Headers, with the head actively being used for influencing a match, are unique in sports and represent an important strategic action in soccer. In current studies, suspicions are expressed that headers may have a negative impact on the brain [1
]. The potential danger of headers is compared to sports such as boxing, american football, and ice hockey, all of which demonstrably promote the development of chronic traumatic encephalopathy [3
]. In particular, the risk of recurring traumatic brain injuries (concussions) is high [5
]. Concussions are said to result from collisions with players or the ground because they strongly accelerate the head [7
]. In addition to these unintentional impacts, another potential danger lies in the high number of headers, i.e., thousands of active headers in the course of a soccer career [10
]. Caccese and colleagues [8
] are talking about cumulative exposure in this case, a function based on force and frequency of impacts. They examined the head acceleration of 23 female college soccer players during matches and training. Depending on the type of header or collision, they measured average accelerations of 26.1 G to 51.3 G [10
From a sports science point of view, many questions on kinetics and kinematics remain to be answered, which would be quite important for understanding and associated preventive approaches [13
]. Fatigue during soccer matches has been a topic of enlarged interest, since the overall intensity of play has increased [14
]. Research concerning the influence of factors such as fatigue has been rather rudimentary to date, especially its effects when heading a ball in soccer [13
]. Current studies show, for example, that a muscular imbalance between neck flexors and extensors is associated with an increased acceleration of the head when performing a header [16
]. In addition, the strength potential of the neck flexors and extensors seems to be a predictor for the accelerations to be expected [17
During ball contact, the cervical spine and head are in neuromuscular stabilization and stiffened with the trunk, while the acceleration required for a header is generated by the core muscles [18
]. Therefore, it seems reasonable to assume that a decoupling of the head–neck–torso alignment takes place due to fatigue or a low training level of the stabilizing muscles, and that in such cases the head is actively moved toward the ball. This in turn results in a reduction of the accelerated mass used for the force impact (head versus head–neck–torso) [19
]. An amplified nodding motion could then compensate for the fatigued core muscles, which would be associated with an increased activation of the neck musculature [21
The aim of this study was to examine the influence of a fatigued core-stabilizing musculature on the acceleration of the head during headers from a jumping and running motion. Additionally, the data collected was to determine whether general differences (before fatigue) exist between headers performed from a standing position, a jumping and a running motion. Moreover, we aimed for a sub-group comparison to identify how acceleration differs between defensive and offensive players and between semi-professional and recreational players.
The acceleration of the head between pre- and post-test caused by the fatigue of the core-stabilizing muscles was reduced by 0.3 G in jump headers and by 0.2 G in run headers (see Table 3
In the post-test, a significant difference (F = 47.67, p = 0.000) was identified between the three header variants standing (N = 68, 6.0 ± 1.1 G), jumping (N = 67, 5.6 ± 1.1 G), and running (N = 64, 7.3 ± 0.9 G). The post hoc Scheffé test showed significant differences between the standing and running variants (p = 0.000; d = 1.29) and between the jumping and running variants (p = 0.000, d = 1.69).
In the sub-group comparison between offensive and defensive players, no significant difference between the standing, jumping, and running variants could be detected. This also applies to the acceleration values in the sub-group comparison between recreational and semi-professional players (Table 4
The aim of this study was to examine the influence of a fatigued core-stabilizing musculature on the acceleration of the head during headers from a jumping and running motion. Furthermore, we investigated the potential general differences (before fatigue) between stand, jump and run headers as well as the differences between different players’ positions and skill levels.
The significance of the core muscles for the head–neck–torso alignment for reducing the strain during headers has already been emphasized by several teams [10
]. The stiffer the head–neck–torso alignment, the larger the accelerated mass and the less the resulting acceleration of the head [31
]. Hip and trunk extensors provide the arched body tension required, and the trunk flexors are essentially responsible for the acceleration of the head–neck–torso system toward the ball [30
]. Therefore, a compensation mechanism for fatigued core muscles is conceivable. It would counterbalance the reduced activity of the core muscles (particularly the trunk flexors) by an increased activity of the neck muscles, i.e., an increased nodding motion [21
]. It needs to be taken into account that this should actually result in dissolution of the head–neck–torso segment stiffness, which, in turn, could lead to increased acceleration of the head. This would increase the potential danger of head injuries [29
]. An increased acceleration of the head should be recorded after fatigue in this case.
