Abstract: In the past decade, significant efforts have been made to understand injury risk in sport using subjective (i.e. rating of perceived exertion) and objective (i.e. inertial sensor outputs) player-monitoring strategies. Particular focus has been placed on the acute:chronic workload ratio (ACWR), defined as the ratio of average acute (1-week) to chronic (4-week) training loads. In the past 5 years, numerous academic papers across multiple sports have concluded that ACWR is predictive of injury risk, and as a result the ACWR has become standard practice in professional sports to manage player workloads.
In this paper, we demonstrate that causal conclusions about the ACWR-injury relationship are prone to confounding from schedule. We use Monte Carlo methods combined with training load data from two sports to illustrate the effect that the yearly training calendar has on the ACWR-injury relationship. We then propose options to mediate this confounding. Our study impacts not only the academic discourse around the ACWR, but also gives practitioners a more realistic expectation of its value in predicting injury.