## 5 Discussion  
The aim of this study was to compare different ICA volumes on CMJ performance among highly trained male volleyball players. The results demonstrated a significant increase in JH and PP from the baseline to 3 and 6 min after the ISO-3 condition, which consisted of three sets lasting 9 s each (total duration: 27 s), partially confirming the stated hypotheses. Contrary to expectations, no significant differences in JH were observed after the ISO-1 condition (single set lasting 9 s). The results emphasize that to increase the JH and PP output during CMJ in highly trained athletes, the ICA protocol should consist of three sets lasting 9 s each rather than a single set. Moreover, it should be noted that despite the lack of significant differences in RSImod and CT between the studied conditions, RSImod in the ISO-3 condition approached significance, which is also important in the context of interpreting the results. Although it did not reach full statistical significance, this suggests that changes in this parameter may potentially influence the CMJ outcomes, which should be considered in future studies. The results of our study, which indicate an increase in JH and PP following the ISO-3 condition, align with previous findings by [PERSON] et al. [1], which also highlight the beneficial impact of three sets of ICA on CMJ performance. According to the previously established hypotheses, ICA leads to different outcomes compared to traditional isotonic CA, as [PERSON] and [PERSON] [3] suggested that single sets of CA in stronger individuals promote superior neuromuscular adaptations compared to multiple sets of CA. These differences may stem from the fact that isometric contractions are characterized by lower metabolic costs compared to concentric contractions, potentially enhancing athletic performance temporarily without inducing additional fatigue [17]. As noted by [PERSON] et al. [18] and [PERSON] et al. [19], a single ICA set may fail to provide a sufficient stimulus to fully recruit high-threshold motor units, which would explain the absence of significant differences in CMJ performance observed after a single ICA set in our study. However, it is not only the  
Figure 3: Change in contraction time between the conditions and time points in ISO-3—condition with 3 sets of ICA. BL—baseline; 3 min—3 rd-minute post-ICA; 6 min—6 th-minute post-ICA; 9 min—9 th-minute post-ICA. number of sets but also the total duration of the effort that contributes to the volume of ICA, which appears to be a significant factor determining the effectiveness of ICA protocols in improving jumping performance and achieving the PAPE effect. Similarly to isotonic training, where time under tension (TUT) is used as a volume indicator in strength training [20], isometric total duration can also serve as a volume marker in the ICA protocol, determining how long the effort lasts, irrespective of the number of contractions performed. This is emphasized by [PERSON] et al. [21], who reported that muscle damage induced by isometric contractions is primarily associated with the total duration of contractions rather than the number of contractions. Consequently, it can be hypothesized that the total duration of ICA may determine the occurrence of other adaptive changes observed with TUT in isotonic training, similar to the previously analyzed number of sets. However, further research is needed to identify the changes in neuromuscular and metabolic mechanisms occurring in muscles in relation to the total duration of ICA. Our results indicate that a total ICA duration of 27 s within three sets of ICA leads to a significant improvement in JH and PP, consistent with the findings of [PERSON] et al. [1]. Conversely, the lack of significant improvement observed with a shorter ICA duration of 9 s partially corresponds to the findings of [PERSON] et al. [12], who also did not report an improvement in CMJ height within one set of ICA. However, these results contradict the findings of [PERSON] et al.