5A 118 CHAPTER 5A injuries risk [4], [6], [7]. The reason for not identifying a relationship in both literature and our study might be found in the employed method of using a nonrelative factor. This emphasizes the importance of including relative measures to a runner’s individual training progression. Therefore, this study also used a relative measure (ACWR). The ACWR as a single factor was not associated with injury risk although other studies showed that spikes in the acute workload, is associated with an increase in injury risk in Australian football [9], rugby [11] and cricket [12] . Contrary to these studies, there were relatively few spikes in the current training data set. In other words, the competitive runners in this study were not regularly exposed to a high increase of acute workloads. Absence of spikes in our dataset does not rule out that there is an association, but we were not able to study this phenomenon when using the acute workload and the related ACWR. In contrast with previous studies in competitive running using average load to identify injury risk, the present study was the first to take the change in the relative workload into account. The study demonstrated an association between an increase in the fortnightly and week-to-week ACWR difference and injury risk. This is consistent with the studies in Australian football, rugby, cricket, and soccer [9]–[13], [19], [37]. A notable finding is the delay of two weeks between the increase of the fortnightly and week-to-week ACWR difference and the injury manifestation. A similar observation was made in cricket and Australian football [9], [12]. Those studies showed an increase in injuries the subsequent week after a high increase of workload. A possible explanation for the difference in delay is the occurrence of spikes in the week before the injury in their study whereas in our study a more cumulative overloading took place. Although an increased risk of sustaining an injury was found, the predictive value of the increase of the fortnightly and week-to-week ACWR difference is low. The fortnightly AWCR difference category ‘Low increase’ had a specificity of 0.62 and a sensitivity of 0.74. The week-to-week AWCR difference category ‘Low increase’ between week three and two before an injury had a specificity of 0.57 and a sensitivity of 0.68.The low specificity and low sensitivity illustrates that the ‘Low increase’ of the fortnightly and week-to-week AWCR difference, though an important signal for an increase in injury risk, is insufficient as a single predictor of an injury. This is consistent with Carey et al. [29] and Fanchini et al. [37] where objective, subjective and relative measures proved to have poor ability to predict an injury. Another limitation of the study is the calculation of the ACWR. The ACWR is only an unbiased measure after 28 days of completing a normal training schedule. Therefore, the first four weeks of data at the start of the running season, the first four weeks of data after recovery, and the data of the rehabilitation period could not be used for monitoring ACWR. The removal of the first four weeks
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