## 4 Discussion
### Most Important Variables for ACWR  
From the 12 studies mentioned, six utilized the rating of perceived exertion (RPE) to calculate the acute-to-chronic workload ratio (ACWR). It is known that this index provides a safe representation of the internal load in trained athletes [34]. The RPE is easy to use and requires minimal equipment, making it one of the most commonly used indices for monitoring workload by coaching staff. In this review, four out of six studies exclusively using this index did not find a significant relationship with the likelihood of injury. However, considering the positive aspects mentioned earlier, we believe that this index can be monitored in conjunction with other variables.  
The most common global positioning system (GPS) variables used in three studies were total distance (TD), low-speed distance (LSD), high-speed distance (HSD), sprint distance (SD), accelerations (ACC), and decelerations (DEC). [PERSON] et al. (2018) [19] observed that when the ACWR for total distance was <0.88 or >1.11, there was an increased likelihood of injuries. [PERSON] et al. (2022) [12] reported a significant difference in the ACWR 1:2 ratio for total distance between injured and non-injured players. [PERSON] et al. (2020) [25] found an increased risk of injuries when the ACWR was >2.14 for TD. Additionally, this study was the only one reporting a relationship between LSD and injury probability (5-6 times higher) when the chronic load was low, and the ACWR was >2.  
Regarding high-speed distance (HSD), [PERSON] et al. (2018) [19] reported an increased risk of injury when the ratio was >1.18. [PERSON] et al. (2022) [12] observed differences in the 1:2 and 1:4 ratios for distances at speeds of 15-20 km/h and 20-25 km/h, indicating that thresholds may need differentiation. In the 1:2 ratio, values were >1, while in the 1:4 ratio, values were >1.07.  
A crucial factor in soccer is sprint distance (SD). Among the three studies, only [PERSON] et al. (2022) [12] investigated the relationship between the ACWR and sprint distance. Specifically, the researchers found that for the 1:4 ratio, a value greater than 1.58 and for the 1:2 ratio, a value greater than 1.47 increased the risk of non-contact injuries.  
The variables included in all three studies were accelerations (ACC) and decelerations (DEC). [PERSON] et al. (2018) [19] observed that medium ratios of ACC (0.87-1.12) and DEC (0.86-1.12) showed a lower risk of injuries compared to lower ratio values. [PERSON] et al. (2020) [25] reported an increased likelihood of injuries (5-7 times) when the ratios for ACC were >2.3 and DEC were >2.32. [PERSON] et al. (2022) [12] noted an increased injury probability when the 1:4 ratio was >1.32 and the 1:2 ratio was >1.16.  
[PERSON] et al. (2022) [30] modified the ACWR and used a workload index from GPS and accelerometers. Their study results indicated that a higher ACWR was associated with more injuries.  
From the above, it is evident that studies applying the ACWR to professional soccer players using GPS variables are limited. No secure conclusions can be drawn regarding which variables can be used by sports scientists to predict injuries. The review revealed studies supporting the relationship between the ACWR and injury probability for all explored factors, as well as studies denying the existence of such a relationship.