the majority of concussions resulted from front and back impacts (9 of 13).
Further, not all athletic endeavors require or allow the use of protective headgear, necessitating the exploration of alternative prevention methods. Neck strengthening protocols are one suggested, but unproven, concussion deterrent intervention. The underlying justification for this type of training is to tighten the neck musculature at the time of impact to create a single rigid unit of both the head and body to increase effective mass and decrease the resulting post-impact head acceleration. Animal based research highlights this principle when the impacted head is and is not secured by external means. Identical force application generating a concussion when the head is allowed to rotate freely did not yield injury when the head was externally secured (25
). Others have shown the same effect by using electrical stimulation to generate neck musculature contractions forceful enough to prevent head motion and consequently concussion (19
). Studies of soccer heading yield similar findings with sternocleidomastoid activation just prior to impact to stabilize the head against the ball (4
). In combination these investigations support the idea that tensing the neck musculature in an effort to increase effective mass at the moment of impact and lessen head acceleration following impact may be an effective concussion prevention strategy.
This investigation provides a biomechanical assessment of concussions sustained in high school football athletes. While our statistical approach differs from previous accounts in both collegiate and professional athletes, the variables of rotational acceleration, linear acceleration, and impact location all appear to play a similarly important role in injury prediction. In fact, the variables and values estimated here appear to be nearly indistinguishable to previous biomechanical studies generated from collegiate and professional athletes. The high school athlete’s tolerance to impacts, given a standardized set of acceleration parameters, appears to be slightly lower than older players, placing them at a slightly elevated risk for injury. Despite this, epidemiological studies support the notion that overall concussion incidence rates are virtually identical across all levels of play: high school (3.6 to 5.6%)(18
), collegiate (4.8 to 6.3%)(15
), and professional (7.7%)(29
). Although the slower and less physical high school game results in lower impact forces, injury incidence similarity may be a consequence of an immature musculoskeletal system and the subsequent diminished ability to control and thusly slow head acceleration following impacts (7
). Additionally, equipment age, quality, and fit are known to be compromised at the high school level and may influence injury rates.