Critical Fluctuations as an Early Warning Signal of Sports Injuries? Applying the Complex Dynamic Systems Toolbox to Football Monitoring Data

Background:
There has been an increasing interest in the development and prevention of sports injuries from a complex dynamic systems perspective. From this perspective, injuries may occur following critical ?uctuations in the psychophysiological state of an athlete. Our objective was to quantify these so-called Early Warning Signals (EWS) to determine their predictive validity for injuries. The sample consisted of 23 professional youth football (soccer) players. Self-reports of psychological and physiological factors as well as data from GPS sensors were gathered on every training and match day over two competitive seasons, which resulted in an average of 339 observations per player (range = 155–430). We calculated the Dynamic Complexity (DC) index of these data, representing a metric of critical ?uctuations. Next, we used this EWS to predict injuries based on different mechanisms (traumatic and overuse) and duration.

Results:
Results showed a signi?cant peak of DC in 31% of the incurred injuries, regardless of mechanism and duration, in the seven data points (roughly one and a half weeks) before the injury. The warning signal exhibited a speci?city of 94%, that is, correctly classifying non-injury instances. We followed up on this promising result with additional calculations to account for the naturally imbalanced data (fewer injuries than non-injuries). The relatively low F1 we obtained (0.08) suggests that the model's overall ability to discriminate between injuries and non-injuries is rather poor, due to the high false positive rate.

Conclusion:
By detecting critical ?uctuations preceding one-third of the injuries, this study provided support for the complex systems theory of injuries. Furthermore, it suggests that increasing critical ?uctuations may be seen as an EWS on which practitioners can intervene. Yet, the relatively high false positive rate on the entire data set, including periods without injuries, suggests critical ?uctuations may also precede transitions to other (e.g., stronger) states. Future research should therefore dig deeper into the meaning of critical ?uctuations in the psychophysiological states of athletes.