However, we were not able to identify an increased acceleration of the head after core muscle fatigue in our test setting. There even was a significant reduction during headers from a jumping motion (p
= 0.011) and a reduction during headers from a running motion (p
= 0.067). These results coincide with the results of other tests [13
]. Two explanations are possible. On the one hand, the reduced acceleration ability or strength development of the trunk seems to be predominant with a fatigued or weak core musculature. On the other hand, the fundamental test setup in the laboratory environment with the motionless pendulum header may be responsible. A field analysis using a ball machine would probably exhibit higher head accelerations and thus higher strain [10
The comparison results of the header variants correspond to the results of earlier tests [18
]. In comparison with the standing and jumping header variants, the header from a running motion correlates with a significantly increased acceleration. A general statement on the difference between the standing and jumping variants seems difficult because numerous factors, such as ball flight curve, ball speed, and timing significantly influence the result. In particular, timing and the coordinately more complex movement pattern are considered the reason for the lower degree of acceleration in the header variant from a jumping motion.
No statistically relevant differences were identified in the sub-group comparison between defensive and offensive players. Despite the different requirement profiles of those groups, the motion technique does not seem to differ significantly, which corresponds to the current findings by Caccese and colleagues [10
]. Similar results were observed in the sub-group comparison between semi-professional and recreational players. Nevertheless, the averaged data for the recreational players show a reduced acceleration of the head in all three variants compared to the semi-professional players (standing: −0.5 G, jumping: −0.4 G, running: −0.4 G). Presumably, technical superiority effectuates quicker acceleration of trunk and ball. Subsequent analyses should include the strength of the neck flexors for a more detailed comparison.
This study showed that fatigued core muscles influence the execution of headers in soccer. In contrast to expectations postulated in literature that the acceleration of the head should increase due to a reduced head–neck–torso alignment, the acceleration of the head actually decreases after fatigue [29
]. Use of the pendulum header and the associated elimination of numerous potential disturbance variables (such as ball speed, ball flight curve, initial position, opponents, teammates, etc.) enable a relatively isolated examination of the header. It remains to be clarified whether the acceleration of the head really increases through the reduced head–neck–torso alignment with accelerated balls [37
]. A preventive approach, for which evidence is yet to be found, would be additional strengthening of the neck flexors and extensors [6
The limitations of the present examinations are the use of a motionless pendulum header in a laboratory setting. The high degree of standardization leads to the limited transferability of the results to the accelerations occurring in actual soccer matches. Therefore, the study results’ generalizability is limited. The large degree of heterogeneity in the sample also needs to be taken into account. Additionally, the treatment does not represent soccer-specific fatigue. However, in order to examine the importance of the core muscles and their fatigue while heading the ball, the choice of exercises is limited; soccer-specific fatigue was not the intention of this study.
Furthermore, data evaluation revealed that the acceleration peaks do not necessarily occur at the time of ball contact. Therefore, the time of measurement seems to be important for improved comparability and interpretation. However, this is often neglected or not considered in other studies. Future examinations pertaining to the acceleration of the head should therefore include a more detailed explanation of the time frame that the maximum acceleration is derived from. The state of research in this regard is still deficient.
Future studies should involve three-dimensional tracking as well as a neuromuscular analysis to get a better understanding on how the core musculature and their fatigue affect the acceleration of the head when heading the ball in soccer. In a different approach, accelerated balls would help to support a general conclusion.
To achieve evidence-based practical applications, further investigations are necessary. So far, the assumption is that an additional strengthening of the neck flexors and extensors, which is not common in soccer, might play an important role in protecting the head from high impacts during headers in soccer. Moreover, the importance of the core musculature for heading, not only for known abilities such as agility, should be underlined [13