Figure 2 illustrates mean initiation, movement, and total times to respond to the target as a function of task for both groups. In Pro-Point, there were no differences between soccer and non-soccer players for initiation times (312 ms vs. 313 ms, t(22) = 0.16, p = 0.87), movement times (445 ms vs. 439 ms, t(22) = 1.18, p = 0.25), or total times (757 ms vs. 752 ms, t(22) = 0.55, p = 0.59) using the mixed effect model. In contrast, in Anti-Point, soccer players were marginally slower than non-soccer players for initiation times (394 ms vs. 378 ms, t(22) = 1.86, p = 0.08) and significantly slower than non-soccer players for movement times (561 ms vs. 531 ms, t(22) = 3.69, p<0.005) and total times (955 ms vs. 909 ms, t(22) = 2.81, p = 0.01) using the mixed effect model.

To further test if Anti-Point response times in soccer players could be accounted for by heading frequency or soccer experience, we included these three variables as independent variables. The mixed effect model with heading ball rate as the independent variable showed marginal effects of heading ball rate for initiation time (t(8) = 1.88, p<0.10) and total time (t(8) = 1.86, p<0.10), but not movement time (t(8) = 1.67, p>0.11) in the Anti-Point task, indicating marginally slower responses with increased number of head balls. With hours of soccer per week as the independent variable, the mixed effect model showed signicant effects for Anti-Point task initiation time (t(10) = 3.51,p<.01), movement time (t(10) = 2.27, p<.05) and total time (t(10) = 2.95, p<02), indicating slower responses with increased hours of soccer per week. Thirdly, with years of soccer as the independent variable, the mixed effect model showed a marginal effect for Anti-Point task initiation time (t(10) = 2.94,p<.07) and signifcant effects for movement time (t(10) = 3.43, p<01) as well as total time (t(10) = 2.71, p<03), indicating slower responses with increased years of soccer experience. Finally, it was determined that the independent variables of heading ball rate, hours of soccer played per week, and years of soccer played were unrelated/uncorrelated by Pearson correlation coefficients.

There were no significant differences in errors in either Pro-Point tasks (0.3% vs. 1.0%, z = 0.83, p = 0.41, logit-link generalized model) or Anti-Point (3.1% vs. 2.5%, z = 1.13, p = 0.26, for soccer and non-soccer players, logit-link generalized model).

Conclusion

The results show that soccer playing in which participants headed the ball did indeed disrupt voluntary performance in female high school soccer players tested immediately following practice. In addition, even in this small sample, this response time slowing on the Anti-Point task was marginally related to number of ball headers (n = 10) and significantly related to hours of soccer per week (n = 12) and years of soccer playing (n = 12). We found no evidence that slowing occurred during reflexive movements under identical sensorimotor conditions (Pro-Point). These findings demonstrate significant and specific cognitive changes in female high school soccer players who head the soccer ball during practice.

One alternative explanation for finding RT slowing after soccer playing is that information processing might be slowed immediately following a bout of aerobic exercise. However, we found no evidence of slowing for the Pro-Point task with identical sensory and motor demands in soccer players. Further, studies that have specifically examined the effects of exercise on cognitive performance have shown reduced RTs and enhanced cognitive functioning and memory storage and retrieval following acute aerobic exercise [24], [25]. Thus, the disruption of cognitive performance in soccer players in our study was not likely due to aerobic activities immediately preceding the testing session. Moreover, such studies, showing enhanced cognitive performance following aerobic exercise, suggest instead that the differences we report might underestimate the cognitive slowing that occurs after soccer playing with ball heading. Nevertheless, given our findings, future work should explore in soccer players the effect of aerobic exercise on cognitive performance independently of ball heading.

Some previous studies in more advanced soccer players have demonstrated correlations in cognitive disruptions with frequency of lifetime heading [26], [27]; however recent studies using computerized testing and more appropriate groups have rebutted these claims and suggested that lifetime heading is not correlated with cognitive deficits [2], [28]. Further, previous studies in young players have also failed to detect any cognitive changes [28], [29]. We found evidence in our small sample of young soccer players that soccer playing with ball heading did result in cognitive changes and that these increases in response time on the Anti-Point task related to both number of headers, hours of soccer per week, and duration of soccer experience. The most conservative interpretation of our findings is that these changes are transient and the result of the immediately preceding soccer session. The trend for an increase in response time with number of ball headers would support such an interpretation. To be consistent with this interpretation, it is possible that the response time slowing that relates to hours of soccer played per week and years of experience is only measurable immediately following these additional subconcussive blows (conceptually similar to the phenomenon of drug sensitization, where there is amplification of a response due to prior experience). Alternatively, the significant slowing that relates to hours of soccer played per week and years of experience could be interpreted to indicate longer lasting changes cumulative across days, the sports’ season, and years of experience. We are unable in our study to tease apart immediate transient effects from longer lasting effects due to the study design. Without additional data, we prefer a more conservative interpretation of transient deficits due to the immediately preceding soccer practice with ball heading. It is also possible that there exists a relationship between the independent variables of heading ball rate, years of soccer played, and hours of soccer played per week that was unattainable with our small sample. Larger scale follow up studies may show that these variables are working together to drive these effects rather than working separately as we have shown here.

The study was carried out at a local high school that was supportive of the study but wanted to minimize any inconvenience to the students, their parents (for minors under 18), and their sports schedule. The soccer sessions were actual varsity training sessions that were on their own tight schedule and it was not possible to run a pre-practice control. The varsity coach controlled the practice, including the heading portion, and we did not have control over what soccer related activity the players performed. There are many potential follow up studies that are possible to try to control for specific differences between soccer and non-soccer players that are difficult to control in a more observational rather than randomized design. Observational designs, where the assignment of treatments is beyond the control of the investigator, can be chosen for a number of reasons, including lack of influence (e.g., on structuring varsity practices) as well as concerns about violation of ethical standards. On the other hand, observational designs provide an accurate assessment of real-world use and practice and help to formulate hypotheses to be tested in following experiments (such as whether the effect we report is transient, and reduced or absent by the next day, accumulative across the week or season, or dependent on a previous history of subconcussive blows).

In sum, the cognitive changes that we report were measured with a simple iPad based application. A simple tool such as this iPad application may be a quick and effective method to screen for and track cognitive deficits in sports players. It could potentially be used to detect, screen, and track other populations for mild traumatic brain injury and development of cognitive comorbidities.

Though the changes we report were robust, they do not necessarily imply sustained changes or brain injury. Further study is needed to track soccer players for longer periods to evaluate if these changes are transient or longer-lasting, if they are dependent upon repeated subconcussive blows, and if they generalize to male soccer players. To our knowledge, these results provide the first evidence that even subconcussive blows in soccer could lead to measureable, even if possibly transient, cognitive changes in young soccer players.