## 4 Discussion
Shibukawa and Hoshikawa [43] found that, by comparing the 2018 and 2020 seasons of the Japanese premiership, goalkeepers were more likely to perform short pass-like kicks instead of long clearing strikes in order to retain possession. The shift in strategy reduced player engagement in aerial heading challenges, leading to a decrease in the exposure to HAEs [43]. However, while these modifications have been beneficial in elite competition, their potential benefits may not be fully realized at lower levels of play. This is partly due to amateur players' limited ability to initiate an attack from the defensive end, resulting in a greater reliance on long goal kicks as a means to effectively progress the ball up the field [40; 44]. Instead, league officials have begun banning heading within youth matches [45], but officials fear that such absolute measures may hinder the proper development of heading technique [6]. Thus, efforts should focus on ensuring appropriate head contact with the ball, which has been shown to significantly reduce the shear forces encountered by the brain [22]. Studies of youth players found that headers performed using the top of the head resulted in larger rotational velocities [1]. To mitigate this, training programs should instruct players to contact the ball using the front of their head, reducing the magnitude of linear head acceleration experienced [1]. However, more research is needed to understand the effects of the heading technique on head kinematics in adult players. Most studies on HAEs in soccer have focused on collegiate women's soccer [1; 6; 7; 46] and female youth players [47; 48], while male players across all ages have been underrepresented. Additionally, few studies have utilized iMGs in conjunction with video review to analyze the context of recorded HAEs [46]. To obtain comparable data between studies and to accurately evaluate the risk of concussion/long-term injury, it is crucial to standardize measurement methodology. For example, Lamond et al. [7] found significant differences in linear head acceleration during different types of play but did not investigate rotational head acceleration, which appears to be a primary contributor to brain injury [46]. The brain's mechanical profile, with a high bulk modulus and low shear modulus, makes it resilient to changes in volume but vulnerable to changes in shape during impact [22]. As a result, the brain primarily deforms in shear when impacted, likely rendering it more susceptible to rotational loading [22]. Until more research is done on the validity of brain injury criteria, it is important to consider both linear and rotational kinematics when assessing the risk of concussion injury and head acceleration exposure. An earlier study [36] that measured both PLA and PAA reported average peak linear (28.2 g) and angular (7.1 krad/s2) accelerations greater than the 95 th percentile values found in the current study (23.9 g and 2.3 krad/s2, respectively). These higher kinematic values reported by Caccese et al. [36] may be partly due to measurement errors caused by relative motion between the skin and skull when using headband-based sensors [22]. Alternatively, iMGs provide a more accurate and reliable measurement of skull acceleration by directly coupling with the upper dentition and, thus, are the favored choice for evaluating HAEs [23]. Compared to previous female iMG kinematics [1; 6], our study found that male collegiate soccer players experienced higher median header magnitudes. However, further studies are needed to understand any potential sex-based discrepancies. Of the 133 HAEs analyzed in this study, 25.6% (33) occurred at an PLA of less than 10 g at the head's center of gravity. The threshold used to measure minimum acceleration can significantly affect the data obtained [22]. Higher thresholds may overlook lower-magnitude HAEs and underestimate cumulative HAE exposure over a player's career [49].