# The Effects of Flywheel Training with a Portable Device on Physical Performance in Soccer Players
## 4 Discussion
We obtained similar improvements to those of other studies, which assessed the COD after applying a training program with traditional RIDs, using half the training volume per session and the same weekly training frequency [6; 21; 28; 31]. It is possible that the similar improvement obtained with half the volume per session was due to the starting strength levels of the different samples. The literature has shown that subjects with more experience achieved better performance with the use of RIDs [32]. However, it has been shown that, both in experienced and inexperienced subjects, when comparing the effects of strength training with traditional systems to training with RIDs, rotary inertial devices yield greater performance improvements in power-related variables [33]. Including a higher volume would increase the total time dedicated to this training and, therefore, decrease the total time available for technical-tactical training. Moreover, RIDs induce a high level of variability per se. Therefore, increasing the level of variability in the exercise will not add benefits to physical performance and training outcomes [34]. Regarding the external load used in inertial systems, the resistance generated by the device is proportional to the force applied by the subject during the concentric phase. This means that if a subject applies less force for any reason, the device will generate proportionally less resistance, and therefore the load is individualized in each set and repetition performed by each subject [35]. As a result, there is no need to manually adjust external loads as would be done in traditional weight training. Further research is needed to corroborate the findings of this study. Sprinting is one of the high-intensity actions that occur in soccer matches [36; 37]. Currently, the literature does not clarify if RID training enhances linear sprint performance. After applying a training program in which the EG included a front start exercise with RIDs in addition to traditional strength exercises, the authors found no significant differences with respect to the CG in the 20 m sprint [38]. [PERSON] et al. [39] also used the front start exercise with RIDs and found improvements in the 10 m sprint but without differences compared to the group that trained without RIDs. On the other hand, [PERSON] et al. [29] did not find conclusive results on the effect of training with RIDs for the improvement of the sprint. In our study, sprint performance even worsened. One possible explanation may be that linear sprints are not the high-intensity action that occurs most often in soccer, since in more than 85% of these high-intensity actions there is a COD [10]. It is possible that the use of a test that is non-specific to the habitual movement of the player may be influenced to a greater extent by the variability in its execution. Another possibility is that the proposed exercises may lead to a selective strengthening of the intrinsic foot musculature [20], with this musculature being more important in the improvement of the COD. Further research is needed to determine the influence of the test specificity, as well as to check whether the type of exercise used can improve the intrinsic foot musculature and consequently the COD performance. This study has some limitations. Firstly, there was no daily control of the training intensity. While the moment of inertia remained consistent throughout the program (0.08 kg/m\({}^{2}\)), the player's acceleration of the device during exercise execution could result in varying levels of effort. Secondly, this study investigated the effect of training with a portable RID on senior amateur soccer players, where it was not possible to quantify the load and volume of soccer training for each player. Thirdly, the strength levels of the participants were not explicitly assessed before and after applying the training protocol.