## 4 Discussion
### ACWR in Relation to RPE or External Variables  
Among the six studies using the RPE for the ACWR, four [21, 26, 27, 29] did not observe a relationship between the ratio and injuries, in contrast to the other two [28, 33]. [PERSON] et al. (2018) [21] found an increased injury frequency when the ratio deviated from the range of 0.85 to 1.25. However, they stated that no combination of ratios (1:2, 1:3, and 1:4) was suitable for injury prediction. In the same year, another study [27] reported a relationship between the ACWR and injury occurrence, but without the ability to predict injuries. In two more recent studies [26, 29], no relationship was observed between the ratio and injury occurrence. Of the six studies using GPS indicators (TD, HSD, SD), two [31, 32] found no relationship between the ACWR value and injury occurrence. In contrast, four studies [12, 19, 25, 30] observed a relationship between the ratio value and the probability of injury. Specifically, [PERSON] (2023) [31] used the uncoupled ACWR 1:3 and noted that three-quarters (3/4) of hamstring injuries occurred after loads with a ratio value less than 1.5. The researchers concluded that the ratio cannot be the primary indicator for assessing the availability of professional football players for training or matches. In a previous study [32], the uncoupled 1:4 ACWR was applied, and no spikes in the ACWR were related to subsequent injuries in professional soccer players. In contrast, [PERSON] et al. (2018) [19] applied the coupled 1:4 ACWR and found an increased injury risk when the ratio in the HSD was >1.18 and a lower risk for medium ratios compared to low ratios for factors like ACC (0.87-1.12), DEC (0.86-1.12), and RPE x duration (0.85-1.12). In a subsequent study [25], when the ratio value exceeded two, the risk of injury increased. [PERSON] et al. (2022) [30] created an alternative form of ACWR (see Table 2) and observed that this ratio is a good indicator for \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Study** & **Year** & **Sample Size** & **Duration** & **Variables** & **Number of Injuries** & **Kind of** & **Results/Conclusions** \\ \hline \multirow{4}{*}{[PERSON] et al. [30]} & \multirow{2}{*}{2022} & \multirow{2}{*}{21} & \multirow{2}{*}{1 season} & Training load & ACWR - & \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al. [31]} & \multirow{2}{*}{2023} & \multirow{2}{*}{48} & \multirow{2}{*}{2 seasons} & TD, HSD, SD & \multirow{2}{*}{20} & Uncoupled 1:3 & & & \\  & & & & & & & & \\  & & & & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al. [32]} & \multirow{4}{*}{2020} & \multirow{4}{*}{15} & \multirow{4}{*}{10 weeks} & TD, HSD, SD & \multirow{4}{*}{1} & Uncoupled 1:4 & & & \\  & & & & & & & & \\ \hline \multirow{4}{*}{[PERSON] et al. [33]} & \multirow{4}{*}{2022} & \multirow{4}{*}{15} & \multirow{4}{*}{1 season} & \multirow{4}{*}{RFE} & \multirow{4}{*}{21} & Uncoupled 1:4 & & & \\  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._assessing the risk of injury. Additionally, [PERSON] et al. (2022) [12] observed that ratios of 1:2 and 1:4 were related to injury occurrence, with the 1:2 ratio appearing to be a more sensitive indicator. An observation regarding the type of variables used for the ACWR and the findings is that the majority of studies using the RPE indicator (4 out of 6) did not observe a relationship between the ratio value and the likelihood of injury. The RPE indicator reflects the internal workload and has been suggested to mirror the external load on the player [34]. However, it is not a direct measurement indicator, and this detail may affect the outcome of the ratio. This ratio flips in studies using external workload indicators from GPS devices (four studies found a relationship, and two did not).