## 4 Discussion
It is interesting to note that, as no force vector is replicated during the execution of previous studies, training-load adaptations every week designed to overload, through elastic bands, 20% of the maximal capacity of the player (which was measured every week before starting) might be an important factor. Despite the previous differences in the force vector and exercises used, one study [21] showed greater enhancements (ES = 0.81 to 0.82 vs. 0.28 to 0.40) than ours. Such differences might be due to some variables like the number of training weeks (3 weeks vs. 4 weeks), player age (U-16 vs. U-18), the tests used (V-cut and curve sprinting vs. the zig-zag test), or the devices used (elastic bands vs. bars + elastic bands). Furthermore, it is possible that when the stretch-shortening cycle or the transition between the eccentric and concentric phases is enhanced, COD ability has more chances to achieve improvements irrespective of the device or force vector used. As such, a previous study [23] focused explicitly on COD tests (0%, 12.5%, and 50% of body mass) on adolescent football players. Interestingly, the 12.5% load exclusively achieved significant improvements (ES = 0.25 to 0.45), similar to those found in the current study. Thus, it seems that both the device and the force vector used may develop greater adaptations in multidirectional ability. However, studies comparing a combination of devices and movement speeds exposed might be needed to elucidate the most appropriate method. So far, we have only analyzed the way of resisting the exercise as a possible cause of the improvement without considering the possible impact of the exercise on improving parameters related to COD performance. [PERSON] et al., 2019 [26] and [PERSON] et al., 2020 [15] associated a decrease in the navicular drop with an improvement in the ability to accelerate and decelerate the intrinsic musculature of the foot, relating this to greater shock absorption and better performance in tasks related to COD. In our study, a simple pre-training activation exercise based on a lateral step resisted with an elastic band has decreased our athletes' navicular drop in just 3 weeks of training, being one of the possible causes of the improvement in their performance in multidirectional ability. Our study shows a general decrease in the navicular drop. However, it only reaches significant values for one of the experimental groups and their dominant foot (i.e., the right foot). As indicatedby [PERSON] et al., 2019 [26], the action of the longitudinal median arch of the foot improves the navicular drop. In our case, these are favored by the choice of training exercise. The lateral step favors the deceleration-acceleration phase of the movement; all the foot pressure falls on the median arch of the foot and the longitudinal line that joins the first toe with the heel. This would also be favored during the squat performance if the foot accelerates until it is supported on the forefoot or if it is jumped with [21]. We hypothesized that, by using a diagonal resistance axis and causing a lateral movement of the body, we would have a more significant impact on the modification factors of the navicular drop. Despite the theoretical framework, small to moderate relationships were found between navicular drop and performance changes. However, when those relationships were individually grouped and analyzed, navicular drops in both right and left improvements were largely (r = 0.53 to 0.54) related to curve sprinting to the left side enhancements in group A between the pre- and mid-test, that is, after the intervention period in said group. Thus, more longitudinal studies are needed to verify the correlation between the decrease in the navicular drop and the improvement in the change of direction using individualized training loads. It is essential to acknowledge some limitations in the present study. Specifically, there is a lack of GPS to monitor the training load daily.